@phdthesis{Kreikenbohm2019,
  author    = {Kreikenbohm, Annika Franziska Eleonore},
  title     = {Classifying the high-energy sky with spectral timing methods},
  doi       = {10.25972/OPUS-19205},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-192054},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Active galactic nuclei (AGN) are among the brightest and most frequent sources on the extragalactic X-ray and gamma-ray sky. Their central supermassive blackhole generates an enormous luminostiy through accretion of the surrounding gas. A few AGN harbor highly collimated, powerful jets in which are observed across the entire electromagnetic spectrum. If their jet axis is seen in a small angle to our line-of-sight (these objects are then called blazars) jet emission can outshine any other emission component from the system. Synchrotron emission from electrons and positrons clearly prove the existence of a relativistic leptonic component in the jet plasma. But until today, it is still an open question whether heavier particles, especially protons, are accelerated as well. If this is the case, AGN would be prime candidates for extragalactic PeV neutrino sources that are observed on Earth. Characteristic signatures for protons can be hidden in the variable high-energy emission of these objects. In this thesis I investigated the broadband emission, particularly the high-energy X-ray and gamma-ray emission of jetted AGN to address open questions regarding the particle acceleration and particle content of AGN jets, or the evolutionary state of the AGN itself. For this purpose I analyzed various multiwavelength observations from optical to gamma-rays over a period of time using a combination of state-of-the-art spectroscopy and timing analysis. By nature, AGN are highly variable. Time-resolved spectral analysis provided a new dynamic view of these sources which helped to determine distinct emission processes that are difficult to disentangle from spectral or timing methods alone. Firstly, this thesis tackles the problem of source classification in order to facilitate the search for interesting sources in large data archives and characterize new transient sources. I use spectral and timing analysis methods and supervised machine learning algorithms to design an automated source classification pipeline. The test and training sample were based on the third XMM-Newton point source catalog (3XMM-DR6). The set of input features for the machine learning algorithm was derived from an automated spectral modeling of all sources in the 3XMM-DR6, summing up to 137200 individual detections. The spectral features were complemented by results of a basic timing analysis as well as multiwavelength information provided by catalog cross-matches. The training of the algorithm and application to a test sample showed that the definition of the training sample was crucial: Despite oversampling minority source types with synthetic data to balance out the training sample, the algorithm preferably predicted majority source types for unclassified objects. In general, the training process showed that the combination of spectral, timing and multiwavelength features performed best with the lowest misclassification rate of \\sim2.4\\\%. The methods of time-resolved spectroscopy was then used in two studies to investigate the properties of two individual AGN, Mrk 421 and PKS 2004-447, in detail. Both objects belong to the class of gamma-ray emitting AGN. A very elusive sub-class are gamma-ray emitting Narrow Line Seyfert 1 (gNLS1) galaxies. These sources have been discovered as gamma-ray sources only recently in 2010 and a connection to young radio galaxies especially compact steep spectrum (CSS) radio sources has been proposed. The only gNLS1 on the Southern Hemisphere so far is PKS2004-447 which lies at the lower end of the luminosity distribution of gNLS1. The source is part of the TANAMI VLBI program and is regularly monitored at radio frequencies. In this thesis, I presented and analyzed data from a dedicated multiwavelength campaign of PKS 2004-447 which I and my collaborators performed during 2012 and which was complemented by individual observations between 2013 and 2016. I focussed on the detailed analysis of the X-ray emission and a first analysis of its broadband spectrum from radio to gamma-rays. Thanks to the dynamic SED I could show that earlier studies misinterpreted the optical spectrum of the source which had led to an underestimation of the high-energy emission and had ignited a discussion on the source class. I show that the overall spectral properties are consistent with dominating jet emission comprised of synchrotron radiation and inverse Compton scattering from accelerated leptons. The broadband emission is very similar to typical examples of a certain type of blazars (flat-spectrum radio quasars) and does not present any unusual properties in comparison. Interestingly, the VLBI data showed a compact jet structure and a steep radio spectrum consistent with a compact steep spectrum source. This classified PKS 2004-447 as a young radio galaxy, in which the jet is still developing. The investigation of Mrk 421 introduced the blazar monitoring program which I and collaborator have started in 2014. By observing a blazar simultaneously from optical, X-ray and gamma-ray bands during a VHE outbursts, the program aims at providing extraordinary data sets to allow for the generation of a series of dynamical SEDs of high spectral and temporal resolution. The program makes use of the dense VHE monitoring by the FACT telescope. So far, there are three sources in our sample that we have been monitoring since 2014. I presented the data and the first analysis of one of the brightest and most variable blazar, Mrk 421, which had a moderate outbreak in 2015 and triggered our program for the first time. With spectral timing analysis, I confirmed a tight correlation between the X-ray and TeV energy bands, which indicated that these jet emission components are causally connected. I discovered that the variations of the optical band were both correlated and anti-correlated with the high-energy emission, which suggested an independent emission component. Furthermore, the dynamic SEDs showed two different flaring behaviors, which differed in the presence or lack of a peak shift of the low-energy emission hump. These results further supported the hypothesis that more than one emission region contributed to the broadband emission of Mrk 421 during the observations. Overall,the studies presented in this thesis demonstrated that time-resolved spectroscopy is a powerful tool to classify both source types and emission processes of astronomical objects, especially relativistic jets in AGN, and thus provide a deeper understanding and new insights of their physics and properties.},
  subject      = {Astronomie},
  language  = {en}
}
@phdthesis{Geissler2017,
  author    = {Geißler, Florian},
  title     = {Transport properties of helical Luttinger liquids},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-153450},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2017},
  abstract  = {The prediction and the experimental discovery of topological insulators has set the stage for a novel type of electronic devices. In contrast to conventional metals or semiconductors, this new class of materials exhibits peculiar transport properties at the sample surface, as conduction channels emerge at the topological boundaries of the system. In specific materials with strong spin-orbit coupling, a particular form of a two-dimensional topological insulator, the quantum spin Hall state, can be observed. Here, the respective one-dimensional edge channels are helical in nature, meaning that there is a locking of the spin orientation of an electron and its direction of motion. Due to the symmetry of time-reversal, elastic backscattering off interspersed impurities is suppressed in such a helical system, and transport is approximately ballistic. This allows in principle for the realization of novel energy-efficient devices, ``spintronic`` applications, or the formation of exotic bound states with non-Abelian statistics, which could be used for quantum computing. The present work is concerned with the general transport properties of one-dimensional helical states. Beyond the topological protection mentioned above, inelastic backscattering can arise from various microscopic sources, of which the most prominent ones will be discussed in this Thesis. As it is characteristic for one-dimensional systems, the role of electron-electron interactions can be of major importance in this context. First, we review well-established techniques of many-body physics in one dimension such as perturbative renormalization group analysis, (Abelian) bosonization, and Luttinger liquid theory. The latter allow us to treat electron interactions in an exact way. Those methods then are employed to derive the corrections to the conductance in a helical transport channel, that arise from various types of perturbations. Particularly, we focus on the interplay of Rashba spin-orbit coupling and electron interactions as a source of inelastic single-particle and two-particle backscattering. It is demonstrated, that microscopic details of the system, such as the existence of a momentum cutoff, that restricts the energy spectrum, or the presence of non-interacting leads attached to the system, can fundamentally alter the transport signature. By comparison of the predicted corrections to the conductance to a transport experiment, one can gain insight about the microscopic processes and the structure of a quantum spin Hall sample. Another important mechanism we analyze is backscattering induced by magnetic moments. Those findings provide an alternative interpretation of recent transport measurements in InAs/GaSb quantum wells.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@article{KernreiterGovernaleZuelickeetal.2016,
  author    = {Kernreiter, T. and Governale, M. and Z{\"u}licke, U. and Hankiewicz, E. M.},
  title     = {Anomalous Spin Response and Virtual-Carrier-Mediated Magnetism in a Topological Insulator},
  series = {Physical Review X},
  volume    = {6},
  journal   = {Physical Review X},
  number    = {021010},
  doi       = {10.1103/PhysRevX.6.021010},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166582},
  year      = {2016},
  abstract  = {We present a comprehensive theoretical study of the static spin response in HgTe quantum wells, revealing distinctive behavior for the topologically nontrivial inverted structure. Most strikingly, the q=0 (long-wavelength) spin susceptibility of the undoped topological-insulator system is constant and equal to the value found for the gapless Dirac-like structure, whereas the same quantity shows the typical decrease with increasing band gap in the normal-insulator regime. We discuss ramifications for the ordering of localized magnetic moments present in the quantum well, both in the insulating and electron-doped situations. The spin response of edge states is also considered, and we extract effective Land{\´e} g factors for the bulk and edge electrons. The variety of counterintuitive spin-response properties revealed in our study arises from the system's versatility in accessing situations where the charge-carrier dynamics can be governed by ordinary Schr{\"o}dinger-type physics; it mimics the behavior of chiral Dirac fermions or reflects the material's symmetry-protected topological order.},
  language  = {en}
}
@article{BechtleCamargoMolinaDeschetal.2016,
  author    = {Bechtle, Philip and Camargo-Molina, Jos{\´e} Eliel and Desch, Klaus and Dreiner, Herbert K. and Hamer, Matthias and Kr{\"a}mer, Michael and O'Leary, Ben and Porod, Werner and Sarrazin, Bj{\"o}rn and Stefaniak, Tim and Uhlenbrock, Mathias and Wienemann, Peter},
  title     = {Killing the cMSSM softly},
  series = {The European Physical Journal C},
  volume    = {76},
  journal   = {The European Physical Journal C},
  number    = {96},
  doi       = {10.1140/epjc/s10052-015-3864-0},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-165045},
  year      = {2016},
  abstract  = {We investigate the constrained Minimal Supersymmetric Standard Model (cMSSM) in the light of constraining experimental and observational data from precision measurements, astrophysics, direct supersymmetry searches at the LHC and measurements of the properties of the Higgs boson, by means of a global fit using the program Fittino. As in previous studies, we find rather poor agreement of the best fit point with the global data. We also investigate the stability of the electro-weak vacuum in the preferred region of parameter space around the best fit point. We find that the vacuum is metastable, with a lifetime significantly longer than the age of the Universe. For the first time in a global fit of supersymmetry, we employ a consistent methodology to evaluate the goodness-of-fit of the cMSSM in a frequentist approach by deriving p values from large sets of toy experiments. We analyse analytically and quantitatively the impact of the choice of the observable set on the p value, and in particular its dilution when confronting the model with a large number of barely constraining measurements. Finally, for the preferred sets of observables, we obtain p values for the cMSSM below 10 \%, i.e. we exclude the cMSSM as a model at the 90 \% confidence level.},
  language  = {en}
}
@phdthesis{Fleckenstein2020,
  author    = {Fleckenstein, Christoph Thomas},
  title     = {Conception and detection of exotic quantum matter in mesoscopic systems},
  doi       = {10.25972/OPUS-21284},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-212847},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2020},
  abstract  = {In this thesis we discuss the potential of nanodevices based on topological insulators. This novel class of matter is characterized by an insulating bulk with simultaneously conducting boundaries. To lowest order, the states that are evoking the conducting behavior in TIs are typically described by a Dirac theory. In the two-dimensional case, together with time- reversal symmetry, this implies a helical nature of respective states. Then, interesting physics appears when two such helical edge state pairs are brought close together in a two-dimensional topological insulator quantum constriction. This has several advantages. Inside the constriction, the system obeys essentially the same number of fermionic fields as a conventional quantum wire, however, it possesses more symmetries. Moreover, such a constriction can be naturally contacted by helical probes, which eventually allows spin- resolved transport measurements. We use these intriguing properties of such devices to predict the formation and detection of several profound physical effects. We demonstrate that narrow trenches in quantum spin Hall materials - a structure we coin anti-wire - are able to show a topological super- conducting phase, hosting isolated non-Abelian Majorana modes. They can be detected by means of a simple conductance experiment using a weak coupling to passing by helical edge states. The presence of Majorana modes implies the formation of unconventional odd-frequency superconductivity. Interestingly, however, we find that regardless of the presence or absence of Majoranas, related (superconducting) devices possess an uncon- ventional odd-frequency superconducting pairing component, which can be associated to a particular transport channel. Eventually, this enables us to prove the existence of odd- frequency pairing in superconducting quantum spin Hall quantum constrictions. The symmetries that are present in quantum spin Hall quantum constrictions play an essen- tial role for many physical effects. As distinguished from quantum wires, quantum spin Hall quantum constrictions additionally possess an inbuilt charge-conjugation symmetry. This can be used to form a non-equilibrium Floquet topological phase in the presence of a time-periodic electro-magnetic field. This non-equilibrium phase is accompanied by topological bound states that are detectable in transport characteristics of the system. Despite single-particle effects, symmetries are particularly important when electronic in- teractions are considered. As such, charge-conjugation symmetry implies the presence of a Dirac point, which in turn enables the formation of interaction induced gaps. Unlike single-particle gaps, interaction induced gaps can lead to large ground state manifolds. In combination with ordinary superconductivity, this eventually evokes exotic non-Abelian anyons beyond the Majorana. In the present case, these interactions gaps can even form in the weakly interacting regime (which is rather untypical), so that the coexistence with superconductivity is no longer contradictory. Eventually this leads to the simultaneous presence of a Z4 parafermion and a Majorana mode bound at interfaces between quantum constrictions and superconducting regions.},
  subject      = {Kondensierte Materie},
  language  = {en}
}
@article{FleszarHanke2015,
  author    = {Fleszar, Andrzej and Hanke, Werner},
  title     = {Two-dimensional metallicity with a large spin-orbit splitting: DFT calculations of the atomic, electronic, and spin structures of the Au/Ge(111)-(√3 x √3)R30° surface},
  series = {Advances in Condensed Matter Physics},
  volume    = {2015},
  journal   = {Advances in Condensed Matter Physics},
  number    = {531498},
  doi       = {10.1155/2015/531498},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149221},
  year      = {2015},
  abstract  = {Density functional theory (DFT) is applied to study the atomic, electronic, and spin structures of the Au monolayer at the Ge(111) surface. It is found that the theoretically determined most stable atomic geometry is described by the conjugated honeycomb-chained-trimer (CHCT) model, in a very good agreement with experimental data. The calculated electronic structure of the system, being in qualitatively good agreement with the photoemission measurements, shows fingerprints of the many-body effects (self-interaction corrections) beyond the LDA or GGA approximations. The most interesting property of this surface system is the large spin splitting of its metallic surface bands and the undulating spin texture along the hexagonal Fermi contours, which highly resembles the spin texture at the Dirac state of the topological insulator Bi\(_{2}\)Te\(_{3}\). These properties make this system particularly interesting from both fundamental and technological points of view.},
  language  = {en}
}
@phdthesis{Northe2019,
  author    = {Northe, Christian},
  title     = {Interfaces and Information in Gauge/Gravity Duality},
  doi       = {10.25972/OPUS-19159},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-191594},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {This dissertation employs gauge/gravity duality to investigate features of ( 2 + 1 ) -dimensional quantum gravity in Anti-de Sitter space (AdS) and its relation to conformal field theory (CFT) in 1 + 1 dimensions. Concretely, we contribute to research on the frontier of gauge/gravity with condensed matter as well as the frontier with quantum informa- tion. The first research topic of this thesis is motivated by the Kondo model, which describes the screening of magnetic impurities in metals by conduction electrons at low temperatures. This process has a de- scription in the language of string theory via fluctuating surfaces in spacetime, called branes. At high temperatures the unscreened Kondo impurity is modelled by a stack of pointlike branes. At low tempera- tures this stack condenses into a single spherical, two-dimensional brane which embodies the screened impurity. This thesis demonstrates how this condensation process is naturally reinvoked in the holographic D1/D5 system. We find brane configu- rations mimicking the Kondo impurities at high and low energies and establish the corresponding brane condensation, where the brane grows two additional dimensions. We construct supergravity solutions, which fully take into account the effect of the brane on its surrounding space- time before and after the condensation takes place. This enables us to compute the full impurity entropies through which we confirm the validity of the g-theorem. The second research topic is rooted in the connection of geometry with quantum information. The motivation stems from the "complexity equals volume" proposal, which relates the volume of wormholes to the cicruit complexity of a thermal quantum state. We approach this proposal from a pragmatic point of view by studying the properties of certain volumes in gravity and their description in the CFT. We study subregion complexities, which are the volumes of the re- gions subtended by Ryu-Takayanagi (RT) geodesics. On the gravity side we reveal their topological properties in the vacuum and in ther- mal states, where they turn out to be temperature independent. On the field theory side we develop and proof a formula using kinematic space which computes subregion complexities without referencing the bulk. We apply our formula to global AdS 3 , the conical defect and a black hole. While entanglement, i.e. minimal boundary anchored geodesics, suffices to produce vacuum geometries, for the conical defect we also need geodesics windings non-trivially around the singularity. The black hole geometry requires additional thermal contributions.},
  subject      = {Information},
  language  = {en}
}
@article{AdrianMartinezAgeronAharonianetal.2016,
  author    = {Adri{\´a}n-Mart{\´i}nez, S. and Ageron, M. and Aharonian, F. and Aiello, S. and Albert, A. and Ameli, F. and Annasontzis, E. and Andre, M. and Androulakis, G. and Anghinolfi, M. and Anton, G. and Ardid, M. and Avgitas, T. and Barbarino, G. and Baret, B. and Barrios-Mart{\´i}, J. and Belhorma, B. and Belias, A. and Berbee, A. and van den Berg, A. and Bertin, V. and Beurthey, S. and van Beeveren, V. and Beverini, N. and Biagi, S. and Biagioni, A. and Billault, M. and Bond{\`i}, M. and Bormuth, R. and Bouhadef, B. and Bourlis, G. and Bourret, S. and Boutonnet, C. and Bouwhuis, M. and Bozza, C. and Bruijn, R. and Brunner, J. and Buis, E. and Busto, J. and Cacopardo, G. and Caillat, L. and Calmai, M. and Calvo, D. and Capone, A. and Caramete, L. and Cecchini, S. and Celli, S. and Champion, C. and Cherkaoui El Moursli, R. and Cherubini, S. and Chiarusi, T. and Circella, M. and Classen, L. and Cocimano, R. and Coelho, J. A. B. and Coleiro, A. and Colonges, S. and Coniglione, R. and Cordelli, M. and Cosquer, A. and Coyle, P. and Creusot, A. and Cuttone, G. and D'Amico, A. and De Bonis, G. and De Rosa, G. and De Sio, C. and Di Capua, F. and Di Palma, I. and D{\´i}az Garc{\´i}a, A. F. and Distefano, C. and Donzaud, C. and Dornic, D. and Dorosti-Hasankiadeh, Q. and Drakopoulou, E. and Drouhin, D. and Drury, L. and Durocher, M. and Eberl, T. and Eichie, S. and van Eijk, D. and El Bojaddaini, I. and El Khayati, N. and Elsaesser, D. and Enzenh{\"o}fer, A. and Fassi, F. and Favali, P. and Fermani, P. and Ferrara, G. and Filippidis, C. and Frascadore, G. and Fusco, L. A. and Gal, T. and Galat{\`a}, S. and Garufi, F. and Gay, P. and Gebyehu, M. and Giordano, V. and Gizani, N. and Gracia, R. and Graf, K. and Gr{\´e}goire, T. and Grella, G. and Habel, R. and Hallmann, S. and van Haren, H. and Harissopulos, S. and Heid, T. and Heijboer, A. and Heine, E. and Henry, S. and Hern{\´a}ndez-Rey, J. J. and Hevinga, M. and Hofest{\"a}dt, J. and Hugon, C. M. F. and Illuminati, G. and James, C. W. and Jansweijer, P. and Jongen, M. and de Jong, M. and Kadler, M. and Kalekin, O. and Kappes, A. and Katz, U. F. and Keller, P. and Kieft, G. and Kießling, D. and Koffeman, E. N. and Kooijman, P. and Kouchner, A. and Kulikovskiy, V. and Lahmann, R. and Lamare, P. and Leisos, A. and Leonora, E. and Lindsey Clark, M. and Liolios, A. and Llorenz Alvarez, C. D. and Lo Presti, D. and L{\"o}hner, H. and Lonardo, A. and Lotze, M. and Loucatos, S. and Maccioni, E. and Mannheim, K. and Margiotta, A. and Marinelli, A. and Mari{\c{s}}, O. and Markou, C. and Mart{\´i}nez-Mora, J. A. and Martini, A. and Mele, R. and Melis, K. W. and Michael, T. and Migliozzi, P. and Migneco, E. and Mijakowski, P. and Miraglia, A. and Mollo, C. M. and Mongelli, M. and Morganti, M. and Moussa, A. and Musico, P. and Musumeci, M. and Navas, S. and Nicoleau, C. A. and Olcina, I. and Olivetto, C. and Orlando, A. and Papaikonomou, A. and Papaleo, R. and Păvăla{\c{s}}, G. E. and Peek, H. and Pellegrino, C. and Perrina, C. and Pfutzner, M. and Piattelli, P. and Pikounis, K. and Poma, G. E. and Popa, V. and Pradier, T. and Pratolongo, F. and P{\"u}hlhofer, G. and Pulvirenti, S. and Quinn, L. and Racca, C. and Raffaelli, F. and Randazzo, N. and Rapidis, P. and Razis, P. and Real, D. and Resvanis, L. and Reubelt, J. and Riccobene, G. and Rossi, C. and Rovelli, A. and Salda{\~n}a, M. and Salvadori, I. and Samtleben, D. F. E. and S{\´a}nchez Garc{\´i}a, A. and S{\´a}nchez Losa, A. and Sanguineti, M. and Santangelo, A. and Santonocito, D. and Sapienza, P. and Schimmel, F. and Schmelling, J. and Sciacca, V. and Sedita, M. and Seitz, T. and Sgura, I. and Simeone, F. and Siotis, I. and Sipala, V. and Spisso, B. and Spurio, M. and Stavropoulos, G. and Steijger, J. and Stellacci, S. M. and Stransky, D. and Taiuti, M. and Tayalati, Y. and T{\´e}zier, D. and Theraube, S. and Thompson, L. and Timmer, P. and T{\"o}nnis, C. and Trasatti, L. and Trovato, A. and Tsirigotis, A. and Tzamarias, S. and Tzamariudaki, E. and Vallage, B. and Van Elewyk, V. and Vermeulen, J. and Vicini, P. and Viola, S. and Vivolo, D. and Volkert, M. and Voulgaris, G. and Wiggers, L. and Wilms, J. and de Wolf, E. and Zachariadou, K. and Zornoza, J. D. and Z{\´u}{\~n}iga, J.},
  title     = {Letter of intent for KM3NeT 2.0},
  series = {Journal of Physics G-Nuclear and Particle Physics},
  volume    = {43},
  journal   = {Journal of Physics G-Nuclear and Particle Physics},
  number    = {8},
  doi       = {10.1088/0954-3899/43/8/084001},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-188050},
  pages     = {84001},
  year      = {2016},
  abstract  = {The main objectives of the KM3NeT Collaboration are (i) the discovery and subsequent observation of high-energy neutrino sources in the Universe and (ii) the determination of the mass hierarchy of neutrinos. These objectives are strongly motivated by two recent important discoveries, namely: (1) the high-energy astrophysical neutrino signal reported by IceCube and (2) the sizable contribution of electron neutrinos to the third neutrino mass eigenstate as reported by Daya Bay, Reno and others. To meet these objectives, the KM3NeT Collaboration plans to build a new Research Infrastructure consisting of a network of deep-sea neutrino telescopes in the Mediterranean Sea. A phased and distributed implementation is pursued which maximises the access to regional funds, the availability of human resources and the synergistic opportunities for the Earth and sea sciences community. Three suitable deep-sea sites are selected, namely off-shore Toulon (France), Capo Passero (Sicily, Italy) and Pylos (Peloponnese, Greece). The infrastructure will consist of three so-called building blocks. A building block comprises 115 strings, each string comprises 18 optical modules and each optical module comprises 31 photo-multiplier tubes. Each building block thus constitutes a three-dimensional array of photo sensors that can be used to detect the Cherenkov light produced by relativistic particles emerging from neutrino interactions. Two building blocks will be sparsely configured to fully explore the IceCube signal with similar instrumented volume, different methodology, improved resolution and},
  language  = {en}
}
@article{GedalinDroege2013,
  author    = {Gedalin, Michael and Dr{\"o}ge, Wolfgang},
  title     = {Ion dynamics in quasi-perpendicular collisionless interplanetary shocks: a case study},
  series = {Frontiers in Physics},
  volume    = {1},
  journal   = {Frontiers in Physics},
  issn      = {2296-424X},
  doi       = {10.3389/fphy.2013.00029},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-189407},
  pages     = {29},
  year      = {2013},
  abstract  = {Interplanetary shocks are believed to play an important role in the acceleration of charged particles in the heliosphere. While the acceleration to high energies proceeds via the diffusive mechanism at the scales exceeding by far the shock width, the initial stage (injection) should occur at the shock itself. Numerical tracing of ions is done in a model quasi-perpendicular shock front with a typical interplanetary shock parameters (Mach number, upstream ion temperature). The analysis of the distribution of the transmitted solar wind is used to adjust the cross-shock potential which is not directly measured. It is found that, for typical upstream ion temperatures, acceleration of the ions from the tail of the solar wind distribution is unlikely. Pickup ions with a shell distribution are found to be effectively energized and may be injected into further diffusive acceleration regime. Pre-accelerated ions are efficiently upscaled in energies. A part of these ions is returned to the upstream region where they can further be diffusively accelerated.},
  language  = {en}
}
@article{AdrianMartinezAlbertAndreetal.2016,
  author    = {Adri{\´a}n-Mart{\´i}nez, S. and Albert, A. and Andr{\´e}, M. and Anton, G. and Ardid, M. and Aubert, J.-J. and Avgitas, T. and Baret, B. and Barrios-Mart{\´i}, J. and Basa, S. and Bertin, V. and Biagi, S. and Bormuth, R. and Bou-Cabo, M. and Bouwhuis, M.C. and Bruijn, R. and Brunner, J. and Busto, J. and Capone, A. and Caramete, L. and Carr, J. and Celli, S. and Chiarusi, T. and Circella, M. and Coleiro, A. and Coniglione, R. and Costantini, H. and Coyle, P. and Creusot, A. and Deschamps, A. and De Bonis, G. and Distefano, C. and Donzaud, C. and Dornic, D. and Drouhin, D. and Eberl, T. and El Bojaddaini, I. and Els{\"a}sser, D. and Enzenh{\"o}fer, A. and Fehn, K. and Felis, I. and Fusco, L.A. and Galat{\`a}, S. and Gay, P. and Geißels{\"o}der, S. and Geyer, K. and Giordano, V. and Gleixner, A. and Glotin, H. and Gracia-Ruiz, R. and Graf, K. and Hallmann, S. and van Haren, H. and Heijboer, A.J. and Hello, Y. and Hern{\´a}ndez-Rey, J.-J. and H{\"o}ßl, J. and Hofest{\"a}dt, J. and Hugon, C. and Illuminati, G. and James, C.W. and de Jong, M. and Kadler, M. and Kalekin, O. and Katz, U. and Kießling, D. and Kouchner, A. and Kreter, M. and Kreykenbohm, I. and Kulikovskiy, V. and Lachaud, C. and Lahmann, R. and Lef{\`e}vre, D. and Leonora, E. and Loucatos, S. and Marcelin, M. and Margiotta, A. and Marinelli, A. and Mart{\´i}nez-Mora, J.A. and Mathieu, A. and Michael, T. and Migliozzi, P. and Moussa, A. and Mueller, C. and Nezri, E. and Păvălaș, G.E. and Pellegrino, C. and Perrina, C. and Piattelli, P. and Popa, V. and Pradier, T. and Racca, C. and Riccobene, G. and Roensch, K. and Salda{\~n}a, M. and Samtleben, D.F.E. and Sanguineti, M. and Sapienza, P. and Schnabel, J. and Sch{\"u}ssler, F. and Seitz, T. and Sieger, C. and Spurio, M. and Stolarczyk, Th. and S{\´a}nchez-Losa, A. and Taiuti, M. and Trovato, A. and Tselengidou, M. and Turpin, D. and T{\"o}nnis, C. and Vallage, B. and Vall{\´e}e, C. and Van Elewyck, V. and Vivolo, D. and Wagner, S. and Wilms, J. and Zornoza, J.D. and Z{\´u}{\~n}iga, J.},
  title     = {A search for Secluded Dark Matter in the Sun with the ANTARES neutrino telescope},
  series = {Journal of Cosmology and Astroparticle Physics},
  volume    = {2016},
  journal   = {Journal of Cosmology and Astroparticle Physics},
  number    = {5},
  organization    = {The ANTARES collaboration},
  doi       = {10.1088/1475-7516/2016/05/016},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-189035},
  pages     = {12},
  year      = {2016},
  abstract  = {A search for Secluded Dark Matter annihilation in the Sun using 2007-2012 data of the ANTARES neutrino telescope is presented. Three different cases are considered: a) detection of dimuons that result from the decay of the mediator, or neutrino detection from: b) mediator that decays into a dimuon and, in turn, into neutrinos, and c) mediator that decays directly into neutrinos. As no significant excess over background is observed, constraints are derived on the dark matter mass and the lifetime of the mediator.},
  language  = {en}
}
@article{DreinerKraussO'Learyetal.2016,
  author    = {Dreiner, Herbi K. and Krauss, Manuel E. and O'Leary, Ben and Opferkuch, Toby and Staub, Florian},
  title     = {Validity of the CMSSM interpretation of the diphoton excess},
  series = {Physical Review D},
  volume    = {94},
  journal   = {Physical Review D},
  number    = {5},
  doi       = {10.1103/PhysRevD.94.055013},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-187429},
  pages     = {055013},
  year      = {2016},
  abstract  = {It has been proposed that the observed diphoton excess at 750 GeV could be explained within the constrained minimal supersymmetric standard model via resonantly produced stop bound states. We reanalyze this scenario critically and extend previous work to include the constraints from the stability of the electroweak vacuum and from the decays of the stoponium into a pair of Higgs bosons. It is shown that the interesting regions of parameter space with a light stop and Higgs of the desired mass are ruled out by these constraints. This conclusion is not affected by the presence of the bound states because the binding energy is usually very small in the regions of parameter space which can explain the Higgs mass. Thus, this also leads to strong constraints on the diphoton production cross section which is in general too small.},
  language  = {en}
}
@article{HirschKraussOpferkuchetal.2016,
  author    = {Hirsch, Martin and Krauss, Manuel E. and Opferkuch, Toby and Porod, Werner and Staub, Florian},
  title     = {A constrained supersymmetric left-right model},
  series = {JOURNAL OF HIGH ENERGY PHYSICS},
  volume    = {03},
  journal   = {JOURNAL OF HIGH ENERGY PHYSICS},
  number    = {009},
  doi       = {10.1007/JHEP03(2016)009},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-168016},
  year      = {2016},
  abstract  = {We present a supersymmetric left-right model which predicts gauge coupling unification close to the string scale and extra vector bosons at the TeV scale. The subtleties in constructing a model which is in agreement with the measured quark masses and mixing for such a low left-right breaking scale are discussed. It is shown that in the constrained version of this model radiative breaking of the gauge symmetries is possible and a SM-like Higgs is obtained. Additional CP-even scalars of a similar mass or even much lighter are possible. The expected mass hierarchies for the supersymmetric states differ clearly from those of the constrained MSSM. In particular, the lightest down-type squark, which is a mixture of the sbottom and extra vector-like states, is always lighter than the stop. We also comment on the model's capability to explain current anomalies observed at the LHC.},
  language  = {en}
}
@article{LiYanThomaleetal.2015,
  author    = {Li, Gang and Yan, Binghai and Thomale, Ronny and Hanke, Werner},
  title     = {Topological nature and the multiple Dirac cones hidden in Bismuth high-Tc superconductors},
  series = {Scientific Reports},
  volume    = {5},
  journal   = {Scientific Reports},
  number    = {10435},
  doi       = {10.1038/srep10435},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-148569},
  year      = {2015},
  abstract  = {Recent theoretical studies employing density-functional theory have predicted BaBiO\(_{3}\) (when doped with electrons) and YBiO\(_{3}\) to become a topological insulator (TI) with a large topological gap (~0.7 eV). This, together with the natural stability against surface oxidation, makes the Bismuth-Oxide family of special interest for possible applications in quantum information and spintronics. The central question, we study here, is whether the hole-doped Bismuth Oxides, i.e. Ba\(_{1-X}\)K\(_{X}\)BiO\(_{3}\) and BaPb\(_{1-X}\)Bi\(_{X}\)O\(_{3}\), which are "high-Tc" bulk superconducting near 30 K, additionally display in the further vicinity of their Fermi energy E\(_{F}\) a topological gap with a Dirac-type of topological surface state. Our electronic structure calculations predict the K-doped family to emerge as a TI, with a topological gap above E\(_{F}\). Thus, these compounds can become superconductors with hole-doping and potential TIs with additional electron doping. Furthermore, we predict the Bismuth-Oxide family to contain an additional Dirac cone below E\(_{F}\) for further hole doping, which manifests these systems to be candidates for both electron-and hole-doped topological insulators.},
  language  = {en}
}
@article{UmHinrichsenKwonetal.2015,
  author    = {Um, Jaegon and Hinrichsen, Haye and Kwon, Chulan and Park, Hyunggyu},
  title     = {Total cost of operating an information engine},
  series = {New Journal of Physics},
  volume    = {17},
  journal   = {New Journal of Physics},
  number    = {085001},
  doi       = {10.1088/1367-2630/17/8/085001},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-148286},
  year      = {2015},
  abstract  = {We study a two-level system controlled in a discrete feedback loop, modeling both the system and the controller in terms of stochastic Markov processes. We find that the extracted work, which is known to be bounded from above by the mutual information acquired during measurement, has to be compensated by an additional energy supply during the measurement process itself, which is bounded by the same mutual information from below. Our results confirm that the total cost of operating an information engine is in full agreement with the conventional second law of thermodynamics. We also consider the efficiency of the information engine as a function of the cycle time and discuss the operating condition for maximal power generation. Moreover, we find that the entropy production of our information engine is maximal for maximal efficiency, in sharp contrast to conventional reversible heat engines.},
  language  = {en}
}
@phdthesis{Kreter2019,
  author    = {Kreter, Michael},
  title     = {Targeting the mystery of extragalactic neutrino sources - A Multi-Messenger Window to the Extreme Universe -},
  doi       = {10.25972/OPUS-17984},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-179845},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Active Galactic Nuclei (AGNs) are among the most powerful and most intensively studied objects in the Universe. AGNs harbor a mass accreting supermassive black hole (SMBH) in their center and emit radiation throughout the entire electromagnetic spectrum. About 10\% show relativistic particle outflows, perpendicular to the so-called accretion disk, which are known as jets. Blazars, a subclass of AGN with jet orientations close to the line-of-sight of the observer, are highly variable sources from radio to TeV energies and dominate the γ- ray sky. The overall observed broadband emission of blazars is characterized by two distinct emission humps. While the low-energy hump is well described by synchrotron radiation of relativistic electrons, both leptonic processes such as inverse Compton scattering and hadronic processes such as pion-photoproduction can explain the radiation measured in the high-energy hump. Neutrinos, neutral, nearly massless particles, which only couple to the weak force 1 are exclusively produced in hadronic interactions of protons accelerated to relativistic energies. The detection of a high-energy neutrino from an AGN would provide an irrefutable proof of hadronic processes happening in jets. Recently, the IceCube neutrino observatory, located at the South Pole with a total instrumented volume of about one km 3 , provided evidence for a diffuse high-energy neutrino flux. Since the atmospheric neutrino spectrum falls steeply with energy, individual events with the clearest signature of coming from an extraterrestrial origin are those at the highest energies. These events are uniformly distributed over the entire sky and are therefore most likely of extragalactic nature. While the neutrino event (known as "BigBird") with a reconstructed energy of ∼ 2 PeV has already been detected in temporal and spatial agreement with a single blazar in an active phase, still, the chance coincidence for such an association is only on the order of ∼ 5\%. The neutrino flux at these high energies is low, so that even the brightest blazars only yield a Poisson probability clearly below unity. Such a small probability is in agreement with the observed all-sky neutrino flux otherwise, the sky would already be populated with numerous confirmed neutrino point sources. In neutrino detectors, events are typically detected in two different signatures 2 . So-called shower-like electron neutrino events produce a large particle cascade, which leads to a pre- cise energy measurement, but causes a large angular uncertainty. Track-like muon neutrino events, however, only produce a single trace in the detector, leading to a precise localization but poor energy reconstruction. The "BigBird" event was a shower-like neutrino event, tem- porally coincident with an activity phase of the blazar PKS 1424-418, lasting several months. Shower-like neutrino events typically lead to an angular resolution of ∼ 10 ◦ , while track-like events show a localization uncertainty of only ∼ 1 ◦ . Considering the potential detection of a track-like neutrino event in agreement with an activity phase of a single blazar lasting only days would significantly decrease the chance coincidence of such an association. In this thesis, a sample of bright blazars, continuously monitored by Fermi/LAT in the MeV to GeV regime, is considered as potential neutrino candidates. I studied the maximum possible neutrino ex- pectation of short-term blazar flares with durations of days to weeks, based on a calorimetric argumentation. I found that the calorimetric neutrino output of most short-term blazar flares is too small to lead to a substantial neutrino detection. However, for the most extreme flares, Poisson probabilities of up to ∼ 2\% are reached, so that the possibility of associated neutrino detections in future data unblindings of IceCube and KM3NeT seems reasonable. On 22 September 2017, IceCube detected the first track-like neutrino event (named IceCube- 170922A) coincident with a single blazar in an active phase. From that time on, the BL Lac object TXS 0506+056 was subject of an enormous multiwavelength campaign, revealing an en- hanced flux state at the time of the neutrino arrival throughout several different wavelengths. In this thesis, I first studied the long-term flaring behavior of TXS 0506+056, using more than nine years of Fermi/LAT data. I found that the activity phase in the MeV to GeV regime already started in early 2017, months before the arrival of IceCube-170922A. I performed a calorimetric analysis on a 3-day period around the neutrino arrival time and found no sub- stantial neutrino expectation from such a short time range. By computing the calorimetric neutrino prediction for the entire activity phase of TXS 0506+056 since early 2017, a possible association seems much more likely. However, the post-trial corrected chance coincidence for a long-term association between IceCube-170922A and the blazar TXS 0506+056 is on the level of ∼ 3.5 σ, establishing TXS 0506+056 as the most promising neutrino point source candidate in the scientific community. Another way to explain a high-energy neutrino signal without an observed astronomical counterpart, would be the consideration of blazars at large cosmological distances. These high-redshift blazars are capable of generating the observed high-energy neutrino flux, while their γ-ray emission would be efficiently downscattered by Extragalactic Background Light (EBL), making them almost undetectable to Fermi/LAT. High-redshift blazars are impor- tant targets, as they serve as cosmological probes and represent one of the most powerful classes of γ-ray sources in the Universe. Unfortunately, only a small number of such objects could be detected with Fermi/LAT so far. In this thesis, I perform a systematic search for flaring events in high-redshift γ-ray blazars, which long-term flux is just below the sensitiv- ity limit of Fermi/LAT. By considering a sample of 176 radio detected high-redshift blazars, undetected at γ-ray energies, I was able to increase the number of previously unknown γ-ray blazars by a total of seven sources. Especially the blazar 5BZQ J2219-2719, at a distance of z = 3.63 was found to be the most distant new γ-ray source identified within this thesis. In the final part of this thesis, I studied the flaring behavior of bright blazars, previously considered as potential neutrino candidates. While the occurrence of flaring intervals in blazars is of purely statistical nature, I found potential differences in the observed flaring behavior of different blazar types. Blazars can be subdivided into BL Lac (BLL) objects, Flat-Spectrum Radio Quasar (FSRQ) and Blazars Candidates of Uncertain type (BCU). FSRQs are typ- ically brighter than BL Lac or BCU type blazars, thus longer flares and more complicated substructures can be resolved. Although BL Lacs and BCUs are capable of generating signifi- cant flaring episodes, they are often identified close to the detection threshold of Fermi/LAT. Long-term outburst periods are exclusively observed in FSRQs, while BCUs can still con- tribute with flare durations of up to ten days. BL Lacs, however, are only detected in flaring states of less than four days. FSRQs are bright enough to be detected multiple times with time gaps between two subsequent flaring intervals ranging between days and months. While BL Lacs can show time gaps of more than 100 days, BCUs are only observed with gaps up to 20 days, indicating that these objects are detected only once in the considered time range of six years. The newly introduced parameter "Boxyness" describes the averaged flux in an identified flaring state and does highly depend on the shape of the considered flare. While perfectly box-like flares (flares which show a constant flux level over the entire time range) correspond to an averaged flux which is equal the maximum flare amplitude, irregular shaped flares generate a smaller averaged flux. While all blazar types show perfectly box-shaped daily flares, BL Lacs and BCUs are typically not bright enough to be resolved for multiple days. The work presented in this thesis illustrates the challenging state of multimessenger neu- trino astronomy and the demanding hunt for the first extragalactic neutrino point sources. In this context, this work discusses the multiwavelength emission behavior of blazars as a promising class of neutrino point sources and allows for predictions of current and future source associations},
  language  = {en}
}
@article{HsuKuegelKemmeretal.2016,
  author    = {Hsu, Pin-Jui and K{\"u}gel, Jens and Kemmer, Jeannette and Toldin, Francesco Parisen and Mauerer, Tobias and Vogt, Matthias and Assaad, Fakher and Bode, Matthias},
  title     = {Coexistence of charge and ferromagnetic order in fcc Fe},
  series = {Nature Communications},
  volume    = {7},
  journal   = {Nature Communications},
  doi       = {10.1038/ncomms10949},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173969},
  year      = {2016},
  abstract  = {Phase coexistence phenomena have been intensively studied in strongly correlated materials where several ordered states simultaneously occur or compete. Material properties critically depend on external parameters and boundary conditions, where tiny changes result in qualitatively different ground states. However, up to date, phase coexistence phenomena have exclusively been reported for complex compounds composed of multiple elements. Here we show that charge- and magnetically ordered states coexist in double-layer Fe/Rh(001). Scanning tunnelling microscopy and spectroscopy measurements reveal periodic charge-order stripes below a temperature of 130 K. Close to liquid helium temperature, they are superimposed by ferromagnetic domains as observed by spin-polarized scanning tunnelling microscopy. Temperature-dependent measurements reveal a pronounced cross-talk between charge and spin order at the ferromagnetic ordering temperature about 70 K, which is successfully modelled within an effective Ginzburg-Landau ansatz including sixth-order terms. Our results show that subtle balance between structural modifications can lead to competing ordering phenomena.},
  language  = {en}
}
@article{ErdmengerFernandezFloryetal.2017,
  author    = {Erdmenger, Johanna and Fern{\´a}ndez, Daniel and Flory, Mario and Meg{\´i}as, Eugenio and Straub, Ann-Kathrin and Witkowski, Piotr},
  title     = {Time evolution of entanglement for holographic steady state formation},
  series = {Journal of High Energy Physics},
  volume    = {2017},
  journal   = {Journal of High Energy Physics},
  number    = {10},
  doi       = {10.1007/JHEP10(2017)034},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173798},
  year      = {2017},
  abstract  = {Within gauge/gravity duality, we consider the local quench-like time evolution obtained by joining two 1+1-dimensional heat baths at different temperatures at time \(t\) = 0. A steady state forms and expands in space. For the 2+1-dimensional gravity dual, we find that the "shockwaves" expanding the steady-state region are of spacelike nature in the bulk despite being null at the boundary. However, they do not transport information. Moreover, by adapting the time-dependent Hubeny-Rangamani-Takayanagi prescription, we holographically calculate the entanglement entropy and also the mutual information for different entangling regions. For general temperatures, we find that the entanglement entropy increase rate satisfies the same bound as in the 'entanglement tsunami' setups. For small temperatures of the two baths, we derive an analytical formula for the time dependence of the entanglement entropy. This replaces the entanglement tsunami-like behaviour seen for high temperatures. Finally, we check that strong subadditivity holds in this time-dependent system, as well as further more general entanglement inequalities for five or more regions recently derived for the static case.},
  language  = {en}
}
@article{LeePapićThomale2015,
  author    = {Lee, Ching Hua and Papić, Zlatko and Thomale, Ronny},
  title     = {Geometric construction of quantum Hall clustering Hamiltonians},
  series = {Physical Review X},
  volume    = {5},
  journal   = {Physical Review X},
  number    = {4},
  doi       = {10.1103/PhysRevX.5.041003},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-145233},
  pages     = {041003},
  year      = {2015},
  abstract  = {Many fractional quantum Hall wave functions are known to be unique highest-density zero modes of certain "pseudopotential" Hamiltonians. While a systematic method to construct such parent Hamiltonians has been available for the infinite plane and sphere geometries, the generalization to manifolds where relative angular momentum is not an exact quantum number, i.e., the cylinder or torus, remains an open problem. This is particularly true for non-Abelian states, such as the Read-Rezayi series (in particular, the Moore-Read and Read-Rezayi Z\(_3\) states) and more exotic nonunitary (Haldane-Rezayi and Gaffnian) or irrational (Haffnian) states, whose parent Hamiltonians involve complicated many-body interactions. Here, we develop a universal geometric approach for constructing pseudopotential Hamiltonians that is applicable to all geometries. Our method straightforwardly generalizes to the multicomponent SU(n) cases with a combination of spin or pseudospin (layer, subband, or valley) degrees of freedom. We demonstrate the utility of our approach through several examples, some of which involve non-Abelian multicomponent states whose parent Hamiltonians were previously unknown, and we verify the results by numerically computing their entanglement properties.},
  language  = {en}
}
@phdthesis{Truestedt2016,
  author    = {Tr{\"u}stedt, Jonas Elias},
  title     = {Long-wavelength radio observations of blazars with the Low-Frequency Array (LOFAR)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-144406},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {Aktive Galaxienkerne (AGN) geh{\"o}ren zu den hellsten Objekten in unserem Universum. Diese Galaxien werden als aktiv bezeichnet, da ihre Zentralregion heller ist als alle Sterne in einer Galaxie zusammen beitragen k{\"o}nnten. Das Zentrum besteht aus einem supermassiven schwarzen Loch, das von einer Akkretionsscheibe und weiter außerhalb von einem Torus aus Staub umgeben ist. Diese AGN k{\"o}nnen {\"u}ber das ganze elektromagnetische Spektrum verteilt gefunden werden, von Radiowellen {\"u}ber Wellenl{\"a}ngen im optischen und R{\"o}ntgenbereich bis hin zur \$\gamma\$-Strahlung. Allerdings sind nicht alle Objekte bei jeder Wellenl{\"a}nge detektierbar. In dieser Arbeit werden {\"u}berwiegend Blazare bei niedrigen Radiofrequenzen untersucht. Blazare geh{\"o}ren zu den radio-lauten AGN, welche {\"u}blicherweise stark kollimierte Jets senkrecht zur Akkretionsscheibe aussenden. Bei Blazaren sind diese Jets in die Richtung des Beobachters gerichtet und ihre Emissionen sind stark variabel. \\ AGN werden anhand ihres Erscheinungsbildes verschiedenen Untergruppen zugeordnet. Diese Untergruppen werden in einem vereinheitlichen AGN Modell zusammengef{\"u}hrt, welches besagt, dass diese Objekte sich nur in ihrer Luminosit{\"a}t und ihrem Winkel zur Sichtlinie unterscheiden. Blazare sind diejenigen Objekte, deren Jets in unsere Sichtrichtung zeigen, w{\"a}hrend die Objekte deren Jets eher senkrecht zur Sichtlinie orientiert sind als Radiogalaxien bezeichnet werden. Daraus folgt, dass Blazare die Gegenst{\"u}cke zu Radiogalaxien mit einem anderen Winkel zur Sichtlinie sind. Diese Beziehung soll unter anderem in dieser Arbeit untersucht werden. \\ Nach ihrer Entdeckung in den 1940er Jahren wurden die aktiven Galaxien bei allen zug{\"a}nglichen Wellenl{\"a}ngen untersucht. Durch die Entwicklung von Interferometern aus Radioteleskopen, welche eine erh{\"o}hte Aufl{\"o}sung bieten, konnten die Beobachtungen stark verbessert werden. In den letzten 20 Jahren wurden viele AGN regelm{\"a}ßig beobachtet. Dies erfolgte unter anderem durch Programme wie dem MOJAVE Programm, welches 274 AGNs regelm{\"a}ßig mithilfe der Technik der ``Very Long Baseline Interferometry" (VLBI) beobachtet. Durch diese Beobachtungen konnten Informationen zur Struktur und Entwicklung der AGN und Jets gesammelt werden. Allerdings sind die Prozesse zur Bildung von Jets und deren Kollimation noch nicht vollst{\"a}ndig bekannt. Durch relativistische Effekte ist es schwierig die eigentlichen Gr{\"o}ßen der Jets anstelle der scheinbaren zu messen. Um die intrinsische Energie von Jets zu messen, sollen die ausgedehnten Emissionsregionen untersucht werden, in denen die Jets enden und mit dem Intergalaktischen Medium interagieren. Beobachtungen bei niedrigen Radiofrequenzen sind empfindlicher um solche ausgedehnte, diffuse Emissionsregionen zu detektieren. \\ Seit Dezember 2012 ist ein neues Radioteleskop f{\"u}r niedrige Frequenzen in Betrieb, dessen Stationen aus Dipolantennen besteht. Die meisten dieser Stationen sind in den Niederlanden verteilt (38 Stationen) und werden durch 12 internationale Stationen in Deutschland, Frankreich, Schweden, Polen und England erg{\"a}nzt. Dieses Instrument tr{\"a}gt den Namen ``Low Frequency Array'' (LOFAR). LOFAR bietet die M{\"o}glichkeit bei Frequenzen von 30--250 MHz bei einer h{\"o}heren Aufl{\"o}sung als bisherige Radioteleskope zu beobachten (Winkelaufl{\"o}sungen unter 1 arcsec f{\"u}r das gesamte Netzwerk aus Teleskopen). \\ Diese Arbeit behandelt die Ergebnisse von Blazaruntersuchungen mithilfe von LOFAR-Beobachtungen. Daf{\"u}r wurden AGNs aus dem MOJAVE Programm verwendet um von den bisherigen Multiwellenl{\"a}ngen-Beobachtungen und Untersuchungen der Kinematik zu profitieren. Das ``Multifrequency Snapshot Sky Survey'' (MSSS) Projekt hat den gesamten Nordhimmel mit kurzen Beobachtungen abgerastert. Aus dem daraus resultierenden vorl{\"a}ufigen Katalog wurden die Flussdichten und Spektralindizes f{\"u}r MOJAVE-Blazare untersucht. In den kurzen Beobachtungen von MSSS sind nur die Stationen in den Niederlanden verwendet worden, wodurch Aufl{\"o}sung und Sensitivit{\"a}t begrenzt sind. F{\"u}r die Erstellung des vorl{\"a}ufigen Kataloges wurde die Aufl{\"o}sung auf \$\sim\$120 arcsec beschr{\"a}nkt. Ein weiterer Vorteil der MOJAVE Objekte ist die regelm{\"a}ßige Beobachtung der AGN mit dem ``Owens Vally Radio Observatory'' zur Erstellung von Lichtkurven bei 15 GHz. Dadurch ist es m{\"o}glich nahezu zeitgleiche Flussdichtemessungen bei 15 GHz zu den entsprechenden MSSS-Beobachtungen zu bekommen. Da diese Beobachtungen zu {\"a}hnlichen Zeitpunkten durchgef{\"u}hrt wurden sind diese Flussdichten weniger von der Variabilit{\"a}t der Blazare beeinflusst. Die Spektralindizes berechnet aus den Flussdichten von MSSS und OVRO k{\"o}nnen verwendet werden um den Anteil an ausgedehnter Emission der AGNs abzusch{\"a}tzen. \\ Im Vergleich der Flussdichten aus dem MSSS Katalog mit den Beobachtungen von OVRO f{\"a}llt auf, dass die Flussdichten bei niedrigen Frequenzen tendenziell h{\"o}her sind, was durch den h{\"o}heren Anteil an ausgedehnter Struktur zu erwarten ist. Die Spektralindexverteilung zwischen MSSS und OVRO zeigt ihren h{\"o}chsten Wert bei \$\sim-0.2\$. In der Verteilung existieren Objekte mit steilerem Spektralindex durch den h{\"o}heren Anteil von ausgedehnter Emission in der Gesamtflussdichte, doch {\"u}ber die H{\"a}lfte der untersuchten Objekte besitzt flache Spektralindizes. Die flachen Spektralindizes bedeuten, dass die Emissionen dieser Objekte gr{\"o}ßtenteils von relativistischen Effekten beeinflusst sind, die schon aus Beobachtungen bei GHz-Frequenzen bekannt sind. \\ Durch neue Auswertung der MSSS Beobachtungsdaten konnten Bilder bei einer verbesserten Aufl{\"o}sung von \$\sim\$20--30 arcsec erstellt werden, wodurch bei einigen Blazaren ausgedehnte Struktur detektiert werden konnte. Diese h{\"o}her aufgel{\"o}sten Bilder sind allerdings nicht komplett kalibriert und k{\"o}nnen somit nur f{\"u}r strukturelle Informationen verwendet werden. Die {\"U}berarbeitung der Beobachtungsdaten konnte f{\"u}r 93 Objekte f{\"u}r ein Frequenzband durchgef{\"u}hrt werden. F{\"u}r 45 der 93 Objekte konnten sogar alle vorhandenen Frequenzb{\"a}nder {\"u}berarbeitet werden und dadurch gemittelte Bilder erstellt werden. Diese Bilder werden in dieser Arbeit vorgestellt. Die resultierenden Bilder mit verbesserter Aufl{\"o}sung wurden verwendet um Objekte auszuw{\"a}hlen, die mit allen LOFAR-Stationen beobachtet und auf ausgedehnte Struktur untersucht werden k{\"o}nnen. \\ Im zweiten Teil der Arbeit werden die Ergebnisse von internationalen LOFAR Beobachtungen von vier Blazaren pr{\"a}sentiert. Da sich die Auswertung und Kalibration von internationalen LOFAR Beobachtungen noch in der Entwicklung befindet, wurde ein Schwerpunkt auf die Kalibration und deren Beschreibung gelegt. Die Kalibration kann zwar noch verbessert werden, aber die Bilder aus der angewandten Kalibration erreichen eine Aufl{\"o}sung von unter 1 arcsec. Die Struktur der untersuchten vier Blazare entspricht den Erwartungen f{\"u}r Radiogalaxien unter einem anderen Sichtwinkel. Durch die gemessenen Flussdichten der ausgedehnten Struktur aus den Helligkeitsverteilungen konnte die Luminosit{\"a}t der ausgedehnten Emissionen berechnet werden. Im Vergleich mit den Luminosit{\"a}ten, die von Radiogalaxien bekannt sind, entsprechen auch diese Werte den Erwartungen des vereinheitlichten AGN Modells. \\ Durch die in dieser Arbeit vorgestellte Kalibration k{\"o}nnen noch mehr Blazare mit LOFAR inklusive den internationalen Stationen beobachtet werden und somit Bilder der Struktur bei {\"a}hnlicher Aufl{\"o}sung erstellt werden. Durch eine erh{\"o}hte Anzahl von untersuchten Blazaren k{\"o}nnten anschließend auch statistisch signifikante Ergebnisse erzielt werden.\\},
  subject      = {Blazar},
  language  = {en}
}
@phdthesis{Schnells2019,
  author    = {Schnells, Vera},
  title     = {Fractional Insulators and their Parent Hamiltonians},
  doi       = {10.25972/OPUS-18561},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-185616},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {In the past few years, two-dimensional quantum liquids with fractional excitations have been a topic of high interest due to their possible application in the emerging field of quantum computation and cryptography. This thesis is devoted to a deeper understanding of known and new fractional quantum Hall states and their stabilization in local models. We pursue two different paths, namely chiral spin liquids and fractionally quantized, topological phases. The chiral spin liquid is one of the few examples of spin liquids with fractional statistics. Despite its numerous promising properties, the microscopic models for this state proposed so far are all based on non-local interactions, making the experimental realization challenging. In the first part of this thesis, we present the first local parent Hamiltonians, for which the Abelian and non-Abelian chiral spin liquids are the exact and, modulo a topological degeneracy, unique ground states. We have developed a systematic approach to find an annihilation operator of the chiral spin liquid and construct from it a many-body interaction which establishes locality. For various system sizes and lattice geometries, we numerically find largely gapped eigenspectra and confirm to an accuracy of machine precision the uniqueness of the chiral spin liquid as ground state of the respective system. Our results provide an exact spin model in which fractional quantization can be studied. Topological insulators are one of the most actively studied topics in current condensed matter physics research. With the discovery of the topological insulator, one question emerged: Is there an interaction-driven set of fractionalized phases with time reversal symmetry? One intuitive approach to the theoretical construction of such a fractional topological insulator is to take the direct product of a fractional quantum Hall state and its time reversal conjugate. However, such states are well studied conceptually and do not lead to new physics, as the idea of taking a state and its mirror image together without any entanglement between the states has been well understood in the context of topological insulators. Therefore, the community has been looking for ways to implement some topological interlocking between different spin species. Yet, for all practical purposes so far, time reversal symmetry has appeared to limit the set of possible fractional states to those with no interlocking between the two spin species. In the second part of this thesis, we propose a new universality class of fractionally quantized, topologically ordered insulators, which we name "fractional insulator". Inspired by the fractional quantum Hall effect, spin liquids, and fractional Chern insulators, we develop a wave function approach to a new class of topological order in a two-dimensional crystal of spin-orbit coupled electrons. The idea is simply to allow the topological order to violate time reversal symmetry, while all locally observable quantities remain time reversal invariant. We refer to this situation as "topological time reversal symmetry breaking". Our state is based on the Halperin double layer states and can be viewed as a two-layer system of an ↑-spin and a ↓-spin sphere. The construction starts off with Laughlin states for the ↑-spin and ↓-spin electrons and an interflavor term, which creates correlations between the two layers. With a careful parameter choice, we obtain a state preserving time reversal symmetry locally, and label it the "311-state". For systems of up to six ↑-spin and six ↓-spin electrons, we manage to construct an approximate parent Hamiltonian with a physically realistic, local interaction.},
  subject      = {Spinfl{\"u}ssigkeit},
  language  = {en}
}
@phdthesis{Ganse2012,
  author    = {Ganse, Urs},
  title     = {Kinetische Simulationen solarer Typ II Radiobursts},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-73676},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {Die Emission solarer Typ II Radiobursts ist ein seit Jahrzehnten beobachtetes Ph{\"a}nomen der heliosph{\"a}rischen Plasmaphysik. Diese Radiobursts, die im Zusammenhang mit der Propagation koronaler Schockfronten auftreten, zeigen ein charakteristisches, zweibandiges Emissionsspektrum. Mit expandierendem Schock driften sie zu niedrigeren Frequenzen. Analytische Theorien dieser Emission sagen nichtlineare Plasmawellenwechselwirkung als Ursache voraus, doch aufgrund des geringen Sonnenabstands der Emissionsregion ist die in-situ Datenlage durch Satellitenmessungen {\"a}usserst schlecht, so dass eine endg{\"u}ltige Verifikation der vorhergesagten Vorg{\"a}nge bisher nicht m{\"o}glich war. Mit Hilfe eines kinetischen Plasma-Simulationscodes nach dem Particle-in-Cell Prinzip wurde in dieser Dissertation die Plasmaumgebung in der Foreshock-Region einer koronalen Schockfront modelliert. Das Propagations- und Kopplungsverhalten elektrostatischer und elektromagnetischer Wellenmoden wurde untersucht. Die vollst{\"a}ndige r{\"a}umliche Information {\"u}ber die Wellenzusammensetzung in der Simulation erlaubt es, die Kinematik nichtlinearer Wellenkopplungen genauestens zu untersuchen. Es zeigte sich ein mit der analytischen Theorie der Drei-Wellen-Wechselwirkung konsistentes Bild der Erzeugung solarer Radiobursts: durch elektromagnetischen Zerfall elektrostatischer Moden kommt es zur Erzeugung fundamentaler, sowie durch Verschmelzung gegenpropagierender elektrostatischer Moden zur Anregung harmonischer Radioemission. Kopplungsst{\"a}rken und Winkelabh{\"a}ngigkeit dieser Prozesse wurden untersucht. Mit dem somit zur Verf{\"u}gung stehenden, numerischen Laborsystem wurde die Parameter-Abh{\"a}ngigkeit der Wellenkopplungen und entstehenden Radioemissionen bez{\"u}glich St{\"a}rke des Elektronenbeams und des solaren Abstandes untersucht.},
  subject      = {Heliosph{\"a}re},
  language  = {de}
}
@phdthesis{Lange2012,
  author    = {Lange, Sebastian},
  title     = {Turbulenz und Teilchentransport in der Heliosph{\"a}re - Simulationen von inkompressiblen MHD-Plasmen und Testteilchen -},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-74012},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {Die Herkunft hochenergetischer solarer Teilchen konnte in den vergangenen Jahren eindeutig auf Schockbeschleunigung an koronalen Masseausw{\"u}rfen zur{\"u}ckgef{\"u}hrt werden. Durch resonante Interaktionen zwischen Wellen und Teilchen werden zum einen geladene Teilchen unter Ver{\"a}nderung ihrer Energie gestreut, zum anderen wird die Dynamik der Plasmawellen in solchen Beschleunigungsregionen durch diese Prozesse von selbstgenerierten Wellenmoden maßgeblich beeinflusst. Mittels numerischer Modellierungen wurden im Rahmen dieser Arbeit die grundlegenden physikalischen Regimes der Turbulenz und des Teilchentransports beschrieben. Die Simulation der Plasmadynamik bedient sich der Methodik der Magnetohydrodynamik, wohingegen kinetische Einzelteilchen durch die elementaren Bewegungsgleichungen der Elektrodynamik berechnet werden. Es konnten die Turbulenztheorien von Goldreich und Sridhar unter heliosph{\"a}rischen Bedingungen bei drei solaren Radien best{\"a}tigt werden. Vor allem zeigten sich Hinweise f{\"u}r das Erreichen der kritischen Balance, einem Schl{\"u}sselparameter dieser Theorien. Weiterhin werden Ergebnisse der dynamischen Entwicklung angeregter Wellenmoden pr{\"a}sentiert, in denen die Bedeutsamkeit f{\"u}r die gesamte Turbulenz gezeigt werden konnte. Als zentraler Prozess bei hohen Energien hat sich das wave-steepening herausgestellt, das als effizienter Energietransportmechanismus in paralleler Richtung zum Hintergrundmagnetfeld identifiziert wurde und somit turbulente Strukturen bei hohen parallelen Wellenzahlen erkl{\"a}rt, deren Entstehung das Goldreich-Sridhar Modell nicht beschreiben kann. Dar{\"u}ber hinaus wurden grundlegende Erkenntnisse {\"u}ber die quasilineare Theorie des Teilchentransports erzielt. Im Speziellen konnte ein tieferes Verst{\"a}ndnis f{\"u}r die Interpretation der Diffusionskoeffizienten von Welle-Teilchen Wechselwirkungen erlangt werden. Simulationen zur Streuung an angeregten Wellenmoden zeigten erstmals komplexe resonante Strukturen die im Rahmen analytischer Modelle nicht mehr ad{\"a}quat beschrieben werden k{\"o}nnen.},
  subject      = {Heliosph{\"a}re},
  language  = {de}
}
@phdthesis{Uhlemann2012,
  author    = {Uhlemann, Christoph Frank},
  title     = {Holographic Description of Curved-Space Quantum Field Theory and Gravity},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-74362},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {The celebrated AdS/CFT dualities provide a window to strongly-coupled quantum field theories (QFTs), which are realized in nature at the most fundamental level on the one hand, but are hardly accessible for the standard mathematical tools on the other hand. The prototype examples of AdS/CFT relate classical supergravity theories on (d+1)-dimensional anti-de Sitter space (AdS) to strongly-coupled d-dimensional conformal field theories (CFTs). The AdS spacetimes admit a timelike conformal boundary, on which the dual CFT is defined. In that sense the AdS/CFT dualities are holographic, and this new approach has led to remarkable progress in understanding strongly-coupled QFTs defined on Minkowski space and on the Einstein cylinder. On the other hand, the study of QFT on more generic curved spacetimes is of fundamental interest and non-trivial already for free theories. Moreover, understanding the properties of gravity as a quantum theory remains among the hardest problems to solve in physics. Both of these issues can be studied holographically and we investigate here generalizations of AdS/CFT involving on the lower-dimensional side QFTs on curved backgrounds and as a further generalization gravity. In the first part we expand on the holographic description of QFT on fixed curved backgrounds, which involves gravity on an asymptotically-AdS space with that prescribed boundary structure. We discuss geometries with de Sitter and AdS as conformal boundary to holographically describe CFTs on these spacetimes. After setting up the procedure of holographic renormalization we study the reflection of CFT unitarity properties in the dual bulk description. The geometry with AdS on the boundary exhibits a number of interesting features, mainly due to the fact that the boundary itself has a boundary. We study both cases and resolve potential tensions between the unitarity properties of the bulk and boundary theories, which would be incompatible with a duality. The origin of these tensions is partly in the structure of the geometry with AdS conformal boundary, while another one arises for a particular limiting case where the bulk and boundary descriptions naively disagree. Besides technical challenges, the hierarchy of boundaries for the geometry with AdS conformal boundary offers an interesting option. Namely, having the dual theory on the conformal boundary itself defined on an AdS space offers the logical possibility of implementing a second instance of AdS/CFT. We discuss an appropriate geometric setting allowing for the notion of the boundary of a boundary and identify limitations for such multi-layered dualities. In the second part we consider five-dimensional supergravities whose solutions can be lifted to actual string-theory backgrounds. We work out the asymptotic structure of the theories on asymptotically-AdS spaces and calculate the Weyl anomaly of the dual CFTs. These holographic calculations confirm the expectations from the field-theory side and provide a non-trivial test of the AdS/CFT conjecture. Moreover, building on the previous results we show that in addition to the usual Dirichlet also more general boundary conditions can be imposed. That allows to promote the boundary metric to a dynamical quantity and is expected to yield a holographic description for a conformal supergravity on the boundary. The boundary theory obtained this way exhibits pathologies such as perturbative ghosts, which is in fact expected for a conformal gravity. The fate of these ghosts beyond perturbation theory is an open question and our setting provides a starting point to study it from the string-theory perspective. That discussion leads to a regime where the holographic description of the boundary theory requires quantization of the bulk supergravity. A necessary ingredient of any supergravity is a number of gravitinos as superpartners of the graviton, for which we thus need an effective-QFT description to make sense of AdS/CFT beyond the limit where bulk theory becomes classical. In particular, quantization should be possible not only on rigid AdS, but also on generic asymptotically-AdS spacetimes which may not be Einstein. In the third part we study the quantization and causality properties of the gravitino on Friedmann-Robertson-Walker spacetimes to explicitly show that a consistent quantization can be carried out also on non-Einstein spaces, in contrast to claims in the recent literature. Furthermore, this reveals interesting non-standard effects for the gravitino propagation, which in certain cases is restricted to regions more narrow than the expected light cones.},
  subject      = {AdS-CFT-Korrespondenz},
  language  = {en}
}
@phdthesis{Schelter2012,
  author    = {Schelter, J{\"o}rg},
  title     = {The Aharonov-Bohm effect and resonant scattering in graphene},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-74662},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {In this thesis, the electronic transport properties of mesoscopic condensed matter systems based on graphene are investigated by means of numerical as well as analytical methods. In particular, it is analyzed how the concepts of quantum interference and disorder, which are essential to mesoscopic devices in general, are affected by the unique electronic and transport properties of the graphene material system. We consider the famous Aharonov-Bohm effect in ring-shaped transport geometries, and, besides providing an overview over the recent developments on the subject, we study the signatures of fundamental phenomena such as Klein tunneling and specular Andreev reflection, which are specific to graphene, in the magnetoconductance oscillations. To this end, we introduce and utilize a variant of the well-known recursive Green's function technique, which is an efficient numerical method for the calculation of transport observables in effectively non-interacting open quantum systems in the framework of a tight binding model. This technique is also applied to study the effects of a specific kind of disorder, namely short-range resonant scatterers, such as strongly bound adatoms or molecules, that can be modeled as vacancies in the graphene lattice. This numerical analysis of the conductance in the presence of resonant scatterers in graphene leads to a non-trivial classification of impurity sites in the graphene lattice and is further substantiated by an independent analytical treatment in the framework of the Dirac equation. The present thesis further contains a formal introduction to the topic of non-equilibrium quantum transport as appropriate for the development of the numerical technique mentioned above, a general introduction to the physics of graphene with a focus on the particular phenomena investigated in this work, and a conclusion where the obtained results are summarized and open questions as well as potential future developments are highlighted.},
  subject      = {Graphen},
  language  = {en}
}
@phdthesis{Edelhaeuser2011,
  author    = {Edelh{\"a}user, Lisa},
  title     = {Model Independent Spin Determination at Hadron Colliders},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-71030},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Mit dem Ende des Jahres 2011 haben die beiden LHC-Experimente ATLAS und CMS jeweils ungef\"ahr 5 inverse Femtobarn an Daten bei einer Energie von 7 TeV aufgenommen. Die bisher analysierten Daten geben nur sehr vage Hinweise auf neue Physik an der TeV-Skala. Trotzdem erwartet man, dass sich an dieser Skala neue Physik zeigt, die bekannte Probleme des Standardmodells behebt. In den letzten Jahrzehnten wurden viele Erweiterungen des Standardmodells der Teilchenphysik und ihre Ph\"anomenologie dazu ausgearbeitet. Sobald sich neue Physik zeigt, stellt sich die Aufgabe, ihre Beschaffenheit und das zugrunde liegende Modell zu finden. Erste Hinweise k\"onnen nat\"urlich schon das Massenspektrum und die Quantenzahlen wie z.B. die elektrische und die Farbladung der neuen Teilchen liefern. \\ In zwei sehr bekannten und gut untersuchten Modellklassen, Supersymmetrie und Extradimensionen, haben neue Teilchen allerdings sehr \"ahnliche Eigenschaften an der erreichbaren Energieskala. Beide Modelle f\"uhren Partnerteilchen zu den bekannten Standardmodell-Teilchen ein, die, abgesehen von der Masse, sehr \"ahnliche Eigenschaften besitzen. Aus diesem Grund ist es n\"otig, weitere Kriterien zu ihrer Unterscheidung einzusetzen.\\ Ein hilfreicher Unterschied ergibt sich aus der Konstruktion beider Modelle: W\"ahrend in Modellen mit Extradimensionen die Partnerteilchen gleichen Spin wie die Standardmodell-Teichen haben, ist der Spin der Partnerteilchen in supersymmetrischen Modellen um 1/2 verschieden. Dieser Unterschied hat nun interessante Auswirkungen auf die jeweilige Ph\"anomenologie der Modelle.\\ Zum Beispiel kann man ausnutzen, dass die unterschiedlichen Spins die absoluten Wirkungsquerschnitte beeinflussen. Diese Methode setzt allerdings voraus, dass man die Massen und Kopplungsst\"arken sehr genau kennt. Eine weitere Herangehensweise nutzt aus, dass Winkelverteilungen vom Spin der involvierten Teilchen abh\"angen k\"onnen. Eine wichtige darauf basierende Methode stellt einen Zusammenhang zwischen der invariante-Masse-Verteilung \$d\Gamma/d\sff\$ zweier Zerfallsprodukte und dem Spin des intermedi\"aren Teilchens, \"uber welches der Zerfall abl\"auft, her.\\ In dieser Arbeit untersuchen wir als erstes den Einfluss von Operatoren h\"oherer Ordnung auf die Spinbestimmung in Zerfallsketten. Wir klassifizieren als erstes die relevanten Operatoren der Dimension 5 und 6. Wir berechnen die neuen Beitr\"age und diskutieren ihre Auswirkungen auf die Bestimmung von Kopplungen und Spin in diesen Zerf\"allen.\\ Im weiteren betrachten wir zwei Szenarien, die nicht die \"ublichen Zerfallsketten beinhalten:\\ In Dreik\"orperzerf\"allen kann die oben erw\"ahnte Methode nicht angewendet werden, da das intermedi\"are Teilchen nicht auf die Massenschale gehen kann. Solche off-shell'' Zerf\"alle k\"onnen in Szenarien wie split-Supersymmetrie oder split-Universal Extra Dimensions'' wichtig sein. Man kann hier die sogenannte Narrow width approximation'' nicht anwenden, welche eine notwendige Voraussetzung f\"ur einen einfachen Zusammenhang zwischen Spin und der invariante-Masse-Verteilung ist. Wir arbeiten eine Strategie f\"ur diese Dreik\"orperzerf\"alle aus, mittels derer man zwischen den unterschiedlichen Spinszenarien unterscheiden kann. Diese Strategie beruht darauf, dass man hier die differentielle Zerfallsbreite als globalen Phasenraumfaktor mal einem Polynom in der invarianten Masse \$\sff\$ schreiben kann. Die hierbei auftretenden Koeffizienten sind nur Funktionen der involvierten Massen und Kopplungen, und wir zeigen, wie beispielsweise ihre Wertebereiche und Vorzeichen dazu benutzt werden k\"onnen, um den zugrunde liegenden Zerfall zu bestimmen. Am Ende testen wir diese Strategie in einer Reihe von Monte Carlo-Simulationen, und diskutieren auch den Einfluss des off-shell'' Teilchens. Im letzten Teil betrachten wir eine Topologie mit sehr kurzen Zefallsketten, in der man den oben genannten Zusammenhang zwischen Spin und invarianter Masse ebenfalls nicht anwenden kann. Wir untersuchen eine bestimmte Variable, die zur Unterscheidung von Supersymmetrie und Universal Extra Dimensions'' eingef\"uhrt wurde. Dabei nutzt man aus, dass sich das Problem im Hochenergielimes auf die zugrunde liegenden Produktionsprozesse reduziert. Wir diskutieren, wie man diese Variable auch in Szenarien anwenden kann, in denen dieser Limes keine gute N\"aherung darstellt. Dazu betrachten wir die m\"oglichen Spinszenarien mit renormierbaren Kopplungen und untersuchen im Detail, wie gut diese Variable zwischen verschiedenen Spin-, Massen- und Kopplungsszenarien unterscheiden kann. Wir finden beispielsweise, dass das Spinszenario, welches den supersymmetrischen Fall beinhaltet, von den meisten anderen Spinszenarien gut unterscheidbar ist.},
  subject      = {Elementarteilchenphysik},
  language  = {en}
}
@misc{ReentsSchiekel2012,
  author    = {Reents, Georg and Schiekel, Bernhard},
  title     = {In memoriam Karl Kraus},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-71296},
  year      = {2012},
  abstract  = {Prof. Dr. Karl Kraus Forscher und Lehrer am Physikalischen Institut der Universit{\"a}t W{\"u}rzburg Curriculum vitae und Publikationsliste},
  subject      = {Kraus},
  language  = {de}
}
@phdthesis{Weidinger2011,
  author    = {Weidinger, Matthias},
  title     = {Variabilit{\"a}t entlang der Blazar-Sequenz - Hinweise auf die Zusammensetzung relativistischer Ausfl{\"u}sse Aktiver Galaxienkerne},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-70508},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Die vorliegende Arbeit besch{\"a}ftigt sich mit der Abstrahlung von Aktiven Galaxienkernen. Das erste Maximum der charakteristischen Doppelpeakstruktur des \$\nu F_{\nu}\$-Spektrums vom Blazaren ist zweifelsfrei Synchrotronstrahlung hochenergetischer Elektronen innerhalb des relativistischen Ausflusses des zugrundeliegenden Aktiven Galaxienkerns. Die zum zweiten (hochenergetischen) Maximum beitragenden Strahlungsprozesse und Teilchenspezies hingegen sind Gegenstand aktueller Diskussionen. In dieser Arbeit wir ein vollst{\"a}ndig selbstkonsistentes und zeitabh{\"a}ngiges hybrides Emissionsmodell, welches auch Teilchenbeschleunigung ber{\"u}cksichtigt, entwickelt und auf verschiedene Blazar-Typen entlang der Blazar-Sequenz, von BL Lac Objekten mit verschiedenen Peakfrequenzen bis hin zu Flachspektrum-Radioquasaren, angewendet. Die spektrale Emission ersterer kann gut im rein leptonischen Grenzfall, d.h. der zweite \$\nu F_{\nu}\$-Peak kommt durch invers Compton-gestreute Synchrotronphotonen der abstrahlenden Elektronen selbst zustande, beschrieben werden. Zur Beschreibung letzterer muss man nicht-thermische Protonen innerhalb des Jets zulassen um die Dominanz des zweiten Maximums im Spektrum konsistent zu erkl{\"a}ren. In diesem Fall besteht der zweite Peak aus Protonensynchrotronstrahlung und Kaskadenstrahlung der photohadronischen Prozesse. Mit dem entwickelten Modell ist es m{\"o}glich auch die zeitliche Information, welche durch Ausbr{\"u}che von Blazaren bereitgestellt wird, auszunutzen um zum einen die freien Modellparameter weiter einzuschr{\"a}nken und -viel wichtiger- zum anderen leptonisch dominierte Blazare von hadronischen zu unterscheiden. Hierzu werden die typischen Zeitunterschiede in den Interbandlichtkurven als hadronischer Fingerabdruck benutzt.\\ Mit einer Stichprobe von 16 Spektren von zehn Blazaren entlang der Blazar-Sequenz, welche in unterschiedlichen Flusszust{\"a}nden und mit starker Variabilit{\"a}t beobachtet wurden, ist es m{\"o}glich die wichtigsten offenen Fragen der Physik relativistischer Ausbr{\"u}che in systematischer Art und Weise zu adressieren. Anhand der modellierten Ausbr{\"u}che kann man erkennen, dass sechs Quellen rein leptonisch dominiert sind, aber vier Protonen bis auf \$\gamma \approx 10^{11}\$ beschleunigen, was Auswirkungen auf die m{\"o}glichen Quellen extragalaktischer kosmischer Strahlung unter den Blazaren hat. Dar{\"u}ber hinaus findet sich eine Abh{\"a}ngigkeit zwischen dem Magnetfeld der Emissionsregion und der injizierten Leuchtkraft, welche unabh{\"a}ngig von den zugrunde liegenden Teilchenpopulationen G{\"u}ltigkeit besitzt. In diesem Zusammenhang l{\"a}sst sich die Blazar-Sequenz als ein evolution{\"a}res Szenario erkl{\"a}ren: die Sequenz \$FSRQ \rightarrow LBL/IBL \rightarrow HBL\$ kommt aufgrund abnehmender Gasdichte der Hostgalaxie und damit einhergehender abnehmender Akkretionsrate zustande, dies wird durch weitere kosmologische Beobachtungen best{\"a}tigt. Eine abnehmende Materiedichte innerhalb des relativistischen Ausflusses wird von einem abnehmenden Magnetfeld begleitet, d.h. aber auch, dass Protonen weit vor den Elektronen nicht mehr im Strahlungsgebiet gehalten werden k{\"o}nnen. Die Blazar-Sequenz ist also ein Maß f{\"u}r die Hadronizit{\"a}t des Jets. Dies erkl{\"a}rt zudem die Dichotomie von FSRQs und BL Lac Objekten sowie die Zweiteilung in anderen Erscheinungsformen von AGN, z.B. FR-I und FR-II Radiogalaxien.\\ W{\"a}hrend der Modellierung wird gezeigt, dass man Blazar-Spektren, speziell im hadronischen Fall, nicht mehr statisch betrachten kann, da es zu kumulierten Effekten aufgrund der langen Protonensynchrotronzeitskala kommt. Die niedrige Luminosit{\"a}t der Quellen und unterschiedlich lange Beobachtungszeiten verschiedener Experimente verlangen bei variablen Blazaren auch im leptonischen Fall eine zeitabh{\"a}ngige Betrachtung. Die Kurzzeitvariabilit{\"a}t scheint bei einzelnen Blazaren stets die selbe Ursache zu haben, unterscheidet sich aber bei der Betrachtung verschiedener Quellen. Zus{\"a}tzlich wird f{\"u}r jeden Blazar, der in verschiedenen Flusszust{\"a}nden beobachtet werden konnte, der Unterschied zwischen Lang- und Kurzzeitvariabilit{\"a}t, auch im Hinblick auf einen m{\"o}glichen globalen Grundzustand hin, betrachtet.},
  subject      = {Blazar},
  language  = {de}
}
@phdthesis{Kiesel2012,
  author    = {Kiesel, Maximilian Ludwig},
  title     = {Unconventional Superconductivity in Cuprates, Cobaltates and Graphene: What is Universal and what is Material-Dependent in strongly versus weakly Correlated Materials?},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-76421},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {Eine allgemeing{\"u}ltige Theorie f{\"u}r alle unterschiedlichen Arten von unkonventionellen Supraleitern ist immer noch eine der ungel{\"o}sten Kernfragen der Festk{\"o}rperphysik. Momentan ist es nicht einmal bewiesen, dass es {\"u}berhaupt einen gemeinsamen grundlegenden Mechanismus gibt, sondern es m{\"u}ssen vielleicht mehrere verschiedene Ursachen f{\"u}r unkonventionelle Supraleitung ber{\"u}cksichtigt werden. Der Einfluss der Elektron-Phonon-Wechselwirkung ist dabei noch nicht abschließend gekl{\"a}rt. In dieser Dissertation wird ein rein elektronischer Paarungsmechanismus untersucht, in welchem die Paarung durch Spin-Fluktuationen vermittelt wird, was nach dem aktuellen Stand der Forschung auf dem Gebiet der unkonventionellen Supraleiter am wahrscheinlichsten ist. Der Schwerpunkt liegt dabei auf der Bestimmung von Material-unabh{\"a}ngigen Eigenschaften der supraleitenden Phase. Diese k{\"o}nnen durch eine Auswahl sehr unterschiedlicher Systeme herausgearbeitet werden. Eine Untersuchung der Phasendiagramme gibt außerdem Auskunft dar{\"u}ber, welche konkurrierenden Quantenfluktuationen den supraleitenden Zustand abschw{\"a}chen oder verst{\"a}rken. F{\"u}r diese Analyse von sehr unterschiedlichen supraleitenden Materialien ist der Einsatz einer einzelnen numerischen L{\"o}sungsmethode unzureichend. F{\"u}r diese Dissertation ist dies aber kein Nachteil, sondern vielmehr ein großer Vorteil, da der Einsatz verschiedener Techniken die Abh{\"a}ngigkeit der Ergebnisse von der verwendeten Numerik reduziert und dadurch der grundlegende Mechanismus besser untersucht werden kann. Im speziellen werden in dieser Dissertation die Kuprate mit der Variationellen Clustern{\"a}herung ausgewertet, weil die Elektronen hier eine starke Wechselwirkung untereinander besitzen. Besonders die Frage eines m{\"o}glichen Klebstoffs f{\"u}r die Cooper-Paare wird ausf{\"u}hrlich diskutiert, auch mit einer Unterscheidung in retardierte und nicht-retardierte Betr{\"a}ge. Den Kupraten werden das Kobaltat NaCoO sowie Graphen gegen{\"u}bergestellt. Diese Materialien sind jedoch schwach korrelierte Systeme, so dass hier die Funkionelle Renormierungsgruppe als numerisches Grundger{\"u}st dient. Die Ergebnisse sind reichhaltige Phasendiagramme mit vielen verschiedenen langreichweitigen Ordnungen, wie zum Beispiel d+id-wellenartige Supraleitung. Diese bricht die Zeitumkehr-Symmetrie und besitzt eine vollst{\"a}ndige Bandl{\"u}cke, welche im Falle von NaCoO jedoch eine stark Dotierungs-abh{\"a}ngige Anisotropie aufweist. Als letztes wird das Kagome-Gitter allgemein diskutiert, ohne ein konkretes Material zu beschreiben. Hier hat eine destruktive Interferenz zwischen den Elektronen auf verschiedenen Untergittern drastische Auswirkungen auf die Instabilit{\"a}ten der Fermi-Fl{\"a}che, so dass die {\"u}bliche Spin-Dichte-Welle und die damit verbundene d+id-wellenartige Supraleitung unterdr{\"u}ckt werden. Dadurch treten ungew{\"o}hnliche Spin- und Ladungsdichte-Ordnungen sowie eine nematische Pomeranchuck Instabilit{\"a}t hervor. Zusammengefasst bietet diese Dissertation einen Einblick in unterschiedliche Materialklassen von unkonventionellen Supraleitern. Dadurch wird es m{\"o}glich, die Material-spezifischen Eigenschaften von den universellen zu trennen.},
  subject      = {Supraleitung},
  language  = {en}
}
@phdthesis{Budich2012,
  author    = {Budich, Jan Carl},
  title     = {Fingerprints of Geometry and Topology on Low Dimensional Mesoscopic Systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-76847},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {In this PhD thesis, the fingerprints of geometry and topology on low dimensional mesoscopic systems are investigated. In particular, holographic non-equilibrium transport properties of the quantum spin Hall phase, a two dimensional time reversal symmetric bulk insulating phase featuring one dimensional gapless helical edge modes are studied. In these metallic helical edge states, the spin and the direction of motion of the charge carriers are locked to each other and counter-propagating states at the same energy are conjugated by time reversal symmetry. This phenomenology entails a so called topological protection against elastic single particle backscattering by time reversal symmetry. We investigate the limitations of this topological protection by studying the influence of inelastic processes as induced by the interplay of phonons and extrinsic spin orbit interaction and by taking into account multi electron processes due to electron-electron interaction, respectively. Furthermore, we propose possible spintronics applications that rely on a spin charge duality that is uniquely associated with the quantum spin Hall phase. This duality is present in the composite system of two helical edge states with opposite helicity as realized on the two opposite edges of a quantum spin Hall sample with ribbon geometry. More conceptually speaking, the quantum spin Hall phase is the first experimentally realized example of a symmetry protected topological state of matter, a non-interacting insulating band structure which preserves an anti-unitary symmetry and is topologically distinct from a trivial insulator in the same symmetry class with totally localized and hence independent atomic orbitals. In the first part of this thesis, the reader is provided with a fairly self-contained introduction into the theoretical concepts underlying the timely research field of topological states of matter. In this context, the topological invariants characterizing these novel states are viewed as global analogues of the geometric phase associated with a cyclic adiabatic evolution. Whereas the detailed discussion of the topological invariants is necessary to gain deeper insight into the nature of the quantum spin Hall effect and related physical phenomena, the non-Abelian version of the local geometric phase is employed in a proposal for holonomic quantum computing with spin qubits in quantum dots.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@article{Winter2012,
  author    = {Winter, Walter},
  title     = {Neutrinos from Cosmic Accelerators Including Magnetic Field and Flavor Effects},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75290},
  year      = {2012},
  abstract  = {We review the particle physics ingredients affecting the normalization, shape, and flavor composition of astrophysical neutrinos fluxes, such as different production modes, magnetic field effects on the secondaries muons, pions, and kaons, and flavor mixing, where we focus on p? interactions. We also discuss the interplay with neutrino propagation and detection, including the possibility to detect flavor and its application in particle physics, and the use of the Glashow resonance to discriminate p? from pp interactions in the source. We illustrate the implications on fluxes and flavor composition with two different models: 1 the target photon spectrum is dominated by synchrotron emission of coaccelerated electrons and 2 the target photon spectrum follows the observed photon spectrum of gamma-ray bursts. In the latter case, the multimessenger extrapolation from the gamma-ray fluence to the expected neutrino flux is highlighted.},
  subject      = {Magnetfeld},
  language  = {en}
}
@article{KaiserRiemerKnopf2011,
  author    = {Kaiser, J. C. and Riemer, N. and Knopf, D. A.},
  title     = {Detailed heterogeneous oxidation of soot surfaces in a particle-resolved aerosol model},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75440},
  year      = {2011},
  abstract  = {Using the particle-resolved aerosol model PartMC-MOSAIC, we simulate the heterogeneous oxidation of a monolayer of polycyclic aromatic hydrocarbons (PAHs) on soot particles in an urban atmosphere. We focus on the interaction of the major atmospheric oxidants (O3, NO2, OH, and NO3) with PAHs and include competitive co-adsorption of water vapour for a range of atmospheric conditions. For the first time detailed heterogeneous chemistry based on the P¨oschl-Rudich-Ammann (PRA) framework is modelled on soot particles with a realistic size distribution and a continuous range of chemical ages. We find PAH half-lives, 1/2, on the order of seconds during the night, when the PAHs are rapidly oxidised by the gas-surface reaction with NO3. During the day, 1/2 is on the order of minutes and determined mostly by the surface layer reaction of PAHs with adsorbed O3. Such short half-lives of surface-bound PAHs may lead to efficient conversion of hydrophobic soot into more hygroscopic particles, thus increasing the particles' aerosol-cloud interaction potential. Despite its high reactivity OH appears to have a negligible effect on PAH degradation which can be explained by its very low concentration in the atmosphere. An increase of relative humidity (RH) from 30\% to 80\% increases PAH half-lives by up to 50\%for daytime degradation and by up to 100\% or more for nighttime degradation. Uptake coefficients, averaged over the particle population, are found to be relatively constant over time for O3 (2×10-7 to 2×10-6) and NO2 (5×10-6 to 10-5) at the different levels of NOx emissions and RH considered in this study. In contrast, those for OH and NO3 depend strongly on the surface concentration of PAHs. We do not find a significant influence of heterogeneous reactions on soot particles on the gas phase composition. The derived half-lives of surfacebound PAHs and the time and particle population averaged uptake coefficients for O3 and NO2 presented in this paper can be used as parameterisations for the treatment of heterogeneous chemistry in large-scale atmospheric chemistry models.},
  subject      = {Physik},
  language  = {en}
}
@phdthesis{Walter2012,
  author    = {Walter, Stefan},
  title     = {Exploring the Quantum Regime of Nanoelectromechanical Systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75188},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {This thesis deals with nanoelectromechanical systems in the quantum regime. Nanoelectromechanical systems are systems where a mechanical degree of freedom of rather macroscopic size is coupled to an electronic degree of freedom. The mechanical degree of freedom can without any constraints be modeled as the fundamental mode of a harmonic oscillator. Due to their size and the energy scales involved in the setting, quantum mechanics plays an important role in their description. We investigate transport through such nanomechanical devices where our focus lies on the quantum regime. We use non-equilibrium methods to fully cover quantum effects in setups where the mechanical oscillator is part of a tunnel junction. In such setups, the mechanical motion influences the tunneling amplitude and thereby the transport properties through the device. The electronics in these setups can then be used to probe and characterize the mechanical oscillator through signatures in transport quantities such as the average current or the current noise. The interplay between the mechanical motion and other physical degrees of freedom can also be used to characterize these other degrees of freedom, i.e., the nanomechanical oscillator can be used as a detector. In this thesis, we will show that a nanomechanical oscillator can be used as a detector for rather exotic degrees of freedom, namely Majorana bound states which recently attracted great interest, theoretically as well as experimentally. Again, the quantum regime plays an essential role in this topic. One of the major manifestations of quantum mechanics is entanglement between two quantum systems. Entanglement of quantum systems with few (discrete) degrees of freedom is a well established and understood subject experimentally as well as theoretically. Here, we investigate quantum entanglement between two macroscopic continuous variable systems. We study different setups where it is possible to entangle two nanomechanical oscillators which are not directly coupled to each other. We conclude with reviewing the obtained results and discuss open questions and possible future developments on the quantum aspects of nanomechanical systems.},
  subject      = {Nanoelektromechanik},
  language  = {en}
}
@phdthesis{Luitz2012,
  author    = {Luitz, David J.},
  title     = {Numerical methods and applications in many fermion systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75927},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {This thesis presents results covering several topics in correlated many fermion systems. A Monte Carlo technique (CT-INT) that has been implemented, used and extended by the author is discussed in great detail in chapter 3. The following chapter discusses how CT-INT can be used to calculate the two particle Green's function and explains how exact frequency summations can be obtained. A benchmark against exact diagonalization is presented. The link to the dynamical cluster approximation is made in the end of chapter 4, where these techniques are of immense importance. In chapter 5 an extensive CT-INT study of a strongly correlated Josephson junction is shown. In particular, the signature of the first order quantum phase transition between a Kondo and a local moment regime in the Josephson current is discussed. The connection to an experimental system is made with great care by developing a parameter extraction strategy. As a final result, we show that it is possible to reproduce experimental data from a numerically exact CT-INT model-calculation. The last topic is a study of graphene edge magnetism. We introduce a general effective model for the edge states, incorporating a complicated interaction Hamiltonian and perform an exact diagonalization study for different parameter regimes. This yields a strong argument for the importance of forbidden umklapp processes and of the strongly momentum dependent interaction vertex for the formation of edge magnetism. Additional fragments concerning the use of a Legendre polynomial basis for the representation of the two particle Green's function, the analytic continuation of the self energy for the Anderson Kane Mele Model, as well as the generation of test data with a given covariance matrix are documented in the appendix. A final appendix provides some very important matrix identities that are used for the discussion of technical details of CT-INT.},
  subject      = {Fermionensystem},
  language  = {en}
}
@article{SchenkelUhlemann2010,
  author    = {Schenkel, Alexander and Uhlemann, Christoph F.},
  title     = {Field Theory on Curved Noncommutative Spacetimes},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-68648},
  year      = {2010},
  abstract  = {We study classical scalar field theories on noncommutative curved spacetimes. Following the approach of Wess et al. [Classical Quantum Gravity 22 (2005), 3511 and Classical Quantum Gravity 23 (2006), 1883], we describe noncommutative spacetimes by using (Abelian) Drinfel'd twists and the associated ?-products and ?-differential geometry. In particular, we allow for position dependent noncommutativity and do not restrict ourselves to the Moyal-Weyl deformation. We construct action functionals for real scalar fields on noncommutative curved spacetimes, and derive the corresponding deformed wave equations. We provide explicit examples of deformed Klein-Gordon operators for noncommutative Minkowski, de Sitter, Schwarzschild and Randall-Sundrum spacetimes, which solve the noncommutative Einstein equations. We study the construction of deformed Green's functions and provide a diagrammatic approach for their perturbative calculation. The leading noncommutative corrections to the Green's functions for our examples are derived.},
  subject      = {Physik},
  language  = {en}
}
@phdthesis{Wisniewski2011,
  author    = {Wisniewski, Martina},
  title     = {Numerische Untersuchung von Turbulenz und Teilchentransport in der Heliosphaere},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-64652},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Hochenergetische solare Teilchen werden bei ihrem Transport durch die Heliosph{\"a}re an turbulenten Magnetfeldern gestreut. F{\"u}r das Verst{\"a}ndnis dieses Streuprozesses ergeben sich aus heutiger Sicht zwei wesentliche Hindernisse: - Bei der Streuung hochenergetischer Teilchen an turbulenten Magnetfeldern handelt es sich um einen nichtlinearen Prozess, der durch analytische Theorien kaum zu beschreiben ist. - Der Streuprozess h{\"a}ngt stark von den tats{\"a}chlichen Magnetfeldern und somit auch von der Magnetfeldturbulenz ab. Unser bisheriges Verst{\"a}ndnis der heliosph{\"a}rischen Turbulenz ist leider aufgrund sp{\"a}rlicher experimenteller Daten deutlich eingeschr{\"a}nkt, was eine qualifizierte Umsetzung in analytischen und numerischen Ans{\"a}tzen deutlich erschwert. Dies machte in der Vergangenheit k{\"u}nstliche Annahmen f{\"u}r die Modellerstellung notwendig. In dieser Arbeit wird der Teilchentransport mit Hilfe der Simulation von Testteilchen in einem turbulenten, magnetohydrodynamischen Plasma untersucht. Durch die Testteilchen werden auch die nichtlinearen Streuprozesse korrekt wiedergegeben, wodurch das erste hier genannte Hindernis {\"u}berwunden wird. Dies wurde auch bereits in fr{\"u}heren numerischen Untersuchungen erfolgreich angewendet. Die Modellierung der Turbulenz f{\"u}r den Fall des Teilchentransports erfolgt in dieser Arbeit erstmalig auf Grundlage der magnetohydrodynamischen Gleichungen. Dabei handelt es sich um die mathematisch korrekte Wiedergabe der Magnetfeldturbulenz unterhalb der Ionen-Gyrofrequenz mit nur geringen numerischen Einschr{\"a}nkungen. Dar{\"u}ber hinaus erlaubt ein auf das physikalische Szenario anpassbarer Turbulenztreiber eine noch realistischere Simulation der Turbulenz. Durch diesen universell g{\"u}ltigen, numerischen Ansatz k{\"o}nnen f{\"u}r das zweite hier angegebene Hindernis jegliche k{\"u}nstlichen Annahmen vermieden werden. Die drei im Rahmen dieser Arbeit erstmals zusammengef{\"u}hrten Methoden (Testteilchen, magnetohydrodynamische Turbulenz, Turbulenztreiber) erm{\"o}glichen somit eine Untersuchung und Analyse von Transport- und Turbulenzph{\"a}nomenen mit herausragender Qualit{\"a}t, die insbesondere f{\"u}r den Fall des Teilchentransports einen direkten Anschluss an experimentelle Ergebnisse erm{\"o}glichen. Wichtige Ergebnisse im Rahmen dieser Arbeit sind: - der Nachweis der Drei-Wellen-Wechselwirkung f{\"u}r schwache und einsetzende starke Turbulenz. - eine Analyse der Anisotropie der Turbulenz im Bezug auf das Hintergrundmagnetfeld in Abh{\"a}ngigkeit vom Treibmodell. Insbesondere die Anisotropie ist experimentell bislang kaum erfassbar. - eine Untersuchung der Auswirkung der Gyroresonanzen auf die Diffusionskoeffizienten hochenergetischer solarer Teilchen in allgemeiner Form. - die Simulation des Teilchentransports in der Heliosph{\"a}re auf Grundlage experimenteller Messdaten. Die genauere Analyse der Simulationsergebnisse erm{\"o}glicht insgesamt einen Zugang zum Verst{\"a}ndnis des Transports, der durch experimentelle Untersuchungen nicht erfassbar ist. Bei der Simulation wurden lediglich die Magnetfeldst{\"a}rke sowie die untersuchte Teilchenenergie vorgegeben. Aus der Analyse der Simulationsergebnisse ergibt sich dieselbe mittlere freie Wegl{\"a}nge, wie sie auch durch andere Verfahren direkt aus den Messergebnissen gewonnen werden konnte. Auch die vorwiegende Ausrichtung der hochenergetischen Teilchen parallel und antiparallel zum Hintergrundmagnetfeld in der Simulation entspricht experimentellen Untersuchungen. Es zeigt sich, dass diese allein aus den resonanten Streuprozessen der Teilchen mit den Magnetfeldern resultiert. Des Weiteren werden die Art der Diffusion, der Energieverlust der Teilchen w{\"a}hrend des Transportprozesses sowie die G{\"u}ltigkeit der quasilinearen Theorie untersucht.},
  subject      = {Sonnenwind},
  language  = {de}
}
@phdthesis{Schenkel2011,
  author    = {Schenkel, Alexander},
  title     = {Noncommutative Gravity and Quantum Field Theory on Noncommutative Curved Spacetimes},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-65823},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {{\"U}ber die letzten Jahrzehnte hat sich die nichtkommutative Geometrie zu einem etablierten Teilgebiet der reinen Mathematik und der theoretischen Physik entwickelt. Die Entdeckung, dass gewisse Grenzf{\"a}lle der Quantengravitation und Stringtheorie zu nichtkommutativer Geometrie f{\"u}hren, motivierte die Suche nach Physik jenseits des Standardmodells der Elementarteilchenphysik und der Einstein'schen allgemeinen Relativit{\"a}tstheorie im Rahmen von nichtkommutativen Geometrien. Einen ergiebigen Ansatz zu letzteren Theorien, welcher Deformationsquantisierung (Sternprodukte) mit Methoden aus der Theorie der Quantengruppen kombiniert, wurde von der Gruppe um Julius Wess entwickelt. Die resultierende Gravitationstheorie ist nicht nur imstande nichtkommutative Effekte der Raumzeit zu beschreiben, sondern sie erf{\"u}llt ebenfalls ein generalisiertes allgemeines Kovarianzprinzip, welches durch eine deformierte Hopf Algebra von Diffeomorphismen beschrieben wird. Gegenstand des ersten Teils dieser Dissertation ist es Symmetriereduktion im Rahmen von nichtkommutativer Gravitation zu verstehen und damit exakte L{\"o}sungen der nichtkommutativen Einstein'schen Gleichungen zu konstruieren. Diese Untersuchungen sind von großer Bedeutung um den physikalischen Inhalt dieser Theorien herauszuarbeiten und den Kontakt zu Anwendungen, z.B. im Rahmen nichtkommutativer Kosmologie und Physik schwarzer L{\"o}cher, herzustellen. Wir verallgemeinern die {\"u}bliche Methode der Symmetriereduktion, welche eine Standardtechnik im Auffinden von L{\"o}sungen der Einstein'schen Gleichungen ist, auf nichtkommutative Gravitation. Es wird gezeigt, dass unsere Methode zur nichtkommutativen Symmetriereduktion f{\"u}r ein gegebenes symmetrisches System zu bevorzugten Deformationen f{\"u}hrt. F{\"u}r Abelsche Drinfel'd Twists klassifizieren wir alle konsistenten Deformationen von r{\"a}umlich flachen Friedmann-Robertson-Walker Kosmologien und des Schwarzschild'schen schwarzen Loches. Aufgrund der deformierten Symmetriestruktur dieser Modelle k{\"o}nnen wir viele Beispiele von exakten L{\"o}sungen der nichtkommutativen Einstein'schen Gleichungen finden, bei welchen das nichtkommutative Metrikfeld mit dem klassischen {\"u}bereinstimmt. Im Fokus des zweiten Teils sind Quantenfeldtheorien auf nichtkommutativen gekr{\"u}mmten Raumzeiten. Dazu entwickeln wir einen neuen Formalismus, welcher algebraische Methoden der Quantenfeldtheorie mit nichtkommutativer Differentialgeometrie verkn{\"u}pft. Als Resultat unseres Ansatzes erhalten wir eine Observablenalgebra f{\"u}r skalare Quantenfeldtheorien auf einer großen Klasse von nichtkommutativen gekr{\"u}mmten Raumzeiten. Es wird eine pr{\"a}zise Relation zwischen dieser Algebra und der Observablenalgebra der undeformierten Quantenfeldtheorie hergeleitet. Wir studieren ebenfalls explizite Beispiele von deformierten Wellenoperatoren und finden, dass im Gegensatz zu dem einfachsten Modell des Moyal-Weyl deformierten Minkowski-Raumes, im Allgemeinen schon die Propagation freier Felder durch die nichtkommutative Geometrie beeinflusst wird. Die Effekte von konvergenten Deformationen werden in einfachen Spezialf{\"a}llen untersucht, und wir beobachten neue Aspekte in diesen Quantenfeldtheorien, welche sich in formalen Deformationen nicht zeigten. Zus{\"a}tzlich zu der erwarteten Nichtlokalit{\"a}t finden wir, dass sich die Beziehung zwischen der deformierten und der undeformierten Quantenfeldtheorie nichttrivial ver{\"a}ndert. Wir beweisen, dass dies zu einem verbesserten Verhalten der nichtkommutativen Theorie bei kurzen Abst{\"a}nden, d.h. im Ultravioletten, f{\"u}hrt. Im dritten Teil dieser Arbeit entwickeln wir Elemente eines leistungsf{\"a}higeren, jedoch abstrakteren, mathematischen Ansatzes zur Beschreibung der nichtkommutativen Gravitation. Das Hauptaugenmerk liegt auf globalen Aspekten von Homomorphismen zwischen und Zusammenh{\"a}ngen auf nichtkommutativen Vektorb{\"u}ndeln, welche fundamentale Objekte in der mathematischen Beschreibung von nichtkommutativer Gravitation sind. Wir beweisen, dass sich alle Homomorphismen und Zusammenh{\"a}nge der deformierten Theorie mittels eines Quantisierungsisomorphismus aus den undeformierten Homomorphismen und Zusammenh{\"a}ngen ableiten lassen. Es wird ebenfalls untersucht wie sich Homomorphismen und Zusammenh{\"a}nge auf Tensorprodukte von Moduln induzieren lassen. Das Verst{\"a}ndnis dieser Induktion erlaubt es uns die nichtkommutative Gravitationstheorie von Wess et al. um allgemeine Tensorfelder zu erweitern. Als eine nichttriviale Anwendung des neuen Formalismus erweitern wir unsere Studien zu exakten L{\"o}sungen der nichtkommutativen Einstein'schen Gleichungen auf allgemeinere Klassen von Deformationen.},
  subject      = {Nichtkommutative Geometrie},
  language  = {en}
}
@phdthesis{Adamek2011,
  author    = {Adamek, Julian},
  title     = {Classical and Quantum Aspects of Anisotropic Cosmology},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-65908},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {The idea that our observable Universe may have originated from a quantum tunneling event out of an eternally inflating false vacuum state is a cornerstone of the multiverse paradigm. Modern theories that are considered as an approach towards the ultraviolet-complete fundamental theory of particles and gravity, such as the various types of string theory, even suggest that a vast landscape of different vacuum configurations exists, and that gravitational tunneling is an important mechanism with which the Universe can explore this landscape. The tunneling scenario also presents a unique framework to address the initial conditions of our observable Universe. In particular, it allows to introduce deviations from the cosmological concordance model in a controlled and well-motivated way. These deviations are a central topic of this work. An important feature in most of the theories mentioned above is the presumed existence of additional space dimensions in excess of the three which we observe in our every-day experience. It was realized that these extra dimensions could avoid our detection if they are compactified to microscopic length scales far beyond the reach of current experiments. There also seem to be natural mechanisms available for dynamical compactification in those theories. These typically lead to a vast landscape of different vacuum configurations which also may differ in the number of macroscopic dimensions, only the total number of dimensions being determined by the theory. Transitions between these vacuum configurations may hence open up new directions which were previously compact, spontaneously compactify some previously macroscopic directions, or otherwise re-arrange the configuration of compact and macroscopic dimensions in a more general way. From within the bubble Universe, such a process may be perceived as an anisotropic background spacetime - intuitively, the dimensions which open up may give rise to preferred directions. If our 3+1 dimensional observable Universe was born in a process as described above, one may expect to find traces of a preferred direction in cosmological observations. For instance, two directions could be curved like on a sphere, while the third space direction is flat. Using a scenario of gravitational tunneling to fix the initial conditions, I show how the primordial signatures in such an anisotropic Universe can be obtained in principle and work out a particular example in more detail. A small deviation from isotropy also has phenomenological consequences for the later evolution of the Universe. I discuss the most important effects and show that backreaction can be dynamically important. In particular, under certain conditions, a buildup of anisotropic stress in different components of the cosmic fluid can lead to a dynamical isotropization of the total stress-energy tensor. The mechanism is again demonstrated with the help of a physical example.},
  subject      = {Kosmologie},
  language  = {en}
}
@phdthesis{Englert2011,
  author    = {Englert, Anja},
  title     = {Chaossynchronisation in Netzwerken mit zeitverz{\"o}gerten Kopplungen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-65454},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Die vorliegende Arbeit besch{\"a}ftigt sich mit der Chaossynchronisation in Netzwerken mit zeitverz{\"o}gerten Kopplungen. Ein Netzwerk chaotischer Einheiten kann isochron und vollst{\"a}ndig synchronisieren, auch wenn der Austausch der Signale einer oder mehreren Verz{\"o}gerungszeiten unterliegt. In einem Netzwerk identischer Einheiten hat sich als Stabilit{\"a}tsanalyse die Methode der Master Stability Funktion von Pecora und Carroll etabliert. Diese entspricht f{\"u}r ein Netzwerk gekoppelter iterativer Bernoulli-Abbildungen Polynomen vom Grade der gr{\"o}ßten Verz{\"o}gerungszeit. Das Stabilit{\"a}tsproblem reduziert sich somit auf die Untersuchung der Nullstellen dieser Polynome hinsichtlich ihrer Lage bez{\"u}glich des Einheitskreises. Eine solche Untersuchung kann beispielsweise numerisch mit dem Schur-Cohn-Theorem erfolgen, doch auch analytische Ergebnisse lassen sich erzielen. In der vorliegenden Arbeit werden Bernoulli-Netzwerke mit einer oder mehreren zeitverz{\"o}gerten Kopplungen und/oder R{\"u}ckkopplungen untersucht. Hierbei werden Aussagen {\"u}ber Teile des Stabilit{\"a}tsgebietes getroffen, welche unabh{\"a}ngig von den Verz{\"o}gerungszeiten sind. Des Weiteren werden Aussagen zu Systemen gemacht, welche sehr große Verz{\"o}gerungszeiten aufweisen. Insbesondere wird gezeigt, dass in einem Bernoulli-Netzwerk keine stabile Chaossynchronisation m{\"o}glich ist, wenn die vorhandene Verz{\"o}gerungszeit sehr viel gr{\"o}ßer ist als die Zeitskala der lokalen Dynamik, bzw. der Lyapunovzeit. Außerdem wird in bestimmten Systemen mit mehreren Verz{\"o}gerungszeiten anhand von Symmetriebetrachtungen stabile Chaossynchronisation ausgeschlossen, wenn die Verz{\"o}gerungszeiten in bestimmten Verh{\"a}ltnissen zueinander stehen. So ist in einem doppelt bidirektional gekoppeltem Paar ohne R{\"u}ckkopplung und mit zwei verschiedenen Verz{\"o}gerungszeiten stabile Chaossynchronisation nicht m{\"o}glich, wenn die Verz{\"o}gerungszeiten in einem Verh{\"a}ltnis von teilerfremden ungeraden ganzen Zahlen zueinander stehen. Es kann zudem Chaossynchronisation ausgeschlossen werden, wenn in einem bipartiten Netzwerk mit zwei großen Verz{\"o}gerungszeiten zwischen diesen eine kleine Differenz herrscht. Schließlich wird ein selbstkonsistentes Argument vorgestellt, das das Auftreten von Chaossynchronisation durch die Mischung der Signale der einzelnen Einheiten interpretiert und sich unter anderem auf die Teilerfremdheit der Zyklen eines Netzes st{\"u}tzt. Abschließend wird untersucht, ob einige der durch die Bernoulli-Netzwerke gefundenen Ergebnisse sich auf andere chaotische Netzwerke {\"u}bertragen lassen. Hervorzuheben ist die sehr gute {\"U}bereinstimmung der Ergebnisse eines Bernoulli-Netzwerkes mit den Ergebnissen eines gleichartigen Netzwerkes gekoppelter Halbleiterlasergleichungen, sowie die {\"U}bereinstimmungen mit experimentellen Ergebnissen eines Systems von Halbleiterlasern.},
  subject      = {Chaos},
  language  = {de}
}
@phdthesis{CardosoBarato2010,
  author    = {Cardoso Barato, Andre},
  title     = {Nonequilibrium phase transitions and surface growth},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-50122},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {This thesis is concerned with the statistical physics of various systems far from thermal equilibrium, focusing on universal critical properties, scaling laws and the role of fluctuations. To this end we study several models which serve as paradigmatic examples, such as surface growth and non-equilibrium wetting as well as phase transitions into absorbing states. As a particular interesting example of a model with a non-conventional scaling behavior, we study a simplified model for pulsed laser deposition by rate equations and Monte Carlo simulations. We consider a set of equations, where islands are assumed to be point-like, as well as an improved one that takes the size of the islands into account. The first set of equations is solved exactly but its predictive power is restricted to the first few pulses. The improved set of equations is integrated numerically, is in excellent agreement with simulations, and fully accounts for the crossover from continuous to pulsed deposition. Moreover, we analyze the scaling of the nucleation density and show numerical results indicating that a previously observed logarithmic scaling does not apply. In order to understand the impact of boundaries on critical phenomena, we introduce particle models displaying a boundary-induced absorbing state phase transition. These are one-dimensional systems consisting of a single site (the boundary) where creation and annihilation of particles occur, while particles move diffusively in the bulk. We study different versions of these models and confirm that, except for one exactly solvable bosonic variant exhibiting a discontinuous transition with trivial exponents, all the others display a non-trivial behavior, with critical exponents differing from their mean-field values, representing a universality class. We show that these systems are related to a \$(0+1)\$-dimensional non-Markovian model, meaning that in nonequilibrium a phase transition can take place even in zero dimensions, if time long-range interactions are considered. We argue that these models constitute the simplest universality class of phase transition into an absorbing state, because the transition is induced by the dynamics of a single site. Moreover, this universality class has a simple field theory, corresponding to a zero dimensional limit of direct percolation with L{\'e}vy flights in time. Another boundary phenomena occurs if a nonequilibrium growing interface is exposed to a substrate, in this case a nonequilibrium wetting transition may take place. This transition can be studied through Langevin equations or discrete growth models. In the first case, the Kardar-Parisi-Zhang equation, which defines a very robust universality class for nonequilibrium moving interfaces, is combined with a soft-wall potential. While in the second, microscopic models, in the corresponding universality class, with evaporation and deposition of particles in the presence of hard-wall are studied. Equilibrium wetting is related to a particular case of the problem, corresponding to the Edwards-Wilkinson equation with a potential in the continuum approach or to the fulfillment of detailed balance in the microscopic models. In this thesis we present the analytical and numerical methods used to investigate the problem and the very rich behavior that is observed with them. The entropy production for a Markov process with a nonequilibrium stationary state is expected to give a quantitative measure of the distance form equilibrium. In the final chapter of this thesis, we consider a Kardar-Parisi-Zhang interface and investigate how entropy production varies with the interface velocity and its dependence on the interface slope, which are quantities that characterize how far the stationary state of the interface is away from equilibrium. We obtain results in agreement with the idea that the entropy production gives a measure of the distance from equilibrium. Moreover we use the same model to study fluctuation relations. The fluctuation relation is a symmetry in the large deviation function associated to the probability of the variation of entropy during a fixed time interval. We argue that the entropy and height are similar quantities within the model we consider and we calculate the Legendre transform of the large deviation function associated to the height for small systems. We observe that there is no fluctuation relation for the height, nevertheless its large deviation function is still symmetric.},
  subject      = {Nichtgleichgewichtsstatistik},
  language  = {en}
}
@phdthesis{Burkart2010,
  author    = {Burkart, Thomas},
  title     = {Der Einfluss des fundamentalen Massenverh{\"a}ltnisses auf die Teilchenbeschleunigung durch Plasmainstabilit{\"a}ten},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56636},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {Im Rahmen dieser Arbeit wurde ein dreidimensionaler vollrelativistischer und parallelisierter Particle-in-Cell Code geschrieben, ausf{\"u}hrlich getestet und angewandt. Der Code ACRONYM ist variabel einsetzbar und von der Genauigkeit und Stabilit{\"a}t her State-of-the-Art und somit konkurrenzf{\"a}hig zu den sonstigen in der Astrophysik eingesetzten Codes anderer Gruppen. Die Energie bleibt bis auf einen Fehler von < 0.03\% erhalten, die Divergenz des Magnetfeldes bleibt immer unter einem Wert von 10^{-12} und die Skalierung wurde mittlerweile bis zu einem Clustergr{\"o}ße von einigen 10000 CPUs getestet. In dieser Arbeit wurde dann, nach der Entwicklung des Codes, der Einfluss des fundamentalen Massenverh{\"a}ltnisses m_p/m_e auf die Teilchenbeschleunigung durch Plasmainstabilit{\"a}ten untersucht. Dies ist relevant und wichtig, da in PiC-Simulationen in den allermeisten F{\"a}llen nicht mit dem realen Massenverh{\"a}ltnis gerechnet wird, da sonst viel zu viel Rechenleistung ben{\"o}tigt w{\"u}rde, um zu sehen, was mit den Protonen geschieht und was ihr Einfluss auf die leichten Teilchen wie Elektronen und Positronen ist. Zu diesem Zweck wurden Simulationen mit Massenverh{\"a}ltnissen zwischen m_p/m_e = 1.0 und 200.0 durchgef{\"u}hrt. Diese haben alle gemeinsam, dass periodische Randbedingungen verwendet wurden und das zur Verf{\"u}gung stehende Simulationsgebiet mit jeweils zwei gegeneinander str{\"o}menden Plasmapopulationen vollst{\"a}ndig gef{\"u}llt wurde, um jegliche Art von auftretenden Schocks auszuschließen. Die Rohdaten der einzelnen Simulationen wurden auf vielf{\"a}ltige Art und Weise analysiert, es wurden z.B. Schnitte durch die Teilchenverteilung erstellt, sowie ein- oder zweidimensionale Histogramme und Energieverl{\"a}ufe betrachtet. Dabei haben sich folgende Kernpunkte ergeben: F{\"u}r Massenverh{\"a}ltnisse bis etwa m_p/m_e = 20 bildet sich die gesamte Zweistrom-Instabilit{\"a}t in nur einer Phase aus, das heißt, es bilden sich von ringf{\"o}rmigen Magnetfeldern umgebene Flussschl{\"a}uche aus, die dann verschmelzen, bis nur noch zwei {\"u}brig sind und alle Teilchen werden {\"u}ber den gesamten Verlauf der Instabilit{\"a}t beschleunigt. Es ist damit zu folgern, dass die unterschiedlich schweren Teilchenspezies Protonen und Elektronen/Positronen durch die relativ nahe beieinander liegenden Massen noch so stark gekoppelt sind, dass sich nur eine Instabilit{\"a}t entwickeln kann. Bei großen Massenverh{\"a}ltnissen (m_p/m_e > 20) ist eine deutliche Trennung in zwei Phasen der Instabilit{\"a}t zu erkennen. Zuerst bilden sich wiederum Flussschl{\"a}uche aus, diese verschmelzen miteinander (zu zweien oder mehr), bevor der erste Teil der Instabilit{\"a}t abflaut. Anschließend entstehen wieder ringf{\"o}rmige Magnetfelder und Flussschl{\"a}uche, von denen einer meist deutlich st{\"a}rker ist als all die anderen, das bedeutet, dass dieser von st{\"a}rkeren Magnetfeldern umgeben ist und eine h{\"o}here Teilchendichte aufweist. Im Rahmen dieser zweigeteilten Instabilit{\"a}t werden die Elektronen und Positronen nur in der ersten Phase signifikant beschleunigt, die deutlich schwereren Protonen gewinnen {\"u}ber den gesamten Zeitraum Energie. Die h{\"o}chstenergetischen Teilchen erreichen im Ruhesystem der jeweiligen Plasmapopulation Werte um gamma = 250. Man kann daraus f{\"u}r zuk{\"u}nftige Untersuchungen mit Hilfe von Particle-in-Cell Codes den Schluss ziehen, dass R{\"u}ckschl{\"u}sse auf das tats{\"a}chliche Verhalten beim realen Massenverh{\"a}ltnis von m_p/m_e = 1836.2 nur aus den Simulationen mit m_p/m_e >> 20 gezogen werden k{\"o}nnen, da die starke Kopplung der leichten und schweren Teilchen bei kleineren Massenverh{\"a}ltnissen die Ergebnisse sehr stark beeinflusst. Es wurde anhand der gemessenen Zeitpunkte der Instabilit{\"a}tsmaxima eine Extrapolation durchgef{\"u}hrt, die zeigt, dass die Instabilit{\"a}t beim realen Massenverh{\"a}ltnis etwa bei t = 1400 omega_{pe}^{-1} auftreten w{\"u}rde. Um dies wirklich zu simulieren m{\"u}sste allerdings mehr als die 1000-fache Anzahl an CPU-Stunden aufgewandt werden. Des weiteren wurde eine Maxwell-J{\"u}ttner-Verteilung an die Teilchenverteilungen der einzelnen Simulationen auf dem H{\"o}hepunkt der Instabilit{\"a}t gefittet, um sowohl die neue Temperatur des Plasmas als auch die Beschleunigungseffizienz des Prozesses zu berechnen. Die Temperatur erh{\"o}ht sich demnach durch die Instabilit{\"a}t von etwa 10^8K auf 10^{10} bis 10^{11}K, der Anteil suprathermischer Teilchen betr{\"a}gt 2 bis 4\%.},
  subject      = {Astrophysik},
  language  = {de}
}
@phdthesis{Rueger2011,
  author    = {R{\"u}ger, Michael},
  title     = {Ein zeitabh{\"a}ngiges, selbstkonsistentes hadronisch-leptonisches Strahlungsmodell zur Modellierung der Multiwellenl{\"a}ngenemission von Blazaren},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56955},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Diese Arbeit besch{\"a}ftigt sich mit Strahlungsprozessen in Blazaren. Bei den Blazaren handelt es sich um eine Unterkategorie der aktiven Galaxienkerne, bei denen die Jetachse in Richtung des Beobachters zeigt. Charakteristisch f{\"u}r die Blazare ist ein Multifrequenzspektrum der Photonen, welches sich vom Radiobereich bis hin zur Gamma-Strahlung mit TeV-Energien erstreckt. Insbesondere der Gamma-Bereich r{\"u}ckt aktuell in den Fokus der Betrachtung mit Experimenten wie zum Beispiel FERMI und MAGIC. Ziel dieser Arbeit ist die Modellierung der auftretenden Strahlungsprozesse und die Beschreibung der Multifrequenzspektren der Blazare mit Hilfe eines hadronisch-leptonischen Modells. Grundlage hierf{\"u}r ist ein selbstkonsistentes Synchrotron-Selbst-Compton-Modell (SSC), welches zur Beschreibung des Spektrums der Quelle 1 ES 1218+30.4 verwendet wird. Dabei wird die Parameterwahl unterst{\"u}tzt durch eine Absch{\"a}tzung der Masse des zentralen schwarzen Loches. Das hier behandelte SSC-Modell wird dahingehend untersucht, wie es sich unter Ver{\"a}nderung der Modellparameter verh{\"a}lt. Dabei werden Abh{\"a}ngigkeiten des Photonenspektrums von {\"A}nderungsfaktoren der Parameter abgeleitet. Außerdem werden diese Abh{\"a}ngigkeiten in Relation gesetzt und aus dieser Betrachtung ergibt sich die Schlussfolgerung, dass unter der Voraussetzung eines festen Spektralindex der Elektronenverteilung die Wahl eines Parametersatzes zur Modellierung eines Photonenspektrums eindeutig ist. Zur Einf{\"u}hrung eines zeitabh{\"a}ngigen, hadronischen Modells wird das SSCModell um die Anwesenheit nichtthermischer Protonen erweitert. Dadurch kann Proton-Synchrotron-Strahlung einen Beitrag im Gamma-Bereich leisten. Außerdem werden durch Proton-Photon-Wechselwirkung Pionen erzeugt. Aus deren Zerfall werden zusammen mit der Paarbildung aus Photon-Photon-Absorption sekund{\"a}re Elektronen und Positronen produziert, die wiederum zum Hochenergiespektrum beitragen. Neben den Pionen werden bei der Proton-Photon- Wechselwirkung außerdem noch Neutrinos und Neutronen erzeugt, die einen direkten Einblick in die Emissionsregion erlauben. Das hier vorgestellte hadronische Modell wird auf die Quelle 3C 279 angewandt. F{\"u}r diese Quelle reicht mit der Detektion im VHE-Bereich der SSCAnsatz nicht aus, um das Photonenspektrum zu beschreiben. Mit dem vorgelegten Modell gelingt die Beschreibung des Spektrums in den SSC-kritischen Bereichen sehr gut. Insbesondere k{\"o}nnen verschiedene Flusszust{\"a}nde modelliert und allein durch Ver{\"a}nderung der Maximalenergien von Protonen und Elektronen ineinander {\"u}berf{\"u}hrt werden. Diese einfache M{\"o}glichkeit der Modellierung der Variabilit{\"a}t der Quelle unterstreicht die Wahl des hadronischen Ansatzes. Somit wird hier ein sehr gutes Werkzeug zur Untersuchung der Emissionsprozesse in Blazaren geliefert. Dar{\"u}ber hinaus ist mit der Absch{\"a}tzung des Neutrino-Flusses zwar die Detektion von 3C 279 als Punktquelle mit IceCube unwahrscheinlich, jedoch liefert das Modell generell die M{\"o}glichkeit im Kontext des Multimessenger-Ansatzes Antworten zu liefern. Im gleichen Kontext wird auch der Beitrag zur kosmischen Strahlung durch entweichende Neutronen untersucht.},
  subject      = {Blazar},
  language  = {de}
}
@phdthesis{Lang2010,
  author    = {Lang, Thomas C.},
  title     = {Quantum Monte Carlo methods and strongly correlated electrons on honeycomb structures},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-53506},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {In this thesis we apply recently developed, as well as sophisticated quantum Monte Carlo methods to numerically investigate models of strongly correlated electron systems on honeycomb structures. The latter are of particular interest owing to their unique properties when simulating electrons on them, like the relativistic dispersion, strong quantum fluctuations and their resistance against instabilities. This work covers several projects including the advancement of the weak-coupling continuous time quantum Monte Carlo and its application to zero temperature and phonons, quantum phase transitions of valence bond solids in spin-1/2 Heisenberg systems using projector quantum Monte Carlo in the valence bond basis, and the magnetic field induced transition to a canted antiferromagnet of the Hubbard model on the honeycomb lattice. The emphasis lies on two projects investigating the phase diagram of the SU(2) and the SU(N)-symmetric Hubbard model on the hexagonal lattice. At sufficiently low temperatures, condensed-matter systems tend to develop order. An exception are quantum spin-liquids, where fluctuations prevent a transition to an ordered state down to the lowest temperatures. Previously elusive in experimentally relevant microscopic two-dimensional models, we show by means of large-scale quantum Monte Carlo simulations of the SU(2) Hubbard model on the honeycomb lattice, that a quantum spin-liquid emerges between the state described by massless Dirac fermions and an antiferromagnetically ordered Mott insulator. This unexpected quantum-disordered state is found to be a short-range resonating valence bond liquid, akin to the one proposed for high temperature superconductors. Inspired by the rich phase diagrams of SU(N) models we study the SU(N)-symmetric Hubbard Heisenberg quantum antiferromagnet on the honeycomb lattice to investigate the reliability of 1/N corrections to large-N results by means of numerically exact QMC simulations. We study the melting of phases as correlations increase with decreasing N and determine whether the quantum spin liquid found in the SU(2) Hubbard model at intermediate coupling is a specific feature, or also exists in the unconstrained t-J model and higher symmetries.},
  subject      = {Monte-Carlo-Simulation},
  language  = {en}
}
@phdthesis{HoehneMoench2010,
  author    = {H{\"o}hne-M{\"o}nch, Daniel},
  title     = {Steady-state emission of blazars at very high energies},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-53700},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {One key scientific program of the MAGIC telescope project is the discovery and detection of blazars. They constitute the most prominent extragalactic source class in the very high energy (VHE) Gamma-ray regime with 29 out of 34 known objects (as of April 2010). Therefore a major part of the available observation time was spent in the last years on high-frequency peaked blazars. The selection criteria were chosen to increase the detection probability. As the X-ray flux is believed to be correlated to the VHE Gamma-ray flux, only X-ray selected sources with a flux F(X) > 2 μJy at 1 keV were considered. To avoid strong attenuation of the Gamma-rays in the extragalactic infrared background, the redshift was restricted to values between z < 0.15 and z < 0.4, depending on the declination of the objects. The latter determines the zenith distance during culmination which should not exceed 30° (for z < 0.4) and 45° (for z < 0.15), respectively. Between August 2005 and April 2009, a sample of 24 X-ray selected high-frequency peaked blazars has been observed with the MAGIC telescope. Three of them were detected including 1ES 1218+304 being the first high-frequency peaked BL Lacertae object (HBL) to be discovered with MAGIC in VHE Gamma-rays. One previously detected object was not confirmed as VHE emitter in this campaign by MAGIC. A set of 20 blazars previously not detected will be treated more closely in this work. In this campaign, during almost four years ~ 450 hrs or ~ 22\% of the available observation time for extragalactic objects were dedicated to investigate the baseline emission of blazars and their broadband spectral properties in this emission state. For the sample of 20 objects in a redshift range of 0.018 < z < 0.361 integral flux upper limits in the VHE range on the 99.7\% confidence level (corresponding to 3 standard deviations) were calculated resulting in values between 2.9\% and 14.7\% of the integral flux of the Crab Nebula. As the distribution of significances of the individual objects shows a clear shift to positive values, a stacking method was applied to the sample. For the whole set of 20 objects, an excess of Gamma-rays was found with a significance of 4.5 standard deviations in 349.5 hours of effective exposure time. For the first time a signal stacking in the VHE regime turned out to be successful. The measured integral flux from the cumulative signal corresponds to 1.4\% of the Crab Nebula flux above 150 GeV with a spectral index α = -3.15±0.57. None of the objects showed any significant variability during the observation time and therefore the detected signal can be interpreted as the baseline emission of these objects. For the individual objects lower limits on the broad-band spectral indices αX-Gamma between the X-ray range at 1 keV and the VHE Gamma-ray regime at 200 GeV were calculated. The majority of objects show a spectral behaviour as expected from the source class of HBLs: The energy output in the VHE regime is in general lower than in X-rays. For the stacked blazar sample the broad-band spectral index was calculated to αX-Gamma = 1.09, confirming the result found for the individual objects. Another evidence for the revelation of the baseline emission is the broad-band spectral energy distribution (SED) comprising archival as well as contemporaneous multi-wavelength data from the radio to the VHE band. The SEDs of known VHE Gamma-ray sources in low flux states matches well the SED of the stacked blazar sample.},
  subject      = {MAGIC-Teleskop},
  language  = {en}
}
@phdthesis{Staub2010,
  author    = {Staub, Florian},
  title     = {Considerations on supersymmetric Dark Matter beyond the MSSM},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-55343},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {The standard model (SM) of particle physics is for the last three decades a very successful description of the properties and interactions of all known elementary particles. Currently, it is again probed with the first collisions at the Large Hadron Collider (LHC). It is widely expected that new physics will be detected at the LHC and the SM has to be extended. The most exhaustive analyzed extension of the SM is supersymmetry (SUSY). SUSY can not only solve intrinsic problems of the SM like the hierarchy problem, but it also postulates new particles which might explain the nature of dark matter in the universe. The majority of all studies about dark matter in the framework of SUSY has focused on the minimal supersymmetric standard model (MSSM). The aim of this work is to consider scenarios beyond that scope. We consider two models which explain not only dark matter but also neutrino masses: the gravitino as dark matter in gauge mediated SUSY breaking (GMSB) with bilinear broken \$R\$-parity as well as different seesaw scenarios with the neutralino as dark matter candidate. Furthermore, we also study the next-to-minimal supersymmetric standard model (NMSSM) which solves the \(\mu\)-problem of the MSSM and discuss the properties of the neutralino as dark matter candidate. In case of \$R\$-parity violation, light gravitinos are often the only remaining candidate for dark matter in SUSY because of their very long life time. We reconsider the cosmological gravitino problem arising for this kind of models. It will be shown that the proposed solution for the overclosure of the universe by light gravitinos, namely the entropy production by decays of GMSB messenger, just works in a small subset of models and in fine-tuned regions of the parameter space. This is a consequence of two effects so far overlooked: the enhanced decay channels in massive vector bosons and the impact of charged messenger particles. Both aspects cause an interplay between different cosmological restrictions which lead to strong constraints on the parameters of GMSB models. Afterwards, a minimal supergravity (mSugra) scenario with additional chiral superfields at high energy scales is considered. These fields are arranged in complete \$SU(5)\$ multiplets in order to maintain gauge unification. The new fields generate a dimension 5 operator to explain neutrino data. Furthermore, they cause large differences in mass spectrum of MSSM fields because of the different evaluation of the renormalization group equations what changes also the properties of the lightest neutralino as dark matter candidate. We discuss the parameter space of all three possible seesaw scenarios with respect to dark matter and the impact on rare lepton flavor violating processes. As we will see, especially in seesaw type~III but also in type~II the mass spectrum and regions of parameter space consistent with dark matter differ significantly in comparison to a common mSugra scenario. Moreover, the experimental bounds, in particular of branching ratios like \(l_i \rightarrow l_j \gamma\), cause large constraints on the seesaw parameters.},
  subject      = {Supersymmetrie},
  language  = {en}
}
@phdthesis{Martin2010,
  author    = {Martin, Lee C.},
  title     = {The Kondo Lattice Model: a Dynamical Cluster Approximation Approach},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-49446},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {We apply an antiferromagnetic symmetry breaking implementation of the dynamical cluster approximation (DCA) to investigate the two-dimensional hole-doped Kondo lattice model (KLM) with hopping \$t\$ and coupling \$J\$. The DCA is an approximation at the level of the self-energy. Short range correlations on a small cluster, which is self-consistently embedded in the remaining bath electrons of the system, are handled exactly whereas longer ranged spacial correlations are incorporated on a mean-field level. The dynamics of the system, however, are retained in full. The strong temporal nature of correlations in the KLM make the model particularly suitable to investigation with the DCA. Our precise DCA calculations of single particle spectral functions compare well with exact lattice QMC results at the particle-hole symmetric point. However, our DCA version, combined with a QMC cluster solver, also allows simulations away from particle-hole symmetry and has enabled us to map out the magnetic phase diagram of the model as a function of doping and coupling \$J/t\$. At half-filling, our results show that the linear behaviour of the quasi-particle gap at small values of \$J/t\$ is a direct consequence of particle-hole symmetry, which leads to nesting of the Fermi surface. Breaking the symmetry, by inclusion of a diagonal hopping term, results in a greatly reduced gap which appears to follow a Kondo scale. Upon doping, the magnetic phase observed at half-filling survives and ultimately gives way to a paramagnetic phase. Across this magnetic order-disorder transition, we track the topology of the Fermi surface. The phase diagram is composed of three distinct regions: Paramagnetic with {\it large} Fermi surface, in which the magnetic moments are included in the Luttinger sum rule, lightly antiferromagnetic with large Fermi surface topology, and strongly antiferromagnetic with {\it small} Fermi surface, where the magnetic moments drop out of the Luttinger volume. We draw on a mean-field Hamiltonian with order parameters for both magnetisation and Kondo screening as a tool for interpretation of our DCA results. Initial results for fixed coupling and doping but varying temperature are also presented, where the aim is look for signals of the energy scales in the system: the Kondo temperature \$T_{K}\$ for initial Kondo screening of the magnetic moments, the Neel temperature \$T_{N}\$ for antiferromagnetic ordering, a possible \$T^{*}\$ at which a reordering of the Fermi surface is observed, and finally, the formation of the coherent heavy fermion state at \$T_{coh}\$.},
  subject      = {Gittermodell},
  language  = {en}
}
@phdthesis{Tang2011,
  author    = {Tang, Jian},
  title     = {Phenomenology of Neutrino Oscillations at the Neutrino Factory},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-66765},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {We consider the prospects for a neutrino factory measuring mixing angles, the CP violating phase and mass-squared differences by detecting wrong-charge muons arising from the chain \$\mu^+\to\nu_e\to\nu_\mu\to\mu^-\$ and the right-charge muons coming from the chain \$\mu^+\to\bar{\nu}_\mu\to\bar{\nu}_\mu\to\mu^+\$ (similar to \$\mu^-\$ chains), where \$\nu_e\to\nu_\mu\$ and \$\bar{\nu}_\mu\to\bar{\nu}_\mu\$ are neutrino oscillation channels through a long baseline. First, we study physics with near detectors and consider the treatment of systematic errors including cross section errors, flux errors, and background uncertainties. We illustrate for which measurements near detectors are required, discuss how many are needed, and what the role of the flux monitoring is. We demonstrate that near detectors are mandatory for the leading atmospheric parameter measurements if the neutrino factory has only one baseline, whereas systematic errors partially cancel if the neutrino factory complex includes the magic baseline. Second, we perform the baseline and energy optimization of the neutrino factory including the latest simulation results from the magnetized iron neutrino detector (MIND). We also consider the impact of \$\tau\$ decays, generated by appearance channels \$\nu_\mu \rightarrow \nu_\tau\$ and \$\nu_e \rightarrow \nu_\tau\$, on the discovery reaches of the mass orderings, the leptonic CP violation, and the non-zero \$\theta_{13}\$, which we find to be negligible for the considered detector. Third, we make a comparison of a high energy neutrino factory to a low energy neutrino factory and find that they are just two versions of the same experiment optimized for different regions of the parameter space. In addition, we briefly comment on whether it is useful to build the bi-magic baseline at the low energy neutrino factory. Finally, the effects of one additional massive sterile neutrino are discussed in the context of a combined short and long baseline setup. It is found that near detectors can provide the required sensitivity at the LSND-motivated \$\Delta m_{41}^2\$-range, while some sensitivity can also be obtained in the region of the atmospheric mass splitting introduced by the sterile neutrino from the long baselines.},
  subject      = {Neutrinooszillation},
  language  = {en}
}
@phdthesis{Simon2011,
  author    = {Simon, Dennis},
  title     = {Aspects in the fate of primordial vacuum bubbles},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-67019},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {At the present day the idea of cosmological inflation constitutes an important extension of Big Bang theory. Since its appearance in the early 1980's many physical mechanisms have been worked out that put the inflationary expansion of space that proceeds the Hot Big Bang on a sound theoretical basis. Among the achievements of the theory of inflation are the explanaition of the almost Euclidean geometry of 'visible'space, the homogeneity of the cosmic background radiation but, in particular, also the tiny inhomogeneity of a relative amplitude of 10-5. In many models of inflation the inflationary phase ends only locally. Hence, there exists the possibility that the inflationary process still goes on in regions beyond our visual horizon. This property is commonly termed 'eternal inflation'. In the framework of a cosmological scalar fields, eternal inflation can manifest itself in a variety of ways. On the one hand fluctuations of the field, if sufficiently large, can work against the classical trajectory and therefore counteract the end of inflation. In regions where this is the case the accelerated expansion of space continues at a higher rate. In parts of this region the process may replicate itself again and in this way may continue throughout all of time. Space and field are said to reproduce themselves. On the other hand, a mechanism that can occur in addition or independent of the latter, is so called vacuum tunneling. If the potential of the scalar field has several local minima, a semi-classical calculation suggests that within a spherical region, a bubble, the field can tunnel to another state. The respective tunneling rates depend on the potential difference and the shape of the potential between the states. Generally, the tunneling rate is exponentially suppressed, which means that the inflation lasts for a long time before tunneling takes place. The ongoing inflationary process effectively reduces local curvature, anistotropy and inhomogeneity, so that this property is known as the 'cosmic no-hair conjecture'. For this reason cosmological considerations of the evolution of bubbles thus far almost entirely involved vacuum (de Sitter) backgrounds. However, new insights in the framework of string theory suggest high tunneling rates which allow for the possibility of bubble nucleation in non-vacuum dominated backgrounds. In this case the evolution of the bubble depends on the properties of the background spacetime. A deeper introduction in chapter 4 is followed by the presentation of the Lema{\^i}tre-Tolman spacetime in chapter 5 which constitutes the background spacetime in the study of the effect of matter and inhomogeneity on the evolution of vacuum bubbles. In chapter 6 we explicitly describe the application of the 'thin-shell' formalism and the resulting system of equations. This is succeeded in chapter 7 by the detailed analysis of bubble evolution in various limits of the Lema{\^i}tre-Tolman spacetime and a Robertson-Walker spacetime with a rapid phase transition. The central observations are that the presence of dust, at a fixed surface energy density, goes along with a smaller nucleation volume and possibly leads to a a collapse of the bubble. In an expanding background, the radially inhomogeneous dust profile is efficiently diluted so that there is essentially no effect on the evolution of the domain wall. This changes in a radially inhomogeneous curvature profile, positive curvature decelerates the expansion of the bubble. Moreover, we point out that the adopted approach does not allow for a treatment of a, physically expected, matter transfer so that the results are to be understood as preliminary under this caveat. In the second part of this thesis we consider potential observable consequences of bubble collisions in the cosmic microwave background radiation. The topological nature of the signal suggests the use of statistics that are well suited to quantify the morphological properties of the temperature fluctuations. In chapter 10 we present Minkowski Functionals (MFs) that exactly provide such statistics. The presented error analysis allows for a higher precision of numerical MFs in comparison to earlier methods. In chapter 12 we present the application of our algorithm to a Gaussian and a collision map. We motivate the expected MFs and extract their numerical counterparts. We find that our least-squares fitting procedure accurately reproduces an underlying signal only when a large number of realizations of maps are averaged over, while for a single WMAP and PLANCK resolution map, only when a highly prominent disk, with |δT| = 2√σG and ϑd = 40◦, we are able to recover the result. This is unfortunate, as it means that MF are intrinsically too noisy to be able to distinguish cold and hot spots in the CMB for small sizes.},
  subject      = {Kosmologie},
  language  = {en}
}
@phdthesis{Liebler2011,
  author    = {Liebler, Stefan},
  title     = {LHC phenomenology and higher order electroweak corrections in supersymmetric models with and without R-parity},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-69367},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {During the last decades the standard model of particle physics has evolved to one of the most precise theories in physics, describing the properties and interactions of fundamental particles in various experiments with a high accuracy. However it lacks on some shortcomings from experimental as well as from theoretical point of view: There is no approved mechanism for the generation of masses of the fundamental particles, in particular also not for the light, but massive neutrinos. In addition the standard model does not provide an explanation for the observance of dark matter in the universe. Moreover the gauge couplings of the three forces in the standard model do not unify, implying that a fundamental theory combining all forces can not be formulated. Within this thesis we address supersymmetric models as answers to these various questions, but instead of focusing on the most simple supersymmetrization of the standard model, we consider basic extensions, namely the next-to-minimal supersymmetric standard model (NMSSM), which contains an additional singlet field, and R-parity violating models. R-parity is a discrete symmetry introduced to guarantee the stability of the proton. Using lepton number violating terms in the context of bilinear R-parity violation and the munuSSM we are able to explain neutrino physics intrinsically supersymmetric, since those terms induce a mixing between the neutralinos and the neutrinos. Since 2009 the Large Hadron Collider (LHC) at CERN explores the new energy regime of Tera-electronvolt, allowing the production of potentially existing heavy particles by the collision of protons. Thus the near future might provide answers to the open questions of mass generation in the standard model and show hints towards physics beyond the standard model. Therefore this thesis works out the phenomenology of the supersymmetric models under consideration and tries to point out differences to the well-known features of the simplest supersymmetric realization of the standard model. In case of the R-parity violating models the decays of the light neutralinos can result in displaced vertices. In combination with a light singlet state these displaced vertices might offer a rich phenomenology like non-standard Higgs decays into a pair of singlinos decaying with displaced vertices. Within this thesis we present some calculations at next order of perturbation theory, since one-loop corrections provide possibly large contributions to the tree-level masses and decay widths. We are using an on-shell renormalization scheme to calculate the masses of neutralinos and charginos including the neutrinos and leptons in case of the R-parity violating models at one-loop level. The discussion shows the similarities and differences to existing calculations in another renormalization scheme, namely the DRbar scheme. Moreover we consider two-body decays of the form chi_j^0 -> chi_l^\pm W^\mp involving a heavy gauge boson in the final state at one-loop level. Corrections are found to be large in case of small or vanishing tree-level decay widths and also for the R-parity violating decay of the lightest neutralino chi_1^0 -> l^\pm W^\mp. An interesting feature of the models based on bilinear R-parity violation is the correlation between the branching ratios of the lightest neutralino decays and the neutrino mixing angles. We discuss these relations at tree-level and for two-body decays chi_1^0 -> l^\pm W^\mp also at one-loop level, since only the full one-loop corrections result in the tree-level expected behavior. The appendix describes the two programs MaCoR and CNNDecays being developed for the analysis carried out in this thesis. MaCoR allows for the calculation of mass matrices and couplings in the models under consideration and CNNDecays is used for the one-loop calculations of neutralino and chargino mass matrices and the two-body decay widths.},
  subject      = {Supersymmetrie},
  language  = {en}
}
@phdthesis{Elsaesser2011,
  author    = {Els{\"a}sser, Dominik Martin},
  title     = {Indirect Search for Dark Matter in the Universe - the Multiwavelength and Multiobject Approach},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-69464},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Dunkle Materie ist ein zentraler Bestandteil der modernen Kosmologie, und damit von entscheidender Bedeutung f{\"u}r unser Verst{\"a}ndnis der Strukturbildung im Universum. Das offensichtliche Fehlen von elektromagnetischer Wechselwirkung in Kombination mit unabh{\"a}ngigen Messungen der Energiedichte der baryonischen Materie {\"u}ber die H{\"a}ufigkeit der primordialen leichten Elemente weisen auf eine nicht-baryonische Natur der Dunklen Materie hin. Die Wirkung der Dunklen Materie bei der Strukturbildung zeigt weiterhin dass ihre Konstituenten kalt sind, also zum Zeitpunkt des Gleichgewichts zwischen Strahlung und Materie eine Temperatur kleine als ihre Ruhemasse aufwiesen. Generische Kandidaten f{\"u}r das Dunkelmaterie-Teilchen sind stabile, schwach wechselwirkende Teilchen mit Ruhemassen von der Gr{\"o}ßenordnung der Skala der elektroschwachen Symmetriebrechung, wie sie zum Beispiel in der Supersymmetrie bei erhaltener R-Parit{\"a}t vorkommen. Derartige Teilchen frieren auf nat{\"u}rliche Weise im fr{\"u}hen Universum mit kosmologisch relevanten Reliktdichten aus. Die fortschreitende Strukturbildung im Universum f{\"u}hrt dann zur Bildung von {\"u}berdichten Regionen, in denen die Dunkelmaterie-Teilchen wiederum in signifikantem Ausmaß annihilieren k{\"o}nnen. Dadurch w{\"u}rde ein potentiell detektierbarer Fluß von Hochenergie-Teilchen einschließlich Photonen aus den instabilen Zwischenprodukten der Annihilationsereignisse erzeugt. Die Spektren dieser Teilchen w{\"u}rden R{\"u}ckschl{\"u}sse auf die Masse und den Annihilations-Querschnitt als wichtige Gr{\"o}ßen zur mikrophysikalischen Identifikation der Dunkelmaterie-Teilchen erlauben. Darin liegt die zentrale Motivation f{\"u}r indirekte Suchen nach der Dunklen Materie. Zum gegenw{\"a}rtigen Zeitpunkt jedoch haben weder diese indirekten Suchen, noch direkte Methoden zur Suche nach elastischen Streuereignissen zwischen Dunkelmaterie-Teilchen und Atomkernen sowie Beschleunigerexperimente einen eindeutigen Nachweis von Dunkelmaterie-Teilchen erbracht. Das an sich stellt keine {\"U}berraschung dar, denn die zu erwartenden Signale sind aufgrund der schwachen Wechselwirkung der Teilchen nur von geringer Intensit{\"a}t. Im Falle der indirekten Suchen steht zu erwarten, dass selbst f{\"u}r die gr{\"o}ßten Massekonzentrationen im Universum die St{\"a}rke des Annihilationssignals der Dunklen Materie den durch astrophysikalische Quellen verursachten Untergrund nicht {\"u}berschreitet. Die M{\"o}glichkeit der sicheren Unterscheidung zwischen einem m{\"o}glichen Signal aus der Annihilation der Dunklen Materie und eben diesem Untergrund ist daher entscheidend f{\"u}r die Erfolgsaussichten der indirekten Suchen. In der vorliegenden Arbeit wird eine neuartige Suchstrategie ausgearbeitet und vorgestellt, deren zentrale Komponente die Auswahl von Beobachtungszielen aus einem breiten Massebereich, die Kontrolle der astrophysikalischen Untergr{\"u}nde, und die Einbeziehung von Daten aus mehreren Wellenl{\"a}ngenbereichen ist. Die durchgef{\"u}hrten Beobachtungen werden vorgestellt und interpretiert. Ein Ergebnis ist, dass die Unsicherheiten in Bezug auf die Verteilung der Dunklen Materie in Halos und deren individuelle Dichtestruktur, sowie in Bezug auf die m{\"o}gliche Verst{\"a}rkung des Annihilationssignales durch Substruktur, im Falle der massearmen Halos (wie zum Beispiel bei den Zwerggalaxien) gr{\"o}ßer ist als bei massereichen Halos, wie denen der Galaxienhaufen. Andererseits weisen die massereichen Halos gr{\"o}ßere Unsicherheiten in Hinblick auf die zu erwartenden rein astrophysikalischen Untergr{\"u}nde auf. Die Unsicherheiten in Bezug auf die bisher unbekannte Teilchenphysik jenseits des Standardmodells schließlich sind unabh{\"a}ngig von der Masse der beobachteten Halos. Im Zusammenspiel erm{\"o}glichen es diese unterschiedlichen Skalierungsverhalten, die globale Unsicherheit durch eine kombinierte Analyse der Beobachtungen von Halos mit verschiedenen Massen, die einen bedeutenden Teil der Masseskala abdecken, nennenswert zu reduzieren. Diese Strategie wurde im Rahmen des wissenschaftlichen Beobachtungsprogrammes des MAGIC Teleskopsystems implementiert. Es wurden Beobachtungen von Zwerggalaxien sowie des Virgo- und des Perseus-Galaxienhaufens durchgef{\"u}hrt. Die resultierenden Grenzen auf Gammastrahlung aus der Annihilation von schwach wechselwirkenden, massereichen Teilchen geh{\"o}ren zum Zeitpunkt dieser Niederschrift zu den st{\"a}rksten Grenzen aus indirekten Suchen nach der Dunklen Materie. Die so gewonnenen Grenzen auf die Annihilations-Fl{\"u}sse schr{\"a}nken einige in der Literatur diskutierte und durch aussergew{\"o}hnlich große Annihilations-Fl{\"u}sse gekennzeichnete Szenarien stark ein.},
  subject      = {Gammastrahlung},
  language  = {en}
}
@article{PinkertSchultzReichardt2010,
  author    = {Pinkert, Stefan and Schultz, Joerg and Reichardt, Joerg},
  title     = {Protein Interaction Networks-More Than Mere Modules},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-68426},
  year      = {2010},
  abstract  = {It is widely believed that the modular organization of cellular function is reflected in a modular structure of molecular networks. A common view is that a ''module'' in a network is a cohesively linked group of nodes, densely connected internally and sparsely interacting with the rest of the network. Many algorithms try to identify functional modules in protein-interaction networks (PIN) by searching for such cohesive groups of proteins. Here, we present an alternative approach independent of any prior definition of what actually constitutes a ''module''. In a self-consistent manner, proteins are grouped into ''functional roles'' if they interact in similar ways with other proteins according to their functional roles. Such grouping may well result in cohesive modules again, but only if the network structure actually supports this. We applied our method to the PIN from the Human Protein Reference Database (HPRD) and found that a representation of the network in terms of cohesive modules, at least on a global scale, does not optimally represent the network's structure because it focuses on finding independent groups of proteins. In contrast, a decomposition into functional roles is able to depict the structure much better as it also takes into account the interdependencies between roles and even allows groupings based on the absence of interactions between proteins in the same functional role. This, for example, is the case for transmembrane proteins, which could never be recognized as a cohesive group of nodes in a PIN. When mapping experimental methods onto the groups, we identified profound differences in the coverage suggesting that our method is able to capture experimental bias in the data, too. For example yeast-two-hybrid data were highly overrepresented in one particular group. Thus, there is more structure in protein-interaction networks than cohesive modules alone and we believe this finding can significantly improve automated function prediction algorithms.},
  subject      = {Netzwerk},
  language  = {en}
}
@phdthesis{Laubach2014,
  author    = {Laubach, Manuel},
  title     = {Nichtmagnetische Isolatoren in Hexagonalen Gittermodellen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-106987},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {Wir untersuchen zunächst das Hubbard-Modell des anisotropen Dreiecksgitters als effektive Beschreibung der Mott-Phase in verschiedenen organischen Verbindungen mit dreieckiger Gitterstruktur. Um die Eigenschaften am absoluten Nullpunkt zu bestimmen benutzen wir die variationelle Cluster Näherung (engl. variational cluster approximation VCA) und erhalten das Phasendiagramm als Funktion der Anisotropie und der Wechselwirkungsstärke. Wir finden f{\"u}r schwache Wechselwirkung ein Metall. F{\"u}r starke Wechselwirkung finden wir je nach Stärke der Anisotropie eine Néel oder eine 120◦-Néel antiferromagnetische Ordnung. In einem Bereich mittlerer Wechselwirkung entsteht in der Nähe des isotropen Dreiecksgitters ein nichtmagnetischer Isolator. Der Metall-Isolator-Übergang hängt maßgeblich von der Anisotropie ab, genauso wie die Art der magnetischen Ordnung und das Erscheinen und die Ausdehnung der nichtmagnetischen Isolatorphase. Spin-Bahn Kopplung ist der ausschlaggebende Parameter, der elektronische Bandmodelle in topologische Isolatoren wandelt. Spin-Bahn Kopplung im Allgemeinen beinhaltet auch den Rashba Term, der die SU(2) Symmetrie vollständig bricht. Sobald man auch Wechselwirkungen ber{\"u}cksichtigt, m{\"u}ssen sich viele theoretische Methoden auf die Analyse vereinfachter Modelle beschränken, die nur Spin-Bahn Kopplungen enthalten, welche die U(1) Symmetrie erhalten und damit eine Rashba Kopplung ausschließen. Wir versuchen diese bisher bestehende L{\"u}cke zu schließen und untersuchen das Kane-Mele Hubbard (KMH) Modell mit Rashba Spin-Bahn Kopplung und präsentieren eine systematische Analyse des Effekts der Rashba Spin-Bahn Kopplung in einem korrelierten zweidimensionalen topologischen Isolator. Wir wenden die VCA auf dieses Problem an und bestimmen das Phasendiagramm mit Wechselwirkung durch die Berechnung der lokalen Zustandsdichte, der Magnetisierung, der Einteilchenspektralfunktion und der Randzustände. Nach einer ausf{\"u}hrlichen Auswertung des KMH-Modells, bei erhaltener U(1) Symmetrie, finden wir auch f{\"u}r endliche Wechselwirkung, dass eine zusätzliche Rashba Kopplung zu neuen elektronischen Phasen f{\"u}hrt, wie eine metallische Phase und eine topologische Isolatorphase ohne Bandl{\"u}cke in der lokalen Zustandsdichte, die aber eine direkte Bandl{\"u}cke f{\"u}r jeden Wellenvektor besitzt. F{\"u}r eine Klasse von 5d Übergangsmetallen untersuchen wir ein KMH ähnliches Modell mit multidirektionaler Spin-Bahn Kopplung, das wegen seiner Relevanz f{\"u}r die Natrium-Iridate (engl. sodium iridate) als SI Modell bezeichnet wird. Diese intrinsische Kopplung bricht die SU(2) Symmetrie bereits vollständig und dennoch erhält man wegen der speziellen Form f{\"u}r starke Wechselwirkung wieder einen rotationssymmetrischen Néel-AFM Isolator. Der topologische Isolator des SIH-Modells ist adiabatisch mit dem des KMH-Modells verbunden, jedoch sind die Randströme hier nicht mehr spinpolarisiert. Wir verallgemeinern das Konzept der Klein-Transformation, das bereits erfolgreich auf Spin-Hamiltonians angewandt wurde, und wenden es auf ein Hubbard-Modell mit rein imaginären spinabhängigen H{\"u}pfen an, das im Grenzfall unendlicher Wechselwirkung in das Kitaev-Heisenberg Modell {\"u}bergeht. Dadurch erhält man ein Modell des Dreiecksgitters mit reellen spinunabhängigen H{\"u}pfen, das aber eine mehratomige Einheitszelle besitzt. F{\"u}r schwache Wechselwirkung ist das System ein Dirac Halbmetall und f{\"u}r starke Wechselwirkung erhält man eine 120◦-Néel antiferromagnetische Ordnung. F{\"u}r mittlere Wechselwirkung findet man aber einen relativ großen Bereich in dem eine nichtmagnetische Isolatorphase stabil ist. Unsere Ergebnisse deuten auf die mögliche Existenz einer Quanten Spinfl{\"u}ssigkeit hin.},
  subject      = {Hexagonaler Kristall},
  language  = {de}
}
@phdthesis{Janotta2014,
  author    = {Janotta, Peter},
  title     = {Nonlocality and entanglement in Generalized Probabilistic Theories and beyond},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-105612},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {Quantum theory is considered to be the most fundamental and most accurate physical theory of today. Although quantum theory is conceptually difficult to understand, its mathematical structure is quite simple. What determines this particularly simple and elegant mathematical structure? In short: Why is quantum theory as it is? Addressing such questions is the aim of investigating the foundations of quantum theory. In the past this field of research was sometimes considered as an academic subject without much practical impact. However, with the emergence of quantum information theory this perception has changed significantly and both fields started to fruitfully influence each other. Today fundamental aspects of quantum theory attract increasing attention and the field belongs to the most exciting subjects of theoretical physics. This thesis is concerned with a particular branch in this field, namely, with so-called Generalized Probabilistic Theories (GPTs), which provide a unified theoretical framework in which classical and quantum theory emerge as special cases. This is used to examine nonlocal features that help to distinguish quantum theory from alternative toy theories. In order to extend the scope of theories that can be examined with the framework, we also introduce several generalizations to the framework itself. We start in Chapter 1 with introducing the standard GPT framework and summarize previous results, based on a review paper of the author [New J. Phys. 13, 063024 (2011)]. To keep the introduction accessible to a broad readership, we follow a constructive approach. Starting from few basic physically motivated assumptions we show how a given set of observations can be manifested in an operational theory. Furthermore, we characterize consistency conditions limiting the range of possible extensions. We point out that non-classical features of single systems can equivalently result from higher dimensional classical theories that have been restricted. Entanglement and non-locality, however, are shown to be genuine non-classical features. We review features that have been found to be specific for quantum theory separably or single and joint systems. Chapter 2 incorporates results published in [J. Phys. A 47(32), pp. 1-32 (2014)] and [Proc. QPL 2011 via EPTCS vol. 95, pp. 183-192 (2012)]. The GPT framework is applied to show how the structure of local state spaces indirectly affects possible nonlocal correlations, which are global properties of a theory. These correlations are stronger than those possible in a classical theory, but happen to show different restrictions that can be linked to the structure of subsystems. We first illustrate the phenomenon with toy theories with particular local state spaces. We than show that a particular class of joint states (inner product states), whose existence depends on geometrical properties of the local subsystems, can only have correlations for a known limited set called Q1. All bipartite correlations of both, quantum and classical correlations, can be mapped to measurement statistics from such joint states. Chapter 3 shows unpublished results on entanglement swapping in GPTs. This protocol, which is well known in quantum information theory, allows to nonlocally transfer entanglement to initially unentangled parties with the help of a third party that shares entanglement with each. We review our approach published in [Proc. QPL 2011 via EPTCS vol. 95, pp. 183-192 (2012)], which mimics the joint systems' structure of quantum theory by modifying a popular toy theory known as boxworld. However, it is illustrated that this approach fails for bigger multipartite systems due to inconsistencies evoked by entanglement swapping. It turns out that the GPT framework does not allow entanglement swapping for general subsystems with two-dimensional state spaces with transitive pure states. Altering the GPT framework to allow completely globally degrees of freedom, however, enables us to construct consistent entanglement swapping for these subsystems. This construction resembles the situation in quantum theory on a real Hilbert space. A questionable assumption usually taken in the standard GPT framework is the so-called no-restriction hypothesis. It states that the measurement that are possible in a theory can be derived from the state space. In fact, this assumption seems to exist for reasons of mathematical convenience, but it seems to lack physical motivation. We generalize the GPT framework to also account for systems that do not obey the no-restriction hypothesis in Chapter 4, which presents results published in [Phys. Rev. A 87, 052131 (2013)] and [Proc. QPL 2013, to be published in EPTCS]. The extended framework includes new classes of probabilistic theories. As an example, we show how to construct theories that include intrinsic noise. We also provide a "self-dualization" procedure that requires the violation of the no-restriction hypothesis. This procedure restricts the measurement of arbitrary theories such that the theories act as if they were self-dual. Self-duality has recently gathered lots of interest, since such theories share many features of quantum theory. For example Tsirelson's bound holds for correlations on the maximally entangled state in these theories. Finally, we characterize the maximal set of joint states that can be consistently defined for given subsystems. This generalizes the maximal tensor product of the standard GPT framework.},
  subject      = {Quantentheorie},
  language  = {en}
}
@article{VainioValtonenHeberetal.2013,
  author    = {Vainio, Rami and Valtonen, Eino and Heber, Bernd and Malandraki, Olga E. and Papaioannou, Athanasios and Klein, Karl-Ludwig and Afanasiev, Alexander and Agueda, Neus and Aurass, Henry and Battarbee, Markus and Braune, Stephan and Dr{\"o}ge, Wolfgang and Ganse, Urs and Hamadache, Clarisse and Heynderickx, Daniel and Huttunen-Heikinmaa, Kalle and Kiener, J{\"u}rgen and Kilian, Patrick and Kopp, Andreas and Kouloumvakos, Athanasios and Maisala, Sami and Mishev, Alexander and Miteva, Rosita and Nindos, Alexander and Oittinen, Tero and Raukunen, Osku and Riihonen, Esa and Rodriguez-Gasen, Rosa and Saloniemi, Oskari and Sanahuja, Blai and Scherer, Renate and Spanier, Felix and Tatischeff, Vincent and Tziotziou, Kostas and Usoskin, Ilya G. and Vilmer, Nicole},
  title     = {The first SEPServer event catalogue similar to ~68-MeV solar proton events observed at 1 AU in 1996-2010},
  series = {Journal of Space Weather and Space Climate},
  volume    = {3},
  journal   = {Journal of Space Weather and Space Climate},
  number    = {A12},
  issn      = {2115-7251},
  doi       = {10.1051/swsc/2013030},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-122847},
  year      = {2013},
  abstract  = {SEPServer is a three-year collaborative project funded by the seventh framework programme (FP7-SPACE) of the European Union. The objective of the project is to provide access to state-of-the-art observations and analysis tools for the scientific community on solar energetic particle (SEP) events and related electromagnetic (EM) emissions. The project will eventually lead to better understanding of the particle acceleration and transport processes at the Sun and in the inner heliosphere. These processes lead to SEP events that form one of the key elements of space weather. In this paper we present the first results from the systematic analysis work performed on the following datasets: SOHO/ERNE, SOHO/EPHIN, ACE/EPAM, Wind/WAVES and GOES X-rays. A catalogue of SEP events at 1 AU, with complete coverage over solar cycle 23, based on high-energy (similar to 68-MeV) protons from SOHO/ERNE and electron recordings of the events by SOHO/EPHIN and ACE/EPAM are presented. A total of 115 energetic particle events have been identified and analysed using velocity dispersion analysis (VDA) for protons and time-shifting analysis (TSA) for electrons and protons in order to infer the SEP release times at the Sun. EM observations during the times of the SEP event onset have been gathered and compared to the release time estimates of particles. Data from those events that occurred during the European day-time, i.e., those that also have observations from ground-based observatories included in SEPServer, are listed and a preliminary analysis of their associations is presented. We find that VDA results for protons can be a useful tool for the analysis of proton release times, but if the derived proton path length is out of a range of 1 AU < s less than or similar to 3 AU, the result of the analysis may be compromised, as indicated by the anti-correlation of the derived path length and release time delay from the associated X-ray flare. The average path length derived from VDA is about 1.9 times the nominal length of the spiral magnetic field line. This implies that the path length of first-arriving MeV to deka-MeV protons is affected by interplanetary scattering. TSA of near-relativistic electrons results in a release time that shows significant scatter with respect to the EM emissions but with a trend of being delayed more with increasing distance between the flare and the nominal footpoint of the Earth-connected field line.},
  language  = {en}
}
@article{CamargoMolinaGarbrechtO'Learyetal.2014,
  author    = {Camargo-Molina, J. E. and Garbrecht, B. and O'Leary, B. and Porod, W. and Staub, F.},
  title     = {Constraining the Natural MSSM through tunneling to color-breaking vacua at zero and non-zero temperature},
  series = {Physics Letters B},
  volume    = {737},
  journal   = {Physics Letters B},
  doi       = {10.1016/j.physletb.2014.08.036},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-118458},
  pages     = {156-161},
  year      = {2014},
  abstract  = {We re-evaluate the constraints on the parameter space of the minimal supersymmetric standard model from tunneling to charge- and/or color-breaking minima, taking into account thermal corrections. We pay particular attention to the region known as the Natural MSSM, where the masses of the scalar partners of the top quarks are within an order of magnitude or so of the electroweak scale. These constraints arise from the interaction between these scalar tops and the Higgs fields, which allows the possibility of parameter points having deep charge- and color-breaking true vacua. In addition to requiring that our electroweak-symmetry-breaking, yet QCD- and electromagnetism-preserving vacuum has a sufficiently long lifetime at zero temperature, also demanding stability against thermal tunneling further restricts the allowed parameter space.},
  language  = {en}
}
@phdthesis{Lewandowska2015,
  author    = {Lewandowska, Natalia Ewelina},
  title     = {A Correlation Study of Radio Giant Pulses and Very High Energy Photons from the Crab Pulsar},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123533},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Pulsars (in short for Pulsating Stars) are magnetized, fast rotating neutron stars. The basic picture of a pulsar describes it as a neutron star which has a rotation axis that is not aligned with its magnetic field axis. The emission is assumed to be generated near the magnetic poles of the neutron star and emitted along the open magnetic field lines. Consequently, the corresponding beam of photons is emitted along the magnetic field line axis. The non-alignment of both, the rotation and the magnetic field axis, results in the effect that the emission of the pulsar is only seen if its beam points towards the observer. The emission from a pulsar is therefore perceived as being pulsed although its generation is not. This rather simple geometrical model is commonly referred to as Lighthouse Model and has been widely accepted. However, it does not deliver an explanation of the precise mechanisms behind the emission from pulsars (see below for more details). Nowadays more than 2000 pulsars are known. They are observed at various wavelengths. Multiwavelength studies have shown that some pulsars are visible only at certain wavelengths while the emission from others can be observed throughout large parts of the electromagnetic spectrum. An example of the latter case is the Crab pulsar which is also the main object of interest in this thesis. Originating from a supernova explosion observed in 1054 A.D. and discovered in 1968, the Crab pulsar has been the central subject of numerous studies. Its pulsed emission is visible throughout the whole electromagnetic spectrum which makes it a key figure in understanding the possible mechanisms of multiwavelength emission from pulsars. The Crab pulsar is also well known for its radio emission strongly varying on long as well as on short time scales. While long time scale behaviour from a pulsar is usually examined through the use of its average profile (a profile resulting from averaging of a large number of individual pulses resulting from single rotations), short time scale behaviour is examined via its single pulses. The short time scale anomalous behaviour of its radio emission is commonly referred to as Giant Pulses and represents the central topic of this thesis. While current theoretical approaches place the origin of the radio emission from a pulsar like the Crab near its magnetic poles (Polar Cap Model) as already indicated by the Lighthouse model, its emission at higher frequencies, especially its gamma-ray emission, is assumed to originate further away in the geometrical region surrounding a pulsar which is commonly referred to as a pulsar magnetosphere (Outer Gap Model). Consequently, the respective emission regions are usually assumed not to be connected. However, past observational results from the Crab pulsar represent a contradiction to this assumption. Radio giant pulses from the Crab pulsar have been observed to emit large amounts of energy on very short time scales implying small emission regions on the surface of the pulsar. Such energetic events might also leave a trace in the gamma-ray emission of the Crab pulsar. The aim of this thesis is to search for this connection in the form of a correlation study between radio giant pulses and gamma-photons from the Crab pulsar. To make such a study possible, a multiwavelength observational campaign was organized for which radio observations were independently applied for, coordinated and carried out with the Effelsberg radio telescope and the Westerbork Synthesis Radio Telescope and gamma-ray observations with the Major Atmospheric Imaging Cherenkov telescopes. The corresponding radio and gamma-ray data sets were reduced and the correlation analysis thereafter consisted of three different approaches: 1) The search for a clustering in the differences of the times of arrival of radio giant pulses and gamma-photons; 2) The search for a linear correlation between radio giant pulses and gamma-photons using the Pearson correlation approach; 3) A search for an increase of the gamma-ray flux around occurring radio giant pulses. In the last part of the correlation study an increase of the number of gamma-photons centered on a radio giant pulse by about 17\% (in contrast with the number of gamma-photons when no radio giant pulse occurs in the same time window) was discovered. This finding suggests that a new theoretical approach for the emission of young pulsars like the Crab pulsar, is necessary.},
  subject      = {Pulsar},
  language  = {en}
}
@phdthesis{Krauss2015,
  author    = {Krauß, Manuel Ernst},
  title     = {Non-minimal supersymmetric models: LHC phenomenology and model discrimination},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123555},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {It is generally agreed upon the fact that the Standard Model of particle physics can only be viewed as an effective theory that needs to be extended as it leaves some essential questions unanswered. The exact realization of the necessary extension is subject to discussion. Supersymmetry is among the most promising approaches to physics beyond the Standard Model as it can simultaneously solve the hierarchy problem and provide an explanation for the dark matter abundance in the universe. Despite further virtues like gauge coupling unification and radiative electroweak symmetry breaking, minimal supersymmetric models cannot be the ultimate answer to the open questions of the Standard Model as they still do not incorporate neutrino masses and are besides heavily constrained by LHC data. This does, however, not derogate the beauty of the concept of supersymmetry. It is therefore time to explore non-minimal supersymmetric models which are able to close these gaps, review their consistency, test them against experimental data and provide prospects for future experiments. The goal of this thesis is to contribute to this process by exploring an extraordinarily well motivated class of models which bases upon a left-right symmetric gauge group. While relaxing the tension with LHC data, those models automatically include the ingredients for neutrino masses. We start with a left-right supersymmetric model at the TeV scale in which scalar \(SU(2)_R\) triplets are responsible for the breaking of left-right symmetry as well as for the generation of neutrino masses. Although a tachyonic doubly-charged scalar is present at tree-level in this kind of models, we show by performing the first complete one-loop evaluation that it gains a real mass at the loop level. The constraints on the predicted additional charged gauge bosons are then evaluated using LHC data, and we find that we can explain small excesses in the data of which the current LHC run will reveal if they are actual new physics signals or just background fluctuations. In a careful evaluation of the loop-corrected scalar potential we then identify parameter regions in which the vacuum with the phenomenologically correct symmetry-breaking properties is stable. Conveniently, those regions favour low left-right symmetry breaking scales which are accessible at the LHC. In a slightly modified version of this model where a \(U(1)_R × U(1)_{B-L}\) gauge symmetry survives down to the TeV scale, we implement a minimal gauge-mediated supersymmetry breaking mechanism for which we calculate the boundary conditions in the presence of gauge kinetic mixing. We show how the presence of the extended gauge group raises the tree-level Higgs mass considerably so that the need for heavy supersymmetric spectra is relaxed. Taking the constraints from the Higgs sector into account, we then explore the LHC phenomenology of this model and point out where the expected collider signatures can be distinguished from standard scenarios. In particular if neutrino masses are explained by low-scale seesaw mechanisms as is done throughout this work, there are potentially spectacular signals at low-energy experiments which search for charged lepton flavour violation. The last part of this thesis is dedicated to the detailed exploration of processes like μ → e γ, μ → 3 e or μ-e conversion in nuclei in a supersymmetric framework with an inverse seesaw mechanism. In particular, we disprove claims about a non-decoupling effect in Z-mediated three-body decays and study the prospects for discovering and distinguishing signals at near-future experiments. In this context we identify the possibility to deduce from ratios like BR(\(τ → 3 μ\))/BR(\(τ → μ e^+ e^-\)) whether the contributions from ν - W loops dominate over supersymmetric contributions or vice versa.},
  subject      = {Supersymmetrie},
  language  = {en}
}
@phdthesis{Kilian2015,
  author    = {Kilian, Patrick},
  title     = {Teilchenbeschleunigung an kollisionsfreien Schockfronten},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119023},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Das Magnetfeld der Sonne ist kein einfaches statisches Dipolfeld, sondern weist wesentlich kompliziertere Strukturen auf. Wenn Rekonnexion die Topologie eines Feldlinienb{\"u}ndels ver{\"a}ndert, wird viel Energie frei, die zuvor im Magnetfeld gespeichert war. Das abgetrennte B{\"u}ndel wird mit dem damit verbundenen Plasma mit großer Geschwindigkeit durch die Korona von der Sonne weg bewegen. Dieser Vorgang wird als koronaler Massenauswurf bezeichnet. Da diese Bewegung mit Geschwindigkeiten deutlich {\"u}ber der Alfv\'en-Geschwindigkeit, der kritischen Geschwindigkeit im Sonnenwind, erfolgen kann, bildet sich eine Schockfront, die durch den Sonnenwind propagiert. Satelliten, die die Bedingungen im Sonnenwind beobachten, detektieren beim Auftreten solcher Schockfronten einen erh{\"o}hten Fluss von hochenergetischen Teilchen. Mit Radioinstrumenten empf{\"a}ngt man zeitgleich elektromagnetische Ph{\"a}nomene, die als Radiobursts bezeichnet werden, und ebenfalls f{\"u}r die Anwesenheit energiereicher Teilchen sprechen. Daher, und aufgrund von theoretischen {\"U}berlegungen liegt es nahe, anzunehmen, daß Teilchen an der Schockfront beschleunigt werden k{\"o}nnen. Die Untersuchung der Teilchenbeschleunigung an kollisionsfreien Schockfronten ist aber noch aus einem zweiten Grund interessant. Die Erde wird kontinuierlich von hochenergetischen Teilchen, die aus historischen Gr{\"u}nden als kosmische Strahlung bezeichnet werden, erreicht. Die g{\"a}ngige Theorie f{\"u}r deren Herkunft besagt, daß zumindest der galaktische Anteil durch die Beschleunigung an Schockfronten, die durch Supernovae ausgel{\"o}st wurden, bis zu den beobachteten hohen Energien gelangt sind. Das Problem bei der Untersuchung der Herkunft der kosmischen Strahlung ist jedoch, daß die Schockfronten um Supernova{\"u}berreste aufgrund der großen Entfernung nicht direkt beobachtbar sind. Es liegt dementsprechend nahe, die Schockbeschleunigung an den wesentlich n{\"a}heren und besser zu beobachtenden Schocks im Sonnensystem zu studieren, um so Modelle und Simulationen entwickeln und testen zu k{\"o}nnen. Die vorliegende Arbeit besch{\"a}ftigt sich daher mit Simulationen von Schockfronten mit Parametern, die etwa denen von CME getriebenen Schocks entsprechen. Um die Entwicklung der Energieverteilung der Teilchen zu studieren, ist ein kinetischer Ansatz n{\"o}tig. Dementsprechend wurden die Simulationen mit einem Particle-in-Cell Code durchgef{\"u}hrt. Die Herausforderung ist dabei die große Spanne zwischen den mikrophysikalischen Zeit- und L{\"a}ngenskalen, die aus Gr{\"u}nden der Genauigkeit und numerischen Stabilit{\"a}t aufgel{\"o}st werden m{\"u}ssen und den wesentlich gr{\"o}ßeren Skalen, die die Schockfront umfasst und auf der Teilchenbeschleunigung stattfindet. Um die Stabilit{\"a}t und physikalische Aussagekraft der Simulationen sicherzustellen, werden die numerischen Bausteine mittels Testf{\"a}llen, deren Verhalten bekannt ist, gr{\"u}ndlich auf ihre Tauglichkeit und korrekte Implementierung gepr{\"u}ft. Bei den resultierenden Simulationen wird das Zutreffen von analytischen Vorhersagen (etwa die Einhaltung der Sprungbedingungen) {\"u}berpr{\"u}ft. Auch die Vorhersagen einfacherer Plasmamodelle, etwa f{\"u}r das elektrostatischen Potential an der Schockfront, das man auch aus einer Zwei-Fluid-Beschreibung erhalten kann, folgen automatisch aus der selbstkonsistenten, kinetischen Beschreibung. Zus{\"a}tzlich erh{\"a}lt man Aussagen {\"u}ber das Spektrum und die Bahnen der beschleunigten Teilchen.},
  subject      = {Stoßfreies Plasma},
  language  = {de}
}
@phdthesis{Boyer2015,
  author    = {Boyer, Sonja},
  title     = {Morphologische und spektroskopische Untersuchungen von Supernova-{\"U}berresten},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119108},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Bis heute ist nicht bekannt, in welcher Umgebung die schwersten Elemente durch Neutroneneinfangprozesse entstehen. Es gibt zwei m{\"o}gliche Szenarien, die in der Literatur diskutiert werden: Supernova-Explosionen und Neutronensternverschmelzungen. Beide tragen zur Elementproduktion bei. Welches Szenario aber die dominierende Umgebung ist, bleibt umstritten. Mehrere Fakten sprechen f{\"u}r Supernova-Explosionen als Entstehungsorte: Wenn ein massereicher Stern kollabiert und anschließend explodiert, sind die Temperatur und die Dichte so hoch, dass Neutronen von den bereits bestehenden Elementen eingefangen und angelagert werden k{\"o}nnen. Obwohl in Simulationen mit kugelsymmetrischen Modellen nur protonen- reiche Ausw{\"u}rfe entstehen, kann es in asymmetrischen Explosionen aufgrund der Rotation und der Magnetfelder vermutlich zu einem neutronenreichen Auswurf kommen. Dieser ist hoch genug, dass der schnelle Neutroneneinfang auftreten kann. In dieser Arbeit habe ich daher die {\"U}berreste solcher Explosionen untersucht, um nach Asymmetrien und ihren m{\"o}glichen Auswirkungen auf die Element-Entstehung und Verteilung zu suchen. Daf{\"u}r wurden die beiden Supernova-{\"U}berreste CTB 109 und RCW 103 ausgew{\"a}hlt. CTB 109 besitzt im Zentrum einen anomale R{\"o}ntgenpulsar, also einen Neutronenstern mit hohem Magnetfeld und starker Rotation, die durch Asymmetrien hervorgerufen worden sein k{\"o}nnten. Auch RCW 103 hat vermutlich einen solchen Pulsar als zentrale Quelle. Beide {\"U}berreste sind noch recht jung und befinden sich in ihrer Sedov-Taylor Phase. Die Distanz zur Erde betr{\"a}gt f{\"u}r beide {\"U}berreste ungef{\"a}hr 3 kpc, womit sie in der n{\"a}heren Umgebung der Erde zu finden sind. Die Elemente bis zur Eisengruppe haben ihre bekanntesten Linien im Bereich der R{\"o}ntgenstrahlung. Deswegen wurden f{\"u}r diese Arbeit archivierte Daten des Satelliten XMM-Newton ausgew{\"a}hlt und die Spektren in definierten Regionen in den bei- den Supernova-{\"U}berresten mit den EPIC MOS-Kameras ausgewertet. Die heutigen R{\"o}ntgensatelliten haben jedoch keine ausreichende Sensitivit{\"a}t, um die schwersten Elemente zu detektieren. In den Spektren der beiden {\"U}berreste wurden deshalb vorwiegend die Elemente Silizium und Magnesium gefunden, in CTB 109 auch Neon. Elemente mit h{\"o}heren Massezahlen konnten leider nicht signifikant aus dem Hintergrund herausgefiltert werden. Deutlich sind die Peaks der drei Elementen sichtbar, aber auch Schwefel ist in den Regionen mit hohen Z{\"a}hlraten zu entdecken. F{\"u}r bei- de Supernova-{\"U}berreste wurde der beste Fit mit dem Modell vpshock gefunden. In diesem Modell wird ein Plasma angenommen, das bei konstanter Temperatur plan-parallel geschockt wird. Um diesen Fit zu erzielen wurden die Parameter f{\"u}r die Elemente Fe, S, Si, Mg, O und Ne variiert. Die restlichen Elemente wurden auf die solare H{\"a}ufigkeit festgelegt. Bei CTB 109 befinden sich die Temperaturen (kT) in den Regionen mit hohen Z{\"a}hlraten im Bereich zwischen 0.6 und 0.7 keV und liegen damit im selben Bereich, der bereits mit anderen Teleskopen f{\"u}r CTB 109 gefunden wurde. In den Regionen mit niedrigen Z{\"a}hlraten liegen die Temperaturen etwas tiefer mit 0.3-0.4 keV. Im Supernova-{\"U}berrest RCW 103 wurde nur eine Region mit hoher Z{\"a}hlrate analysiert und eine Temperatur von 0.57 keV gefunden, w{\"a}hrend in der Region mit niedriger Z{\"a}hlrate die Temperatur kT = 0.36 ± 0.08 keV betr{\"a}gt. Beide Werte passen zu den Werten in CTB 109. Die einzelnen Elementlinien wurden zus{\"a}tzlich mit einer Gauß-Verteilung angepasst und die Fl{\"u}sse ermittelt. Diese wurden in Intensit{\"a}tskarten aufgetragen, in denen die unterschiedlichen Verteilungen der Elemente {\"u}ber den Supernova-{\"U}berrest zu sehen sind. W{\"a}hrend Silizium in einigen wenigen Regionen geklumpt auftritt, ist Magnesium {\"u}ber die {\"U}berreste verteilt und hat in einigen Regionen h{\"o}here Werte als Silizium. Das l{\"a}sst den Schluss zu, dass die beiden Elemente auf unterschiedliche Weise aus der Explosion herausgeschleudert wurden. Die Verteilung ist hier durchaus asymmetrisch, es ist jedoch nicht m{\"o}glich dies auf eine asymmetrische Explosion der Supernova zur{\"u}ckzuf{\"u}hren. Daf{\"u}r m{\"u}ssen mehr als zwei Supernova-{\"U}berreste mit dieser Methode untersucht werden und mit einer noch nicht vorhandenen Theorie zur Verteilung der Elemente in {\"U}berresten verglichen werden. Im direkten Vergleich der beiden bisher untersuchten Supernova-{\"U}berreste CTB 109 und RCW 103 sieht man, dass die beiden {\"U}berreste sich sehr in der Temperatur und der Verteilung der Elemente {\"a}hneln. Das l{\"a}sst auf eine einheitliche Ausbreitung der Elemente innerhalb der Supernova-{\"U}berreste schließen. Silizium wird aufgrund der Explosion in fingerartigen Strukturen, die Rayleigh-Taylor-Instabilit{\"a}ten, nach außen transportiert. Dabei bildet es Klumpen, die mit den weiter außen liegenden Schalen reagieren. Magnesium und Neon hingegen werden haupts{\"a}chlich in den Brennphasen vor der Explosion und in den {\"a}ußeren Schichten des Sterns, der Zwiebelschalenstruktur, produziert. Dadurch ist eine ausgedehnte Verteilung zu er- warten. Diese Verteilungen der drei Elemente ist in dieser Arbeit best{\"a}tigt worden. W{\"a}hrend Magnesium und Neon {\"u}ber den gesamten {\"U}berrest hohe Fl{\"u}sse aufweisen, ist Silizium sehr lokal im Lobe von CTB 109 und im hellen S{\"u}den von RCW 103 zu finden. Mit zuk{\"u}nftigen R{\"o}ntgenteleskopen, die eine h{\"o}here r{\"a}umliche Aufl{\"o}sung erm{\"o}glichen, k{\"o}nnten die beobachteten Zusammenh{\"a}nge zwischen der asymmetrischen Elementverteilung im Supernova{\"u}berrest und den Mechanismen der Elemententstehung in der Supernova weiter untersucht werden.},
  subject      = {Supernova{\"u}berrest},
  language  = {de}
}
@article{EdgecockCarettaDavenneetal.2013,
  author    = {Edgecock, T. R. and Caretta, O. and Davenne, T. and Densam, C. and Fitton, M. and Kelliher, D. and Loveridge, P. and Machida, S. and Prior, C. and Rogers, C. and Rooney, M. and Thomason, J. and Wilcox, D. and Wildner, E. and Efthymiopoulos, I. and Garoby, R. and Gilardoni, S. and Hansen, C. and Benedetto, E. and Jensen, E. and Kosmicki, A. and Martini, M. and Osborne, J. and Prior, G. and Stora, T. and Melo Mendonca, T. and Vlachoudis, V. and Waaijer, C. and Cupial, P. and Chanc{\´e}, A. and Longhin, A. and Payet, J. and Zito, M. and Baussan, E. and Bobeth, C. and Bouquerel, E. and Dracos, M. and Gaudiot, G. and Lepers, B. and Osswald, F. and Poussot, P. and Vassilopoulos, N. and Wurtz, J. and Zeter, V. and Bielski, J. and Kozien, M. and Lacny, L. and Skoczen, B. and Szybinski, B. and Ustrycka, A. and Wroblewski, A. and Marie-Jeanne, M. and Balint, P. and Fourel, C. and Giraud, J. and Jacob, J. and Lamy, T. and Latrasse, L. and Sortais, P. and Thuillier, T. and Mitrofanov, S. and Loiselet, M. and Keutgen, Th. and Delbar, Th. and Debray, F. and Trophine, C. and Veys, S. and Daversin, C. and Zorin, V. and Izotov, I. and Skalyga, V. and Burt, G. and Dexter, A. C. and Kravchuk, V. L. and Marchi, T. and Cinausero, M. and Gramegna, F. and De Angelis, G. and Prete, G. and Collazuol, G. and Laveder, M. and Mazzocco, M. and Mezzetto, M. and Signorini, C. and Vardaci, E. and Di Nitto, A. and Brondi, A. and La Rana, G. and Migliozzi, P. and Moro, R. and Palladino, V. and Gelli, N. and Berkovits, D. and Hass, M. and Hirsh, T. Y. and Schuhmann, M. and Stahl, A. and Wehner, J. and Bross, A. and Kopp, J. and Neuffer, D. and Wands, R. and Bayes, R. and Laing, A. and Soler, P. and Agarwalla, S. K. and Cervera Villanueva, A. and Donini, A. and Ghosh, T. and G{\´o}mez Cadenas, J. J. and Hern{\´a}ndez, P. and Mart{\´i}n-Albo, J. and Mena, O. and Burguet-Castell, J. and Agostino, L. and Buizza-Avanzini, M. and Marafini, M. and Patzak, T. and Tonazzo, A. and Duchesneau, D. and Mosca, L. and Bogomilov, M. and Karadzhov, Y. and Matev, R. and Tsenov, R. and Akhmedov, E. and Blennow, M. and Lindner, M. and Schwetz, T. and Fern{\´a}ndez Martinez, E. and Maltoni, M. and Men{\´e}ndez, J. and Giunti, C. and Gonz{\´a}lez Garc{\´i}a, M. C. and Salvado, J. and Coloma, P. and Huber, P. and Li, T. and L{\´o}pez Pav{\´o}n, J. and Orme, C. and Pascoli, S. and Meloni, D. and Tang, J. and Winter, W. and Ohlsson, T. and Zhang, H. and Scotto-Lavina, L. and Terranova, F. and Bonesini, M. and Tortora, L. and Alekou, A. and Aslaninejad, M. and Bontoiu, C. and Kurup, A. and Jenner, L. J. and Long, K. and Pasternak, J. and Pozimski, J. and Back, J. J. and Harrison, P. and Beard, K. and Bogacz, A. and Berg, J. S. and Stratakis, D. and Witte, H. and Snopok, P. and Bliss, N. and Cordwell, M. and Moss, A. and Pattalwar, S. and Apollonio, M.},
  title     = {High intensity neutrino oscillation facilities in Europe},
  series = {Physical Review Special Topics-Accelerators and Beams},
  volume    = {16},
  journal   = {Physical Review Special Topics-Accelerators and Beams},
  number    = {2},
  doi       = {10.1103/PhysRevSTAB.16.021002},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-126611},
  pages     = {21002},
  year      = {2013},
  abstract  = {The EUROnu project has studied three possible options for future, high intensity neutrino oscillation facilities in Europe. The first is a Super Beam, in which the neutrinos come from the decay of pions created by bombarding targets with a 4 MW proton beam from the CERN High Power Superconducting Proton Linac. The far detector for this facility is the 500 kt MEMPHYS water Cherenkov, located in the Frejus tunnel. The second facility is the Neutrino Factory, in which the neutrinos come from the decay of mu(+) and mu(-) beams in a storage ring. The far detector in this case is a 100 kt magnetized iron neutrino detector at a baseline of 2000 km. The third option is a Beta Beam, in which the neutrinos come from the decay of beta emitting isotopes, in particular He-6 and Ne-18, also stored in a ring. The far detector is also the MEMPHYS detector in the Frejus tunnel. EUROnu has undertaken conceptual designs of these facilities and studied the performance of the detectors. Based on this, it has determined the physics reach of each facility, in particular for the measurement of CP violation in the lepton sector, and estimated the cost of construction. These have demonstrated that the best facility to build is the Neutrino Factory. However, if a powerful proton driver is constructed for another purpose or if the MEMPHYS detector is built for astroparticle physics, the Super Beam also becomes very attractive.},
  language  = {en}
}
@phdthesis{Werner2014,
  author    = {Werner, Jan},
  title     = {Numerical Simulations of Heavy Fermion Systems: From He-3 Bilayers to Topological Kondo Insulators},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-112039},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {Even though heavy fermion systems have been studied for a long time, a strong interest in heavy fermions persists to this day. While the basic principles of local moment formation, Kondo effect and formation of composite quasiparticles leading to a Fermi liquid, are under- stood, there remain many interesting open questions. A number of issues arise due to the interplay of heavy fermion physics with other phenomena like magnetism and superconduc- tivity. In this regard, experimental and theoretical investigations of He-3 can provide valuable insights. He-3 represents a unique realization of a quantum liquid. The fermionic nature of He-3 atoms, in conjunction with the absence of long-range Coulomb repulsion, makes this material an ideal model system to study Fermi liquid behavior. Bulk He-3 has been investigated for quite some time. More recently, it became possible to prepare and study layered He-3 systems, in particular single layers and bilayers. The pos- sibility of tuning various physical properties of the system by changing the density of He-3 and using different substrate materials makes layers of He-3 an ideal quantum simulator for investigating two-dimensional Fermi liquid phenomenology. In particular, bilayers of He-3 have recently been found to exhibit heavy fermion behavior. As a function of temperature, a crossover from an incoherent state with decoupled layers to a coherent Fermi liquid of composite quasiparticles was observed. This behavior has its roots in the hybridization of the two layers. The first is almost completely filled and subject to strong correlation effects, while the second layer is only partially filled and weakly correlated. The quasiparticles are formed due to the Kondo screening of localized moments in the first layer by the second-layer delocalized fermions, which takes place at a characteristic temperature scale, the coherence scale Tcoh. Tcoh can be tuned by changing the He-3 density. In particular, at a certain critical filling, the coherence scale is expected to vanish, corresponding to a divergence of the quasiparticle effective mass, and a breakdown of the Kondo effect at a quantum critical point. Beyond the critical point, the layers are decoupled. The first layer is a local moment magnet, while the second layer is an itinerant overlayer. However, already at a filling smaller than the critical value, preempting the critical point, the onset of a finite sample magnetization was observed. The character of this intervening phase remained unclear. Motivated by these experimental observations, in this thesis the results of model calcula- tions based on an extended Periodic Anderson Model are presented. The three particle ring exchange, which is the dominant magnetic exchange process in layered He-3, is included in the model. It leads to an effective ferromagnetic interaction between spins on neighboring sites. In addition, the model incorporates the constraint of no double occupancy by taking the limit of large local Coulomb repulsion. By means of Cellular DMFT, the model is investigated for a range of values of the chemical potential µ and inverse temperature β = 1/T . The method is a cluster extension to the Dy- namical Mean-Field Theory (DMFT), and allows to systematically include non-local correla- tions beyond the DMFT. The auxiliary cluster model is solved by a hybridization expansion CTQMC cluster solver, which provides unbiased, numerically exact results for the Green's function and other observables of interest. As a first step, the onset of Fermi liquid coherence is studied. At low enough temperature, the self-energy is found to exhibit a linear dependence on Matsubara frequency. Meanwhile, the spin susceptibility crossed over from a Curie-Weiss law to a Pauli law. Both observations serve as fingerprints of the Fermi liquid state. The heavy fermion state appears at a characteristic coherence scale Tcoh. This scale depends strongly on the density. While it is rather high for small filling, for larger filling Tcoh is increas- ingly suppressed. This involves a decreasing quasiparticle residue Z ∼ Tcoh and an enhanced mass renormalization m∗/m ∼ Tcoh-1. Extrapolation leads to a critical filling, where the co- herence scale is expected to vanish at a quantum critical point. At the same time, the effective mass diverges. This corresponds to a breakdown of the Kondo effect, which is responsible for the formation of quasiparticles, due to a vanishing of the effective hybridization between the layers. Taking only single-site DMFT results into account, the above scenario seems plausible. However, paramagnetic DMFT neglects the ring exchange interaction completely. In or- der to improve on this, Cellular DMFT simulations are conducted for small clusters of size Nc = 2 and 3. The results paint a different physical picture. The ring exchange, by favor- ing a ferromagnetic alignment of spins, competes with the Kondo screening. As a result, strong short-range ferromagnetic fluctuations appear at larger values of µ. By lowering the temperature, these fluctuations are enhanced at first. However, for T < Tcoh they are increas- ingly suppressed, which is consistent with Fermi liquid coherence. However, beyond a certain threshold value of µ, fluctuations persist to the lowest temperatures. At the same time, while not apparent in the DMFT results, the total occupation n increases quite strongly in a very narrow range around the same value of µ. The evolution of n with µ is always continuous, but hints at a discontinuity in the limit Nc → ∞. This first-order transition breaks the Kondo effect. Beyond the transition, a ferromagnetic state in the first layer is established, and the second layer becomes a decoupled overlayer. These observations provide a quite appealing interpretation of the experimental results. As a function of chemical potential, the Kondo breakdown quantum critical point is preempted by a first-order transition, where the layers decouple and the first layer turns into a ferromagnet. In the experimental situation, where the filling can be tuned directly, the discontinuous transition is mirrored by a phase separation, which interpolates between the Fermi liquid ground state at lower filling and the magnetic state at higher filling. This is precisely the range of the intervening phase found in the experiments, which is characterized by an onset of a finite sample magnetization. Besides the interplay of heavy fermion physics and magnetic exchange, recently the spin- orbit coupling, which is present in many heavy fermion materials, attracted a lot of interest. In the presence of time-reversal symmetry, due to spin-orbit coupling, there is the possibility of a topological ground state. It was recently conjectured that the energy scale of spin-orbit coupling can become dom- inant in heavy fermion materials, since the coherence scale and quasiparticle bandwidth are rather small. This can lead to a heavy fermion ground state with a nontrivial band topology; that is, a topological Kondo insulator (TKI). While being subject to strong correlation effects, this state must be adiabatically connected to a non-interacting, topological state. The idea of the topological ground state realized in prototypical Kondo insulators, in par- ticular SmB6, promises to shed light on some of the peculiarities of these materials, like a residual conductivity at the lowest temperatures, which have remained unresolved so far. In this work, a simple two-band model for two-dimensional topological Kondo insulators is devised, which is based on a single Kramer's doublet coupled to a single conduction band. The model is investigated in the presence of a Hubbard interaction as a function of interaction strength U and inverse temperature β. The bulk properties of the model are obtained by DMFT, with a hybridization expansion CTQMC impurity solver. The DMFT approximation of a local self-energy leads to a very simple way of computing the topological invariant. The results show that with increasing U the system can be driven through a topological phase transition. Interestingly, the transition is between distinct topological insulating states, namely the Γ-phase and M-phase. This appearance of different topological phases is possible due to the symmetry of the underlying square lattice. By adiabatically connecting both in- teracting states with the respective non-interacting state, it is shown that the transition indeed drives the system from the Γ-phase to the M-phase. A different behavior can be observed by pushing the bare position of the Kramer's doublet to higher binding energies. In this case, the non-interacting starting point has a trivial band topology. By switching on the interaction, the system can be tuned through a quantum phase transition, with a closing of the band gap. Upon reopening of the band gap, the system is in the Γ-phase, i. e. a topological insulator. By increasing the interaction strength further, the system moves into a strongly correlated regime. In fact, close to the expected transition to the M phase, the mass renormalization becomes quite substantial. While absent in the para- magnetic DMFT simulations conducted, it is conceivable that instead of a topological phase transition, the system undergoes a time-reversal symmetry breaking, magnetic transition. The regime of strong correlations is studied in more detail as a function of temperature, both in the bulk and with open boundary conditions. A quantity which proved very useful is the bulk topological invariant Ns, which can be generalized to finite interaction strength and temperature. In particular, it can be used to define a temperature scale T ∗ for the onset of the topological state. Rescaling the results for Ns, a nice data collapse of the results for different values of U, from the local moment regime to strongly mixed valence, is obtained. This hints at T ∗ being a universal low energy scale in topological Kondo insulators. Indeed, by comparing T ∗ with the coherence scale extracted from the self-energy mass renormalization, it is found that both scales are equivalent up to a constant prefactor. Hence, the scale T ∗ obtained from the temperature dependence of topological properties, can be used as an independent measure for Fermi liquid coherence. This is particularly useful in the experimentally relevant mixed valence regime, where charge fluctuations cannot be neglected. Here, a separation of the energy scales related to spin and charge fluctuations is not possible. The importance of charge fluctuations becomes evident in the extent of spectral weight transfer as the temperature is lowered. For mixed valence, while the hybridization gap emerges, a substantial amount of spectral weight is shifted from the vicinity of the Fermi level to the lower Hubbard band. In contrast, this effect is strongly suppressed in the local moment regime. In addition to the bulk properties, the spectral function for open boundaries is studied as a function of temperature, both in the local moment and mixed valence regime. This allows an investigation of the emergence of topological edge states with temperature. The method used here is the site-dependent DMFT, which is a generalization of the conventional DMFT to inhomogeneous systems. The hybridization expansion CTQMC algorithm is used as impurity solver. By comparison with the bulk results for the topological quantity Ns, it is found that the temperature scale for the appearance of the topological edge states is T ∗, both in the mixed valence and local moment regime.},
  subject      = {Fermionensystem},
  language  = {en}
}
@phdthesis{Bustamante2014,
  author    = {Bustamante, Mauricio},
  title     = {Ultra-high-energy neutrinos and cosmic rays from gamma-ray bursts: exploring and updating the connection},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-112480},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {It is natural to consider the possibility that the most energetic particles detected (> 10^18 eV), ultra-high-energy cosmic rays (UHECRs), are originated at the most luminous transient events observed (> 10^52 erg s^-1), gamma-ray bursts (GRBs). As a result of the interaction of highly-accelerated, magnetically-confined protons and ions with the photon field inside the burst, both neutrons and UHE neutrinos are expected to be created: the former escape the source and beta-decay into protons which propagate to Earth, where they are detected as UHECRs, while the latter, if detected, would constitute the smoking gun of hadronic acceleration in the sources. Recently, km-scale neutrino telescopes such as IceCube have finally reached the sensitivities required to probe the neutrino predictions of some of the existing GRB models. On that account, we present here a revised, self-consistent model of joint UHE proton and neutrino production at GRBs that includes a state-of-the-art, improved numerical calculation of the neutrino flux (NeuCosmA); that uses a generalised UHECR emission model where some of the protons in the sources are able to "leak out" of their magnetic confinement before having interacted; and that takes into account the energy losses of the protons during their propagation to Earth. We use our predictions to take a close look at the cosmic ray-neutrino connection and find that the current UHECR observations by giant air shower detectors, together with the upper bounds on the flux of neutrinos from GRBs, are already sufficient to put tension on several possibilities of particle emission and propagation, and to point us towards some requirements that should be fulfilled by GRBs if they are to be the sources of the UHECRs. We further refine our analysis by studying a dynamical burst model, where we find that the different particle species originate at distinct stages of the expanding GRB, each under particular conditions. Finally, we consider a possibility of new physics: the effect of neutrino decay in the flux of UHE neutrinos from GRBs. On the whole, our results demonstrate that self-consistent models of particle production are now integral to the advancement of the field, given that the full picture of the UHE Universe will only emerge as a result of looking at the multi-messenger sky, i.e., at gamma-rays, cosmic rays, and neutrinos simultaneously.},
  subject      = {Gamma-Burst},
  language  = {en}
}
@phdthesis{Rothe2015,
  author    = {Rothe, Dietrich Gernot},
  title     = {Spin Transport in Topological Insulators and Geometrical Spin Control},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-125628},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {In the field of spintronics, spin manipulation and spin transport are the main principles that need to be implemented. The main focus of this thesis is to analyse semiconductor systems where high fidelity in these principles can be achieved. To this end, we use numerical methods for precise results, supplemented by simpler analytical models for interpretation. The material system of 2D topological insulators, HgTe/CdTe quantum wells, is interesting not only because it provides a topologically distinct phase of matter, physically manifested in its protected transport properties, but also since within this system, ballistic transport of high quality can be realized, with Rashba spin-orbit coupling and electron densities that are tunable by electrical gating. Extending the Bernvevig-Hughes-Zhang model for 2D topological insulators, we derive an effective four-band model including Rashba spin-orbit terms due to an applied potential that breaks the spatial inversion symmetry of the quantum well. Spin transport in this system shows interesting physics because the effects of Rashba spin-orbit terms and the intrinsic Dirac-like spin-orbit terms compete. We show that the resulting spin Hall signal can be dominated by the effect of Rashba spin-orbit coupling. Based on spin splitting due to the latter, we propose a beam splitter setup for all-electrical generation and detection of spin currents. Its working principle is similar to optical birefringence. In this setup, we analyse spin current and spin polarization signals of different spin vector components and show that large in-plane spin polarization of the current can be obtained. Since spin is not a conserved quantity of the model, we first analyse the transport of helicity, a conserved quantity even in presence of Rashba spin-orbit terms. The polarization defined in terms of helicity is related to in-plane polarization of the physical spin. Further, we analyse thermoelectric transport in a setup showing the spin Hall effect. Due to spin-orbit coupling, an applied temperature gradient generates a transverse spin current, i.e. a spin Nernst effect, which is related to the spin Hall effect by a Mott-like relation. In the metallic energy regimes, the signals are qualitatively explained by simple analytic models. In the insulating regime, we observe a spin Nernst signal that originates from the finite-size induced overlap of edge states. In the part on methods, we discuss two complementary methods for construction of effective semiconductor models, the envelope function theory and the method of invariants. Further, we present elements of transport theory, with some emphasis on spin-dependent signals. We show the connections of the adiabatic theorem of quantum mechanics to the semiclassical theory of electronic transport and to the characterization of topological phases. Further, as application of the adiabatic theorem to a control problem, we show that universal control of a single spin in a heavy-hole quantum dot is experimentally realizable without breaking time reversal invariance, but using a quadrupole field which is adiabatically changed as control knob. For experimental realization, we propose a GaAs/GaAlAs quantum well system.},
  subject      = {Elektronischer Transport},
  language  = {en}
}
@phdthesis{Franke1995,
  author    = {Franke, Fabian},
  title     = {Produktion und Zerfall von Neutralinos im Nichtminimalen Supersymmetrischen Standardmodell},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-25666},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {1995},
  abstract  = {Das Ziel der vorliegenden Arbeit ist eine umfassende Analyse von Erzeugung und anschließenden Zerf{\"a}llen von Neutralinos im Nichtminimalen Supersymmetrischen Standardmodell (NMSSM) speziell f{\"u}r den n{\"a}chsten verf{\"u}gbaren Elektron-Positron-Speicherring LEP2 am CERN mit einer voraussichtlichen Schwerpunktsenergie von 190 GeV. Das NMSSM ist die einfachste Erweiterung des Minimalen Supersymmetrischen Standardmodells MSSM mit einem Singlett-Superfeld, so dass der Higgs-Sektor insgesamt sieben physikalische Higgs-Teilchen enth{\"a}lt, und zwar drei neutrale skalare, zwei pseudoskalare und zwei geladene. Weiterhin enth{\"a}lt das NMSSM f{\"u}nf Neutralinos gegen{\"u}ber vier im MSSM. In dieser Arbeit pr{\"a}sentieren wir die 5 x 5 Neutralinomischungsmatrix, stellen die Eigenwertgleichung auf und analysieren das Massenspektrum und die Parameterabh{\"a}ngigkeit m{\"o}glicher masseloser Zust{\"a}nde. F{\"u}r die Untersuchung von Neutralinoproduktion und -zerfall wurden verschiedene Szenarien gew{\"a}hlt, in denen das leichteste Neutralino eine Masse von 10 GeV und eine Singlettkomponente von {\"u}ber 90\% besitzt oder in denen das leichteste Neutralino bis zu 50 Gev schwer ist und sich der Singlettanteil auf die beiden leichtesten Neutralinos verteilt. Die Wirkungsquerschnitte f{\"u}r die Neutralinoproduktion wurden in den gew{\"a}hlten Szenarien f{\"u}r Schwerpunktsenergien von 100 GeV bis 600 GeV berechnet, also bis zu einem Bereich, den ein geplanter Elektron-Positron-Linearbeschleuniger erreichen kann. Typische Wirkungsquerschnitte f{\"u}r die direkte Produktion vorwiegend singlettartiger Neutralinos liegen im Bereich von 100 fb. Selbst wenn das leichteste Neutralino sehr leicht ist, kann das n{\"a}chste bereits so schwer sein, dass bei LEP2 nur die nicht nachtweisbare Paarproduktion des leichtesten supersymmetrischen Teilchens m{\"o}glich ist. Somit ist bei LEP2 keine Erh{\"o}hung der unteren Neutralinomassengrenzen im NMSSM zu erwarten, falls kein Neutralino gefunden wird. In Szenarien mit leichten singlettartigen Neutralinos k{\"o}nnen sehr oft auch sehr leichte Higgs-Bosonen mit Massen unterhalb der im MSSM vorhandenen Grenzen existieren. Somit kann in allen unseren Szenarien der Neutralinozerfall in ein skalares oder pseudoskalares Higgs-Boson m{\"o}glich sein und dann Verweigungsverh{\"a}ltnisse bis zu fast 100\% erreichen. Wir berechnen in dieser Arbeit f{\"u}r die bei LEP2 produzierbaren Neutralinos die Verwzeigungsverh{\"a}ltnisse f{\"u}r die Zweik{\"o}rperzerf{\"a}lle in Higgs-Bosonen, die Dreik{\"o}rperzerf{\"a}lle in zwei Fermionen und den Schleifenzerfall in ein Photon. In allen F{\"a}llen befindet sich im Endzustand außerdem das unsichtbare leichteste Neutralino, dass sich experimentell als fehlende Energie niederschl{\"a}gt. Zur Bestimmung der Signaturen betrachten wir außerdem die anschließenden Zerfallsmodi der leichten Higgs-Bosonen. Der Nachweis von leichten singlettartigen Neutralinos im NMSSM kann einerseits unm{\"o}glich sein, wenn entweder die schweren Neutralinos bei der verf{\"u}gbaren Schwerpunktsenergie nicht produziert werden k{\"o}nnen oder {\"u}ber Higgs-Bosonen vollkommen in das LSP zerfallen, andererseits aber auch durch klare Signaturen mit einem Photon oder mit Jets im Endzustand erleichtert werden. Bei LEP2 sollten also durchaus Chancen bestehen, auch im Rahmen des NMSSM ein Neutralino zu entdecken. Zumindest werden sich weitere Einschr{\"a}nkungen des Parameterraums ergeben. Der Dissertation ist ein Anhang beigef{\"u}gt, der eine vollst{\"a}ndige Liste aller Feynman-Regeln des NMSSM enth{\"a}lt, die sich von denjenigen des MSSM unterscheiden.},
  subject      = {Supersymmetrie},
  language  = {de}
}
@phdthesis{Bruenger2007,
  author    = {Br{\"u}nger, Christian},
  title     = {Numerical Studies of Quantum Spin Systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26439},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2007},
  abstract  = {Der erste Teil der Arbeit widmet sich der Untersuchung des Bilayer-Heisenberg-Modells und des zweidimensionalen Kondo-Necklace-Modells. Beide Modelle weisen einen Quantenphasen{\"u}bergang zwischen einer geordneten und einer ungeordneten Phase auf. In dieser Arbeit richtet sich das Interesse insbesondere auf die Kopplung der kritischen Fluktuationen an ein in das System eingebundenes Loch. Mittels eines selbstkonsistenten Born'schen N{\"a}herungsverfahrens wird gezeigt, dass das Loch mit den Magnonen derart wechselwirkt, dass dessen Quasiteilchengewicht am quantenkritischen Punkt verschwindet. Um diesen Aspekt weiter zu untersuchen, wird das Verhalten des Quasiteilchengewichts im Bereich der kritischen Kopplung auch mit Quanten-Monte-Carlo-Methoden analysiert. Desweiteren werden die dynamischen Eigenschaften des Loches im magnetischen Hintergrund untersucht. Im zweiten Teil dieser Arbeit gilt das Interesse der Untersuchung des Spiral-Staircase-Heisenberg-Modells. Dieses besteht aus zwei, zu einer Spinleiter ferromagnetisch gekopplten Spin-1/2-Ketten, wobei die antiferromagnetische Kopplung innerhalb der zweiten Kette durch Windung der Leiter variiert werden kann. Dieses Model eignet sich, den {\"U}bergang zwischen einer Spin-1/2-Kette ohne Spinl{\"u}cke und einer Spin-1-Kette mit Spinl{\"u}cke zu studieren. Besondere Beachtung ist dem {\"O}ffnen der Spinl{\"u}cke in Abh{\"a}ngigkeit der ferromagnetischen Kopplung zwischen den Leiterbeinen geboten. Es stellt sich heraus, dass das System, abh{\"a}ngig von der Leiterwindung, wesentliche Unterschiede im Skalierungsverhalten der Spinl{\"u}cke aufweist. Desweiteren wird mittels der String-Order-Parameter gezeigt, dass das Spiral-Staircase-Heisenberg-Modell trotz des unterschiedlichen Skalierungsverhaltens der Spinl{\"u}cke und unabh{\"a}ngig von der Wahl der Parameter sich stets in der Haldane-Phase befindet. Die Analyse der Modelle bedient sich haupts{\"a}chlich Quanten-Monte-Carlo-Methoden, aber auch exakter Diagonalisierungstechniken, sowie auf Molekularfeldn{\"a}herungen gest{\"u}tzten Rechnungen.},
  subject      = {Spinsystem},
  language  = {en}
}
@phdthesis{Paul2010,
  author    = {Paul, Surajit},
  title     = {Evolution of shocks and turbulence in major galaxy-cluster mergers},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-47266},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {Mergers between rich clusters of galaxies represent the most violent events in the Universe. The merger events initiate a complex chain of processes that leads to the dissipation of the collisional energy. This phase of violent relaxation is accompanied by turbulence and shock waves as well as non-thermal particle acceleration. This thesis aims at the interpretation of multi-wavelength observations of the merging cluster of galaxies Abell 3376 in the framework of a theoretical model of the involved effects. Observations with the Very Large Array radio interferometer were carried out and analyzed to clarify the morphology of the non-thermal particle distribution in Abell 3376, in particular about the shocked regions. The dissipation in the hot intra-cluster gas was studied using archival X-ray observations with ROSAT and XMM. Results were compared with constrained numerical simulations of the evolution of the merger process in the framework of cosmological structure formation. For this purpose, the ENZO-Code was employed for the computation of the gas dynamics and self-gravity of the colliding mass distribution. The non-thermal properties of the intra-cluster gas could be indirectly inferred from the local Mach number and the strength of the turbulence.},
  subject      = {Galaxienhaufen},
  language  = {en}
}
@phdthesis{Speckner2009,
  author    = {Speckner, Christian},
  title     = {LHC Phenomenology of the Three-Site Higgsless Model},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-45931},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {The Three-Site Higgsless Model is alternative implementation of electroweak symmetry breaking which in the Standard Model is mediated by the Higgs mechanism. The main features of this model is the appearance of two new heavy vector resonances W' and Z' with masses > 380 GeV as well as a set of new heavy fermions (> 1.8 TeV). In this model, unitarity of the amplitudes for the scattering of longitudinal gauge bosons is maintained by the exchange of the W' and Z' up to a scale of ~2 TeV. Consistency with the electroweak precision observables from the LEP / LEP-II experiments implies an exceedingly small coupling of the new vector bosons to the light Standard Model fermions (about 3\% of the isospin gauge coupling). In this thesis, the LHC phenomenology of this scenario is explored. To this end, we calculated the couplings and widths of all the new particles and implemented the model into the Monte-Carlo eventgenerator WHIZARD / O'Mega. With this implementation, we simulated the parton-level production of the gauge boson and fermion partners in different channels possibly suitable for their discovery at the LHC. The results are presented together with an introduction to the model and a discussion of its properties. We find that, while the fermiophobic nature of the new heavy gauge bosons does make them intrinsically difficult to observe at a collider, the LHC should be able to establish the existence of both resonances and even give some hints about the properties of their couplings which would be a vital test of the consistency of such a scenario. For the heavy fermions, we find that their large mass is accompanied by relative widths of more than \$10\\%\$, making them ill-suited for a direct discovery at the LHC. Nevertheless, our simulations reveal that there is a part of parameter space where, given enough time, patience and a good understanding of detector and backgrounds, a direct discovery might be possible.},
  subject      = {LHC},
  language  = {en}
}
@phdthesis{Boettcher2021,
  author    = {B{\"o}ttcher, Jan Frederic},
  title     = {Fate of Topological States of Matter in the Presence of External Magnetic Fields},
  doi       = {10.25972/OPUS-22045},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-220451},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {The quantum Hall (QH) effect, which can be induced in a two-dimensional (2D) electron gas by an external magnetic field, paved the way for topological concepts in condensed matter physics. While the QH effect can for that reason not exist without Landau levels, there is a plethora of topological phases of matter that can exist even in the absence of a magnetic field. For instance, the quantum spin Hall (QSH), the quantum anomalous Hall (QAH), and the three-dimensional (3D) topological insulator (TI) phase are insulating phases of matter that owe their nontrivial topology to an inverted band structure. The latter results from a strong spin-orbit interaction or, generally, from strong relativistic corrections. The main objective of this thesis is to explore the fate of these preexisting topological states of matter, when they are subjected to an external magnetic field, and analyze their connection to quantum anomalies. In particular, the realization of the parity anomaly in solid state systems is discussed. Furthermore, band structure engineering, i.e., changing the quantum well thickness, the strain, and the material composition, is employed to manipulate and investigate various topological properties of the prototype TI HgTe. Like the QH phase, the QAH phase exhibits unidirectionally propagating metallic edge channels. But in contrast to the QH phase, it can exist without Landau levels. As such, the QAH phase is a condensed matter analog of the parity anomaly. We demonstrate that this connection facilitates a distinction between QH and QAH states in the presence of a magnetic field. We debunk therefore the widespread belief that these two topological phases of matter cannot be distinguished, since they are both described by a \$\mathbb{Z}\$ topological invariant. To be more precise, we demonstrate that the QAH topology remains encoded in a peculiar topological quantity, the spectral asymmetry, which quantifies the differences in the number of states between the conduction and valence band. Deriving the effective action of QAH insulators in magnetic fields, we show that the spectral asymmetry is thereby linked to a unique Chern-Simons term which contains the information about the QAH edge states. As a consequence, we reveal that counterpropagating QH and QAH edge states can emerge when a QAH insulator is subjected to an external magnetic field. These helical-like states exhibit exotic properties which make it possible to disentangle QH and QAH phases. Our findings are of particular importance for paramagnetic TIs in which an external magnetic field is required to induce the QAH phase. A byproduct of the band inversion is the formation of additional extrema in the valence band dispersion at large momenta (the `camelback'). We develop a numerical implementation of the \$8 \times 8\$ Kane model to investigate signatures of the camelback in (Hg,Mn)Te quantum wells. Varying the quantum well thickness, as well as the Mn-concentration, we show that the class of topologically nontrivial quantum wells can be subdivided into direct gap and indirect gap TIs. In direct gap TIs, we show that, in the bulk \$p\$-regime, pinning of the chemical potential to the camelback can cause an onset to QH plateaus at exceptionally low magnetic fields (tens of mT). In contrast, in indirect gap TIs, the camelback prevents the observation of QH plateaus in the bulk \$p\$-regime up to large magnetic fields (a few tesla). These findings allowed us to attribute recent experimental observations in (Hg,Mn)Te quantum wells to the camelback. Although our discussion focuses on (Hg,Mn)Te, our model should likewise apply to other topological materials which exhibit a camelback feature in their valence band dispersion. Furthermore, we employ the numerical implementation of the \$8\times 8\$ Kane model to explore the crossover from a 2D QSH to a 3D TI phase in strained HgTe quantum wells. The latter exhibit 2D topological surface states at their interfaces which, as we demonstrate, are very sensitive to the local symmetry of the crystal lattice and electrostatic gating. We determine the classical cyclotron frequency of surface electrons and compare our findings with experiments on strained HgTe.},
  subject      = {Topologie},
  language  = {en}
}
@phdthesis{Wang2021,
  author    = {Wang, Zhenjiu},
  title     = {Numerical simulations of continuum field theories and exotic quantum phase transitions},
  doi       = {10.25972/OPUS-23800},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-238001},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {In this thesis, we investigate several topics pertaining to emergent collective quantum phenomena in the domain of correlated fermions, using the quantum Monte Carlo method. They display exotic low temperature phases as well as phase transitions which are beyond the Landau-Ginzburg theory. The interplay between three key points is crucial for us: fermion statistics, many body effects and topology. We highlight the following several achievements: 1. Successful modeling of continuum field theories with lattice Hamiltonians, 2. their sign-problem-free Monte Carlo simulations of these models, 3. and numerical results beyond mean field descriptions. First, we consider a model of Dirac fermions with a spin rotational invariant inter- action term that dynamically generates a quantum spin Hall insulator. Surprisingly, an s-wave superconducting phase emerges due to the condensation of topological de- fects of the spin Hall order parameter. When particle-hole symmetry is present, the phase transition between the topological insulator and the superconducting phase is an example of a deconfined quantum critical point(DQCP). Although its low energy effec- tive field theory is purely bosonic, the exact conservation law of the skyrmion number operator rules out the possibility of realizing this critical point in lattice boson models. This work is published in Ref. [1]. Second, we dope the dynamically generated quantum spin Hall insulator mentioned above. Hence it is described by a field theory without Lorentz invariance due to the lack of particle-hole symmetry. This sheds light on the extremely hot topic of twisted bilayergraphene: Why is superconductivity generated when the repulsive Coulomb interaction is much stronger than the electron-phonon coupling energy scale? In our case, Cooper pairs come from the topological skyrmion defects of the spin current order parameter, which are charged. Remarkably, the nature of the phase transition is highly non-mean-field-like: one is not allowed to simply view pairs of electrons as single bosons in a superfluid-Mott insulator transition, since the spin-current order parameter can not be ignored. Again, due to the aforementioned skyrmions, the two order parameters are intertwined: One phase transition occurs between the two symmetry breaking states. This work is summarized in Ref. [2]. Third, we investigate the 2 + 1 dimensional O(5) nonlinear sigma model with a topological Wess-Zumino-Witten term. Remarkably, we are able to perform Monte Carlo calculations with a UV cutoff given by the Dirac Landau level quantization. It is a successful example of simulating a continuous field theory without lattice regularization which leads to an additional symmetry breaking. The Dirac background and the five anti-commuting Dirac mass terms naturally introduce the picture of a non-trivial Berry phase contribution in the parameter space of the five component order parameter. Using the finite size scaling method given by the flux quantization, we find a stable critical phase in the low stiffness region of the sigma model. This is a candidate ground state of DQCP when the O(5) symmetry breaking terms are irrelevant at the critical point. Again, it has a bosonic low energy field theory which is seemingly unable to be realized in pure boson Hamiltonians. This work is summarized in Ref. [3].},
  subject      = {Quanten-Monte-Carlo-Methode},
  language  = {en}
}
@phdthesis{Klett2021,
  author    = {Klett, Michael},
  title     = {Auxiliary particle approach for strongly correlated electrons : How interaction shapes order},
  doi       = {10.25972/OPUS-24812},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-248121},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {Since the genesis of condensed matter physics, strongly correlated fermionic systems have shown a variety of fascinating properties and remain a vital topic in the field. Such systems arise through electronic interaction, and despite decades of intensive research, no holistic approach to solving this problem has been found. During that time, physicists have compiled a wealth of individual experimental and theoretical results, which together give an invaluable insight into these materials, and, in some instances, can explain correlated phenomena. However, there are several systems that stubbornly refuse to fall completely in line with current theoretical descriptions, among them the high-\( T_c{}\) cuprates and heavy fermion compounds. Although the two material classes have been around for the better part of the last 50 years, large portions of their respective phase diagram are still under intensive debate. Recent experiments in several electron-doped cuprates compounds, e.g. neodymium cerium copper oxide (Nd\(_{2x}\)Ce\(_x\)CuO\(_4\)), reveal a charge ordering about an antiferromagnetic ground state. So far, it has not been conclusively clarified how this intertwining of charge and spin polarization comes about and how it can be reconciled with a rigorous theoretical description. The heavy-fermion semimetals, on the other hand, have enjoyed renewed scientific interest with the discovery of topological Kondo insulators, a new material class offering a unique interface of topology, symmetry breaking, and correlated phenomena. In this context, samarium hexaboride (SmB\(_6\)) has emerged as a prototypical system, which may feature a topological ground state. In this thesis, we present a spin rotational invariant auxiliary particle approach to investigate the propensities of interacting electrons towards forming new states of order. In particular, we study the onset of spin and charge order in high-\( T_c{}\) cuprate systems and Kondo lattices, as well as the interplay of magnetism and topology. To that end, we use a sophisticated mean-field approximation of bosonic auxiliary particles augmented by a stability analysis of the saddle point via Gaussian fluctuations. The latter enables the derivation of dynamic susceptibilities, which describe the response of the system under external fields and offer a direct comparison to experiments. Both the mean-field and fluctuation formalisms require a numerical tool that is capable of extremizing the saddle point equations, on the one hand, and reliably solving a loop integral of the susceptibility-type, on the other. A full, from scratch derivation of the formalism tailored towards a software implementation, is provided and pedagogically reviewed. The auxiliary particle method allows for a rigorous description of incommensurate magnetic order and compares well to other established numerical and analytical techniques. Within our analysis, we employ the two-dimensional one-band Hubbard as well as the periodic Anderson model as minimal Hamiltonians for the high-\( T_c{}\) cuprates and Kondo systems, respectively. For the former, we observe a regime of intertwined charge- and spin-order in the electron-doped regime, which matches recent experimental observations in the cuprate material Nd\(_{2x}\)Ce\(_x\)CuO\(_4\). Furthermore, we localize the emergence of a Kondo regime in the periodic Anderson model and establish the magnetic phase diagram of the two-band model for topological Kondo insulators. The emerging antiferromagnetic ground state can be characterized by its topological properties and shows, for a non-trivial phase, topologically protected hinge modes.},
  subject      = {Festk{\"o}rpertheorie},
  language  = {en}
}
@phdthesis{Reinthaler2015,
  author    = {Reinthaler, Rolf Walter},
  title     = {Charge and Spin Transport in Topological Insulator Heterojunctions},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-135611},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Over the last decade, the field of topological insulators has become one of the most vivid areas in solid state physics. This novel class of materials is characterized by an insulating bulk gap, which, in two-dimensional, time-reversal symmetric systems, is closed by helical edge states. The latter make topological insulators promising candidates for applications in high fidelity spintronics and topological quantum computing. This thesis contributes to bringing these fascinating concepts to life by analyzing transport through heterostructures formed by two-dimensional topological insulators in contact with metals or superconductors. To this end, analytical and numerical calculations are employed. Especially, a generalized wave matching approach is used to describe the edge and bulk states in finite size tunneling junctions on the same footing. The numerical study of non-superconducting systems focuses on two-terminal metal/topological insulator/metal junctions. Unexpectedly, the conductance signals originating from the bulk and the edge contributions are not additive. While for a long junction, the transport is determined purely by edge states, for a short junction, the conductance signal is built from both bulk and edge states in a ratio, which depends on the width of the sample. Further, short junctions show a non-monotonic conductance as a function of the sample length, which distinguishes the topologically non-trivial regime from the trivial one. Surprisingly, the non-monotonic conductance of the topological insulator can be traced to the formation of an effectively propagating solution, which is robust against scalar disorder. The analysis of the competition of edge and bulk contributions in nanostructures is extended to transport through topological insulator/superconductor/topological insulator tunneling junctions. If the dimensions of the superconductor are small enough, its evanescent bulk modes can couple edge states at opposite sample borders, generating significant and tunable crossed Andreev reflection. In experiments, the latter process is normally disguised by simultaneous electron transmission. However, the helical edge states enforce a spatial separation of both competing processes for each Kramers' partner, allowing to propose an all-electrical measurement of crossed Andreev reflection. Further, an analytical study of the hybrid system of helical edge states and conventional superconductors in finite magnetic fields leads to the novel superconducting quantum spin Hall effect. It is characterized by edge states. Both the helicity and the protection against scalar disorder of these edge states are unaffected by an in-plane magnetic field. At the same time its superconducting gap and its magnetotransport signals can be tuned in weak magnetic fields, because the combination of helical edge states and superconductivity results in a giant g-factor. This is manifested in a non-monotonic excess current and peak splitting of the dI/dV characteristics as a function of the magnetic field. In consequence, the superconducting quantum spin Hall effect is an effective generator and detector for spin currents. The research presented here deepens the understanding of the competition of bulk and edge transport in heterostructures based on topological insulators. Moreover it proposes feasible experiments to all-electrically measure crossed Andreev reflection and to test the spin polarization of helical edge states.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@article{SchottdorfKeilCoppolaetal.2015,
  author    = {Schottdorf, Manuel and Keil, Wolfgang and Coppola, David and White, Leonard E. and Wolf, Fred},
  title     = {Random Wiring, Ganglion Cell Mosaics, and the Functional Architecture of the Visual Cortex},
  series = {PLoS Computational Biology},
  volume    = {11},
  journal   = {PLoS Computational Biology},
  number    = {11},
  doi       = {10.1371/journal.pcbi.1004602},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-138879},
  pages     = {e1004602},
  year      = {2015},
  abstract  = {The architecture of iso-orientation domains in the primary visual cortex (V1) of placental carnivores and primates apparently follows species invariant quantitative laws. Dynamical optimization models assuming that neurons coordinate their stimulus preferences throughout cortical circuits linking millions of cells specifically predict these invariants. This might indicate that V1's intrinsic connectome and its functional architecture adhere to a single optimization principle with high precision and robustness. To validate this hypothesis, it is critical to closely examine the quantitative predictions of alternative candidate theories. Random feedforward wiring within the retino-cortical pathway represents a conceptually appealing alternative to dynamical circuit optimization because random dimension-expanding projections are believed to generically exhibit computationally favorable properties for stimulus representations. Here, we ask whether the quantitative invariants of V1 architecture can be explained as a generic emergent property of random wiring. We generalize and examine the stochastic wiring model proposed by Ringach and coworkers, in which iso-orientation domains in the visual cortex arise through random feedforward connections between semi-regular mosaics of retinal ganglion cells (RGCs) and visual cortical neurons. We derive closed-form expressions for cortical receptive fields and domain layouts predicted by the model for perfectly hexagonal RGC mosaics. Including spatial disorder in the RGC positions considerably changes the domain layout properties as a function of disorder parameters such as position scatter and its correlations across the retina. However, independent of parameter choice, we find that the model predictions substantially deviate from the layout laws of iso-orientation domains observed experimentally. Considering random wiring with the currently most realistic model of RGC mosaic layouts, a pairwise interacting point process, the predicted layouts remain distinct from experimental observations and resemble Gaussian random fields. We conclude that V1 layout invariants are specific quantitative signatures of visual cortical optimization, which cannot be explained by generic random feedforward-wiring models.},
  language  = {en}
}
@article{RousochatzakisReutherThomaleetal.2015,
  author    = {Rousochatzakis, Ioannis and Reuther, Johannes and Thomale, Ronny and Rachel, Stephan and Perkins, N. B.},
  title     = {Phase Diagram and Quantum Order by Disorder in the Kitaev K\(_1\) - K\(_2\) Honeycomb Magnet},
  series = {Physical Review X},
  volume    = {5},
  journal   = {Physical Review X},
  number    = {041035},
  doi       = {10.1103/PhysRevX.5.041035},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-137235},
  year      = {2015},
  abstract  = {We show that the topological Kitaev spin liquid on the honeycomb lattice is extremely fragile against the second-neighbor Kitaev coupling K\(_2\), which has recently been shown to be the dominant perturbation away from the nearest-neighbor model in iridate Na\(_2\)IrO\(_3\), and may also play a role in \(\alpha\)-RuCl\(_3\) and Li\(_2\)IrO\(_3\). This coupling naturally explains the zigzag ordering (without introducing unrealistically large longer-range Heisenberg exchange terms) and the special entanglement between real and spin space observed recently in Na\(_2\)IrO\(_3\). Moreover, the minimal K\(_1\) - K\(_2\) model that we present here holds the unique property that the classical and quantum phase diagrams and their respective order-by-disorder mechanisms are qualitatively different due to the fundamentally different symmetries of the classical and quantum counterparts.},
  language  = {en}
}
@phdthesis{Schulz2016,
  author    = {Schulz, Robert Frank},
  title     = {A radio view of high-energy emitting AGNs},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-137358},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {The most energetic versions of active galactic nuclei (AGNs) feature two highly-relativistic plasma outflows, so-called jets, that are created in the vicinity of the central supermassive black hole and evolve in opposite directions. In blazars, which dominate the extragalactic gamma-ray sky, the jets are aligned close to the observer's line of sight leading to strong relativistic beaming effects of the jet emission. Radio observations especially using very long baseline interferometry (VLBI) provide the best way to gain direct information on the intrinsic properties of jets down to sub-parsec scales, close to their formation region. In this thesis, I focus on the properties of three AGNs, IC 310, PKS 2004-447, and 3C 111 that belong to the small non-blazar population of gamma-ray-loud AGNs. In these kinds of AGNs, the jets are less strongly aligned with respect to the observer than in blazars. I study them in detail with a variety of radio astronomical instruments with respect to their high-energy emission and in the context of the large samples in the monitoring programmes MOJAVE and TANAMI. My analysis of radio interferometric observations and flux density monitoring data reveal very different characteristics of the jet emission in these sources. The work presented in this thesis illustrates the diversity of the radio properties of gamma-ray-loud AGNs that do not belong to the dominating class of blazars.},
  subject      = {Aktiver galaktischer Kern},
  language  = {en}
}
@phdthesis{Pasold2016,
  author    = {Pasold, Christian},
  title     = {QCD and electroweak NLO corrections to W + Photon and Z + Photon production including leptonic decays},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-137456},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {At a hadron collider as the LHC or the Tevatron the production of a photon in association with a leptonically decaying vector boson represents an important class of processes. These processes stand out due to a very clean signal of a photon and two leptons. Furthermore they provide direct access to the photon-vector-boson couplings and thus an easy opportunity to test the gauge sector of the Standard Model. Within the scope of this work we present a full calculation of the next-to-leading-order corrections which include the O (αs) corrections of the strong interaction as well as the electroweak corrections of O (α) including all photon-induced contributions. For the creation of matrix elements we use methods based on Feynman diagrams. The IR singularities are treated with the dipole subtraction technique. In order to separate photons from jets, a quark-to-photon fragmentation function ´a la Glover / Morgan or Frixione's cone isolation is employed. Moreover, two different scenarios for charged leptons in the fi state were considered. The fi scenario for dressed leptons assumes that a charged lepton and a photon will be recombined if they are collinear. In the second scenario for bare muons it is assumed that leptons and photon can be separated in a detector also if they are collinear. For our calculation we implemented all corrections into a fl Monte Carlo program. Be- sides the computation of the total cross section this program is also able to generate diff tial distributions of several experimentally motivated observables. Apart from the expected large electroweak corrections in the high transverse-momentum regions and sizeable corrections in the resonance regions of the transverse or the invariant masses we found photon-induced corrections up to several 10\% for high transverse momenta. Within run I at the LHC for 7/8 TeV the experimental accuracy for Vγ production was roughly 10\%. Due to the higher luminosity at run II this accuracy will be reduced to the level of a few percent so that corrections of the same order within the theoretical predictions might become relevant. In this work we present results for the total cross section at the LHC for 7, 8 and 14 TeV and the corresponding distributions for 14 TeV.},
  subject      = {Quantenchromodynamik},
  language  = {en}
}
@phdthesis{Fuchs2016,
  author    = {Fuchs, Moritz Jakob},
  title     = {Spin dynamics in the central spin model: Application to graphene quantum dots},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-136079},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {Due to their potential application for quantum computation, quantum dots have attracted a lot of interest in recent years. In these devices single electrons can be captured, whose spin can be used to define a quantum bit (qubit). However, the information stored in these quantum bits is fragile due to the interaction of the electron spin with its environment. While many of the resulting problems have already been solved, even on the experimental side, the hyperfine interaction between the nuclear spins of the host material and the electron spin in their center remains as one of the major obstacles. As a consequence, the reduction of the number of nuclear spins is a promising way to minimize this effect. However, most quantum dots have a fixed number of nuclear spins due to the presence of group III and V elements of the periodic table in the host material. In contrast, group IV elements such as carbon allow for a variable size of the nuclear spin environment through isotopic purification. Motivated by this possibility, we theoretically investigate the physics of the central spin model in carbon based quantum dots. In particular, we focus on the consequences of a variable number of nuclear spins on the decoherence of the electron spin in graphene quantum dots. Since our models are, in many aspects, based upon actual experimental setups, we provide an overview of the most important achievements of spin qubits in quantum dots in the first part of this Thesis. To this end, we discuss the spin interactions in semiconductors on a rather general ground. Subsequently, we elaborate on their effect in GaAs and graphene, which can be considered as prototype materials. Moreover, we also explain how the central spin model can be described in terms of open and closed quantum systems and which theoretical tools are suited to analyze such models. Based on these prerequisites, we then investigate the physics of the electron spin using analytical and numerical methods. We find an intriguing thermal flip of the electron spin using standard statistical physics. Subsequently, we analyze the dynamics of the electron spin under influence of a variable number of nuclear spins. The limit of a large nuclear spin environment is investigated using the Nakajima-Zwanzig quantum master equation, which reveals a decoherence of the electron spin with a power-law decay on short timescales. Interestingly, we find a dependence of the details of this decay on the orientation of an external magnetic field with respect to the graphene plane. By restricting to a small number of nuclear spins, we are able to analyze the dynamics of the electron spin by exact diagonalization, which provides us with more insight into the microscopic details of the decoherence. In particular, we find a fast initial decay of the electron spin, which asymptotically reaches a regime governed by small fluctuations around a finite long-time average value. Finally, we analytically predict upper bounds on the size of these fluctuations in the framework of quantum thermodynamics.},
  subject      = {Elektronenspin},
  language  = {en}
}
@article{VarykhalovMarchenkoSanchezBarrigaetal.2012,
  author    = {Varykhalov, A. and Marchenko, D. and S{\´a}nchez-Barriga, J. and Scholz, M. R. and Verberck, B. and Trauzettel, B. and Wehling, T. O. and Carbone, C. and Rader, O.},
  title     = {Intact Dirac Cones at Broken Sublattice Symmetry: Photoemission Study of Graphene on Ni and Co},
  series = {Physical Review X},
  volume    = {2},
  journal   = {Physical Review X},
  number    = {041017},
  doi       = {10.1103/PhysRevX.2.041017},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-135732},
  year      = {2012},
  abstract  = {The appearance of massless Dirac fermions in graphene requires two equivalent carbon sublattices of trigonal shape. While the generation of an effective mass and a band gap at the Dirac point remains an unresolved problem for freestanding extended graphene, it is well established by breaking translational symmetry by confinement and by breaking sublattice symmetry by interaction with a substrate. One of the strongest sublattice-symmetry-breaking interactions with predicted and measured band gaps ranging from 400 meV to more than 3 eV has been attributed to the interfaces of graphene with Ni and Co, which are also promising spin-filter interfaces. Here, we apply angle-resolved photoemission to epitaxial graphene on Ni (111) and Co(0001) to show the presence of intact Dirac cones 2.8 eV below the Fermi level. Our results challenge the common belief that the breaking of sublattice symmetry by a substrate and the opening of the band gap at the Dirac energy are in a straightforward relation. A simple effective model of a biased bilayer structure composed of graphene and a sublattice-symmetry-broken layer, corroborated by density-functional-theory calculations, demonstrates the general validity of our conclusions.},
  language  = {en}
}
@phdthesis{Posske2015,
  author    = {Posske, Thore Hagen},
  title     = {Dressed Topological Insulators: Rashba Impurity, Kondo Effect, Magnetic Impurities, Proximity-Induced Superconductivity, Hybrid Systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131249},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Topological insulators are electronic phases that insulate in the bulk and accommodate a peculiar, metallic edge liquid with a spin-dependent dispersion. They are regarded to be of considerable future use in spintronics and for quantum computation. Besides determining the intrinsic properties of this rather novel electronic phase, considering its combination with well-known physical systems can generate genuinely new physics. In this thesis, we report on such combinations including topological insulators. Specifically, we analyze an attached Rashba impurity, a Kondo dot in the two channel setup, magnetic impurities on the surface of a strong three-dimensional topological insulator, the proximity coupling of the latter system to a superconductor, and hybrid systems consisting of a topological insulator and a semimetal. Let us summarize our primary results. Firstly, we determine an analytical formula for the Kondo cloud and describe its possible detection in current correlations far away from the Kondo region. We thereby rely on and extend the method of refermionizable points. Furthermore, we find a class of gapless topological superconductors and semimetals, which accommodate edge states that behave similarly to the ones of globally gapped topological phases. Unexpectedly, we also find edge states that change their chirality when affected by sufficiently strong disorder. We regard the presented research helpful in future classifications and applications of systems containing topological insulators, of which we propose some examples.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@phdthesis{BolanosRosales2016,
  author    = {Bola{\~n}os-Rosales, Alejandro},
  title     = {Low Mach Number Simulations of Convective Boundary Mixing in Classical Novae},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-132863},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {Classical novae are thermonuclear explosions occurring on the surface of white dwarfs. When co-existing in a binary system with a main sequence or more evolved star, mass accretion from the companion star to the white dwarf can take place if the companion overflows its Roche lobe. The envelope of hydrogen-rich matter which builds on top of the white dwarf eventually ignites under degenerate conditions, leading to a thermonuclear runaway and an explosion in the order of 1046 erg, while leaving the white dwarf intact. Spectral analyses from the debris indicate an abundance of isotopes that are tracers of nuclear burning via the hot CNO cycle, which in turn reveal some sort of mixing between the envelope and the white dwarf underneath. The exact mechanism is still a matter of debate. The convection and deflagration in novae develop in the low Mach number regime. We used the Seven League Hydro code (SLH ), which employs numerical schemes designed to correctly simulate low Mach number flows, to perform two and three- dimensional simulations of classical novae. Based on a spherically-symmetric model created with aid of a stellar evolution code, we developed our own nova model and tested it on a variety of numerical grids and boundary conditions for validation. We focused on the evolution of temperature, density and nuclear energy generation rate at the layers between white dwarf and envelope, where most of the energy is generated, to understand the structure of the transition region, and its effect on the nuclear burning. We analyzed the resulting dredge-up efficiency stemming from the convective motions in the envelope. Our models yield similar results to the literature, but seem to depend very strongly on the numerical resolution. We followed the evolution of the nuclear species involved in the CNO cycle and concluded that the thermonuclear reactions primarily taking place are those of the cold and not the hot CNO cycle. The reason behind this could be that under the conditions generally assumed for multi-dimensional simulations, the envelope is in fact not degenerate. We performed initial tests for 3D simulations and realized that alternative boundary conditions are needed.},
  subject      = {Nova},
  language  = {en}
}
@article{BechtleBringmannDeschetal.2012,
  author    = {Bechtle, Philip and Bringmann, Torsten and Desch, Klaus and Dreiner, Herbi and Hamer, Matthias and Hensel, Carsten and Kr{\"a}mer, Michael and Nguyen, Nelly and Porod, Werner and Prudent, Xavier and Sarrazin, Bj{\"o}rn and Uhlenbrock, Mathias and Wienemann, Peter},
  title     = {Constrained supersymmetry after two years of LHC data: a global view with Fittino},
  series = {Journal of High Energy Physics},
  volume    = {06},
  journal   = {Journal of High Energy Physics},
  number    = {098},
  doi       = {10.1007/JHEP06(2012)098},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129573},
  year      = {2012},
  abstract  = {We perform global fits to the parameters of the Constrained Minimal Super-symmetric Standard Model (CMSSM) and to a variant with non-universal Higgs masses (NUHM1). In addition to constraints from low-energy precision observables and the cosmological dark matter density, we take into account the LHC exclusions from searches in jets plus missing transverse energy signatures with about 5 fb\(^{-1}\) of integrated luminosity. We also include the most recent upper bound on the branching ratio B\(_s\)  → μμ from LHCb. Furthermore, constraints from and implications for direct and indirect dark matter searches are discussed. The best fit of the CMSSM prefers a light Higgs boson just above the experimentally excluded mass. We find that the description of the low-energy observables, (g - 2)\(_μ\) in particular, and the non-observation of SUSY at the LHC become more and more incompatible within the CMSSM. A potential SM-like Higgs boson with mass around 126 GeV can barely be accommodated. Values for B(B\(_s\)→μμ) just around the Standard Model prediction are naturally expected in the best fit region. The most-preferred region is not yet affected by limits on direct WIMP searches, but the next generation of experiments will probe this region. Finally, we discuss implications from fine-tuning for the best fit regions.},
  language  = {en}
}
@article{CamagoMolinaO'LearyPorodetal.2013,
  author    = {Camago-Molina, J.E. and O'Leary, B. and Porod, W. and Staub, F.},
  title     = {Vevacious: a tool for finding the global minima of one-loop effective potentials with many scalars},
  series = {European Physical Journal C},
  volume    = {73},
  journal   = {European Physical Journal C},
  number    = {2588},
  doi       = {10.1140/epjc/s10052-013-2588-2},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-132110},
  year      = {2013},
  abstract  = {Several extensions of the Standard Model of particle physics contain additional scalars implying a more complex scalar potential compared to that of the Standard Model. In general these potentials allow for charge- and/or color-breaking minima besides the desired one with correctly broken SU(2) L ×U(1) Y . Even if one assumes that a metastable local minimum is realized, one has to ensure that its lifetime exceeds that of our universe. We introduce a new program called Vevacious which takes a generic expression for a one-loop effective potential energy function and finds all the tree-level extrema, which are then used as the starting points for gradient-based minimization of the one-loop effective potential. The tunneling time from a given input vacuum to the deepest minimum, if different from the input vacuum, can be calculated. The parameter points are given as files in the SLHA format (though is not restricted to supersymmetric models), and new model files can be easily generated automatically by the Mathematica package SARAH. This code uses HOM4PS2 to find all the minima of the tree-level potential, PyMinuit to follow gradients to the minima of the one-loop potential, and CosmoTransitions to calculate tunneling times.},
  language  = {en}
}
@article{KilianOhlReuteretal.2012,
  author    = {Kilian, W. and Ohl, T. and Reuter, J. and Speckner, C.},
  title     = {QCD in the color-flow representation},
  series = {Journal of High Energy Physics},
  volume    = {10},
  journal   = {Journal of High Energy Physics},
  number    = {022},
  doi       = {10.1007/JHEP10(2012)022},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129583},
  year      = {2012},
  abstract  = {For many practical purposes, it is convenient to formulate unbroken non-abelian gauge theories like QCD in a color-flow basis. We present a new derivation of SU(N) interactions in the color-flow basis by extending the gauge group to U(N) × U(1)′ in such a way that the two U(1) factors cancel each other. We use the quantum action principles to show the equivalence to the usual basis to all orders in perturbation theory. We extend the known Feynman rules to exotic color representations (e.g. sextets) and discuss practical applications as they occur in automatic computation programs.},
  language  = {en}
}
@article{LangenfeldMochPfoh2012,
  author    = {Langenfeld, Ulrich and Moch, Sven-Olaf and Pfoh, Torsten},
  title     = {QCD threshold corrections for gluino pair production at hadron colliders},
  series = {Journal of High Energy Physics},
  volume    = {11},
  journal   = {Journal of High Energy Physics},
  number    = {070},
  doi       = {10.1007/JHEP11(2012)070},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129609},
  year      = {2012},
  abstract  = {We present the complete threshold enhanced predictions in QCD for the total cross section of gluino pair production at hadron colliders at next-to-next-to-leading order. Thanks to the computation of the required one-loop hard matching coefficients our results are accurate to the next-to-next-to-leading logarithm. In a brief phenomenological study we provide predictions for the total hadronic cross sections at the LHC and we discuss the uncertainties arising from scale variations and the parton distribution functions.},
  language  = {en}
}
@article{Winter2013,
  author    = {Winter, Walter},
  title     = {Long-baseline sensitivity studies and comparison (discussion session)},
  series = {Journal of Physics: Conference Series},
  volume    = {408},
  journal   = {Journal of Physics: Conference Series},
  number    = {012020},
  doi       = {10.1088/1742-6596/408/1/012020},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129440},
  year      = {2013},
  abstract  = {In this discussion session, the sensitivity and optimization of future long-baseline experiments is addressed, with a special emphasis on feasible projects and the description in terms of the error on the parameters. In addition, a statement on the precision interesting for \(ν_e → ν_τ\) and \(ν_μ → ν_τ\)oscillation measurements is obtained. A special topic is the impact of the recent T2K hint for non-zero \(θ_{13}\).},
  language  = {en}
}
@article{AssaadHerbut2013,
  author    = {Assaad, Fakher F. and Herbut, Igor F.},
  title     = {Pinning the Order: The Nature of Quantum Criticality in the Hubbard Model on Honeycomb Lattice},
  series = {Physical Review X},
  volume    = {3},
  journal   = {Physical Review X},
  number    = {031010},
  doi       = {10.1103/PhysRevX.3.031010},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129829},
  year      = {2013},
  abstract  = {In numerical simulations, spontaneously broken symmetry is often detected by computing two-point correlation functions of the appropriate local order parameter. This approach, however, computes the square of the local order parameter, and so when it is small, very large system sizes at high precisions are required to obtain reliable results. Alternatively, one can pin the order by introducing a local symmetrybreaking field and then measure the induced local order parameter infinitely far from the pinning center. The method is tested here at length for the Hubbard model on honeycomb lattice, within the realm of the projective auxiliary-field quantum Monte Carlo algorithm. With our enhanced resolution, we find a direct and continuous quantum phase transition between the semimetallic and the insulating antiferromagnetic states with increase of the interaction. The single-particle gap, measured in units of Hubbard U, tracks the staggered magnetization. An excellent data collapse is obtained by finite-size scaling, with the values of the critical exponents in accord with the Gross-Neveu universality class of the transition.},
  language  = {en}
}
@article{AssaadBercxHohenadler2013,
  author    = {Assaad, F. F. and Bercx, M. and Hohenadler, M.},
  title     = {Topological Invariant and Quantum Spin Models from Magnetic pi Fluxes in Correlated Topological Insulators},
  series = {Physical Review X},
  volume    = {3},
  journal   = {Physical Review X},
  number    = {1},
  doi       = {10.1103/PhysRevX.3.011015},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129849},
  year      = {2013},
  abstract  = {The adiabatic insertion of a \(\pi\) flux into a quantum spin Hall insulator gives rise to localized spin and charge fluxon states. We demonstrate that \(\pi\) fluxes can be used in exact quantum Monte Carlo simulations to identify a correlated \(Z_2\) topological insulator using the example of the Kane-Mele-Hubbard model. In the presence of repulsive interactions, a \(\pi\) flux gives rise to a Kramers doublet of spin-fluxon states with a Curie-law signature in the magnetic susceptibility. Electronic correlations also provide a bosonic mode of magnetic excitons with tunable energy that act as exchange particles and mediate a dynamical interaction of adjustable range and strength between spin fluxons. \(\pi\) fluxes can therefore be used to build models of interacting spins. This idea is applied to a three-spin ring and to one-dimensional spin chains. Because of the freedom to create almost arbitrary spin lattices, correlated topological insulators with \(\pi\) fluxes represent a novel kind of quantum simulator, potentially useful for numerical simulations and experiments.},
  language  = {en}
}
@article{GeisslerBudichTrauzettel2013,
  author    = {Geissler, F. and Budich, J. C. and Trauzettel, B.},
  title     = {Group theoretical and topological analysis of the quantum spin Hall effect in silicene},
  series = {New Journal of Physics},
  volume    = {15},
  journal   = {New Journal of Physics},
  number    = {085030},
  doi       = {10.1088/1367-2630/15/8/085030},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129732},
  year      = {2013},
  abstract  = {Silicene consists of a monolayer of silicon atoms in a buckled honeycomb structure. It was recently discovered that the symmetry of such a system allows for interesting Rashba spin-orbit effects. A perpendicular electric field is able to couple to the sublattice pseudospin, making it possible to electrically tune and close the band gap. Therefore, external electric fields may generate a topological phase transition from a topological insulator to a normal insulator (or semimetal) and vice versa. The contribution of the present paper to the study of silicene is twofold. Firstly, we perform a group theoretical analysis to systematically construct the Hamiltonian in the vicinity of the K points of the Brillouin zone and find an additional, electric field induced spin-orbit term, that is allowed by symmetry. Subsequently, we identify a tight-binding model that corresponds to the group theoretically derived Hamiltonian near the K points. Secondly, we start from this tight-binding model to analyze the topological phase diagram of silicene by an explicit calculation of the Z2 topological invariant of the band structure. To this end, we calculate the Z2 topological invariant of the honeycomb lattice in a manifestly gauge invariant way which allows us to include Sz symmetry breaking terms—like Rashba spin-orbit interaction—into the topological analysis. Interestingly, we find that the interplay of a Rashba and an intrinsic spin-orbit term can generate a non-trivial quantum spin Hall phase in silicene. This is in sharp contrast to the more extensively studied honeycomb system graphene where Rashba spin-orbit interaction is known to compete with the quantum spin Hall effect in a detrimental way.},
  language  = {en}
}
@article{BudichTrauzettel2013,
  author    = {Budich, Jan Carl and Trauzettel, Bj{\"o}rn},
  title     = {Z(2) Green's function topology of Majorana wires},
  series = {New Journal of Physics},
  volume    = {15},
  journal   = {New Journal of Physics},
  number    = {065006},
  doi       = {10.1088/1367-2630/15/6/065006},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129751},
  year      = {2013},
  abstract  = {We represent the Z2 topological invariant characterizing a one-dimensional topological superconductor using a Wess-Zumino-Witten dimensional extension. The invariant is formulated in terms of the single-particle Green's function which allows us to classify interacting systems. Employing a recently proposed generalized Berry curvature method, the topological invariant is represented independent of the extra dimension requiring only the single-particle Green's function at zero frequency of the interacting system. Furthermore, a modified twisted boundary conditions approach is used to rigorously define the topological invariant for disordered interacting systems.},
  language  = {en}
}
@phdthesis{Brehm2009,
  author    = {Brehm, Sascha},
  title     = {Two-Particle Excitations in the Hubbard Model for High-Temperature Superconductors: A Quantum Cluster Study},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38719},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {Two-particle excitations, such as spin and charge excitations, play a key role in high-Tc cuprate superconductors (HTSC). Due to the antiferromagnetism of the parent compound the magnetic excitations are supposed to be directly related to the mechanism of superconductivity. In particular, the so-called resonance mode is a promising candidate for the pairing glue, a bosonic excitation mediating the electronic pairing. In addition, its interactions with itinerant electrons may be responsible for some of the observed properties of HTSC. Hence, getting to the bottom of the resonance mode is crucial for a deeper understanding of the cuprate materials . To analyze the corresponding two-particle correlation functions we develop in the present thesis a new, non-perturbative and parameter-free technique for T=0 which is based on the Variational Cluster Approach (VCA, an embedded cluster method for one-particle Green's functions). Guided by the spirit of the VCA we extract an effective electron-hole vertex from an isolated cluster and use a fully renormalized bubble susceptibility chi0 including the VCA one-particle propagators.Within our new approach, the magnetic excitations of HTSC are shown to be reproduced for the Hubbard model within the relevant strong-coupling regime. Exceptionally, the famous resonance mode occurring in the underdoped regime within the superconductivity-induced gap of spin-flip electron-hole excitations is obtained. Its intensity and hourglass dispersion are in good overall agreement with experiments. Furthermore, characteristic features such as the position in energy of the resonance mode and the difference of the imaginary part of the susceptibility in the superconducting and the normal states are in accord with Inelastic Neutron Scattering (INS) experiments. For the first time, a strongly-correlated parameter-free calculation revealed these salient magnetic properties supporting the S=1 magnetic exciton scenario for the resonance mode. Besides the INS data on magnetic properties further important new insights were gained recently via ARPES (Angle-Resolved Photoemission-Spectroscopy) and Raman experiments which disclosed a quite different doping dependence of the antinodal compared to the near-nodal gap. This thesis provides an approach to the Raman response similar to the magnetic case for inspecting this gap dichotomy. In agreement with experiments and one-particle data obtained in the VCA, we recover the antinodal gap decreasing and the near-nodal gap increasing as a function of doping. Hence, our results prove the Hubbard model to account for these salient gap features. In summary, we develop a two-particle cluster approach which is appropriate for the strongly-correlated regime and contains no free parameter. Our results obtained with this new approach combined with the phase diagram and the one-particle excitations obtained in the VCA strongly constitute a Hubbard model description of HTSC cuprate materials.},
  subject      = {Hochtemperatursupraleiter},
  language  = {en}
}
@phdthesis{Plentinger2009,
  author    = {Plentinger, Florian},
  title     = {Systematic Model Building with Flavor Symmetries},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38077},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {Die Beobachtung von Neutrinomassen und Leptonenmischungen haben gezeigt, dass das Standard-Modell unvollst{\"a}ndig ist. Im Zuge dieser Entdeckung tauchen neue Fragestellungen auf: warum sind die Neutrinomassen so klein, wie sieht ihre Massenhierarchie aus, warum sind die Mischungen im Quark- und Leptonen-Sektor so unterschiedlich oder welche Form hat der Higgs-Sektor. Um diese Fragen zu beantworten und um zuk{\"u}nftige experimentelle Daten vorherzusagen, werden verschiedene Ans{\"a}tze betrachtet. Besonders interessant sind Grand Unified Theories, wie SU(5) oder SO(10). GUTs sind vertikale Symmetrien, da sie die SM-Teilchen in Multipletts vereinheitlichen und {\"u}blicherweise neue Elementarteilchen vorhersagen, die durch den Seesaw-Mechanismus, auf nat{\"u}rliche Weise die Kleinheit der Neutrinomassen erkl{\"a}ren. Dar{\"u}berhinaus sind auch horizontale Symmetrien, d.h. Flavor-Symmetrien, welche auf den Generationen-Raum der SM-Teilchen wirken, interessant. Sie k{\"o}nnen die Quark- und Leptonen-Massenhierarchien, sowie die unterschiedlichen Quark- und Leptonenmischungen, erkl{\"a}ren. Ausserdem beeinflussen Flavor-Symmetrien massgeblich den Higgs-Sektor und sagen bestimmte Formen von Massenmatrizen vorher. Diese hohe Vorhersagekraft machen GUTs und Flavor-Symmetrien sowohl f{\"u}r Theoretiker, als auch f{\"u}r Experimentalphysiker interessant. Solche Erweiterungen des SM k{\"o}nnen mit weiteren Konzepten wie Supersymmetrie oder extra Dimensionen kombiniert werden. Hinzu kommt, dass sie f{\"u}r gew{\"o}hnlich Auswirkungen auf die beobachtete Materie-Antimaterie Asymmetrie des Universums haben und einen dunkle Materie Kandidaten beinhalten k{\"o}nnen. Im Allgemeinen sagen sie auch die seltene Leptonenzahl verletzenden Zerf{\"a}lle mu -> e gamma, tau -> mu gamma und tau -> e gamma vorher, die stark von Experimenten eingeschr{\"a}nkt sind, aber m{\"o}glicherweise in der Zukunft beobachtet werden. In dieser Arbeit kombinieren wir all diese Zug{\"a}nge, d.h. GUTs, den Seesaw-Mechanismus und Flavor-Symmetrien. Dr{\"u}ber hinaus ist unser Anliegen einen systematischen Zugang zum Modellbau zu entwickeln und durchzuf{\"u}hren, sowie die Suche nach ph{\"a}nomenologischen Implikationen. Dies stellt eine neue Sichtweise im Modellbau dar, da es uns erlaubt bestimmte Modelle durch ihre theoretischen und ph{\"a}nomenologischen Vorhersagen zu filtern. D.h. wir k{\"o}nnen weitere Einschr{\"a}nkungen an Modelle fordern, um ein bestimmtes auszuw{\"a}hlen. Die Ergebnisse unserer Herangehensweise sind zum Beispiel mannigfaltige Leptonen-Flavor- und GUT-Modelle, ein systematischer Scan von Leptonenzahl verletzenden Prozessen, neue Massenmatrizen, eine neues Vest{\"a}ndnis der Leptonenmischungswinkel, eine Verallgemeinerung der Idee der Quark-Leptonen-Komplementarit{\"a}t theta_12=pi/4-epsilon/sqrt{2} und zum ersten Mal die QLC-Relation in einer SU(5) GUT.},
  subject      = {Symmetrie},
  language  = {en}
}
@phdthesis{Csallner2006,
  author    = {Csallner, Sigrun},
  title     = {Produktion und Nachweis schwerer Selektronen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-22433},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2006},
  abstract  = {Wir studieren die Produktion und den Nachweis von Selektronen mit Massen jenseits der Schwelle zur Paarerzeugung an k{\"u}nftigen Linearbeschleunigern mit Schwerpunktsenergien von 500 GeV und 800 GeV. Hierzu betrachten wir die Produktion von linken und rechten Selektronen in Assoziation mit dem jeweils leichtesten Neutralino oder Chargino durch Elektron-Elektron-, Elektron-Positron- und Elektron-Photon-Streuung im Rahmen des MSSM. Die Produktion durch Elektron-Elektron-Streuung untersuchen wir zus{\"a}tzlich in zwei erweiterten Modellen, dem NMSSM und einem E6-Modell mit einem zus{\"a}tzlichen U(1)-Eichfaktor.},
  subject      = {Linearbeschleuniger},
  language  = {de}
}
@phdthesis{Elbracht2009,
  author    = {Elbracht, Oliver},
  title     = {Wave Extraction in Numerical Relativity},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-40672},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {Diese Arbeit konzentriert sich auf eine fundamentale Problematik der numerischen Relativit{\"a}tstheorie: Die Extraktion von Gravitationswellen in einer eich- und koordinateninvarianten Formulierung, um ein physikalisch interpretierbares Objekt zu erhalten. Es wird eine neue Methodik entwickelt, um die physikalisch relevanten Gr{\"o}ßen aus einer numerisch erzeugten Raumzeit zu extrahieren. Wir pr{\"a}sentieren eine allgemeing{\"u}ltige kanonische Formulierung der Weyl Skalare im Newman-Penrose Formalismus als eine Funktion von fundamentalen Raumzeit-Invarianten. Dadurch zeigt sich, dass mit Hilfe dieser Methodik die explizite Konstruktion eines Vierbeins vollst{\"a}ndig redundant ist. Als weiteren Schwerpunkt charakterisieren wir innerhalb des Newman-Penrose Formalismus eine spezielle Untergruppe von Tetraden, die transversen Frames. Es wird eine bisher unbekannte Verbindung zwischen den prim{\"a}r genutzen Vierbeinen f{\"u}r die Extraktion der Wellenform abgeleitet, dem Gram-Schmidt Vierbein und dem quasi-Kinnersley Vierbein. Abschliessend studieren wir die Abh{\"a}ngigkeit der Gravitationswellen eines gest{\"o}rten Schwarzen Loches vom verwendeten Vierbein. Wir berechnen die Form der Gravitationswellen in dieser Raumzeit und demonstrieren inwieweit unsere neue Methodik robustere und exaktere Ergebnisse liefert, als die gew{\"o}hnlich verwendeten Ans{\"a}tze zur Extraktion des Signals.},
  subject      = {Allgemeine Relativit{\"a}tstheorie},
  language  = {en}
}
@phdthesis{Schmidt2008,
  author    = {Schmidt, Manuel J.},
  title     = {Replica Symmetry Breaking at Low Temperatures},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30660},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {In this thesis, the low-temperature regime of replica symmetry breaking in the SK-model has been thoroughly investigated. In order to access this regime and to perform self-consistence calculations with high accuracy at high orders of replica symmetry breaking, a formalism has been developed which reduces the numerical effort to the absolute minimum. The central idea of its derivation is the identification of asymptotic regions in which the recursion relations can be solved analytically. The new object in the numerical treatment is then the correction to this asymptotic regime, represented by a sequence of so-called kernel correction functions. This method increased the effciency of the numerics considerably so that up to 200 orders of RSB could be calculated at zero temperature and zero external field, and up to 60 (65) orders of RSB for finite temperature (external field). The remarkable high precision of these calculations allowed the extraction of several quantities with accuracy exceeding the literature values by several orders of magnitude. The results of the numerical calculations have been analyzed in great detail. Especially the convergence behavior of various observables and of the order function with respect to the RSB order has been investigated since the high but finite RSB regime has been addressed in the present work for the first time. Several unexpected features of finite order replica symmetry breaking have been observed.},
  subject      = {Spin-Spin-Wechselwirkung},
  language  = {en}
}
@phdthesis{Maier2008,
  author    = {Maier, Andreas},
  title     = {Adaptively Refined Large-Eddy Simulations of Galaxy Clusters},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-32274},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {It is aim of this work to develop, implement, and apply a new numerical scheme for modeling turbulent, multiphase astrophysical flows such as galaxy cluster cores and star forming regions. The method combines the capabilities of adaptive mesh refinement (AMR) and large-eddy simulations (LES) to capture localized features and to represent unresolved turbulence, respectively; it will be referred to as Fluid mEchanics with Adaptively Refined Large-Eddy SimulationS or FEARLESS.},
  subject      = {Turbulenz},
  language  = {en}
}
@phdthesis{Berger2009,
  author    = {Berger, Karsten},
  title     = {Discovery and Characterization of the first Low-Peaked and Intermediate-Peaked BL Lacertae Objects in the Very High Energy Gamma-Ray Regime},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37431},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {20 years after the discovery of the Crab Nebula as a source of very high energy gamma-rays, the number of sources newly discovered above 100 GeV using ground-based Cherenkov telescopes has considerably grown, at the time of writing of this thesis to a total of 81. The sources are of different types, including galactic sources such as supernova remnants, pulsars, binary systems, or so-far unidentified accelerators and extragalactic sources such as blazars and radio galaxies. The goal of this thesis work was to search for gamma-ray emission from a particular type of blazars previously undetected at very high gamma-ray energies, by using the MAGIC telescope. Those blazars previously detected were all of the same type, the so-called high-peaked BL Lacertae objects. The sources emit purely non-thermal emission, and exhibit a peak in their radio-to-X-ray spectral energy distribution at X-ray energies. The entire blazar population extends from these rare, low-luminosity BL Lacertae objects with peaks at X-ray energies to the much more numerous, high-luminosity infrared-peaked radio quasars. Indeed, the low-peaked sources dominate the source counts obtained from space-borne observations at gamma-ray energies up to 10 GeV. Their spectra observed at lower gamma-ray energies show power-law extensions to higher energies, although theoretical models suggest them to turn over at energies below 100 GeV. This opened the quest for MAGIC as the Cherenkov telescope with the currently lowest energy threshold. In the framework of this thesis, the search was focused on the prominent sources BL Lac, W Comae and S5 0716+714, respectively. Two of the sources were unambiguously discovered at very high energy gamma-rays with the MAGIC telescope, based on the analysis of a total of about 150 hours worth of data collected between 2005 and 2008. The analysis of this very large data set required novel techniques for treating the effects of twilight conditions on the data quality. This was successfully achieved and resulted in a vastly improved performance of the MAGIC telescope in monitoring campaigns. The detections of low-peaked and intermediate-peaked BL Lac objects are in line with theoretical expectations, but push the models based on electron shock acceleration and inverse-Compton cooling to their limits. The short variability time scales of the order of one day observed at very high energies show that the gamma-rays originate rather close to the putative supermassive black holes in the centers of blazars, corresponding to less than 1000 Schwarzschild radii when taking into account relativistic bulk motion.},
  subject      = {Aktiver galaktischer Kern},
  language  = {en}
}
@article{RoyAssaadHerbut2014,
  author    = {Roy, Bitan and Assaad, Fakher F. and Herbut, Igor F.},
  title     = {Zero Modes and Global Antiferromagnetism in Strained Graphene},
  series = {Physical Review X},
  volume    = {4},
  journal   = {Physical Review X},
  number    = {2},
  issn      = {2160-3308},
  doi       = {10.1103/PhysRevX.4.021042},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-116108},
  pages     = {21042},
  year      = {2014},
  abstract  = {A novel magnetic ground state is reported for the Hubbard Hamiltonian in strained graphene. When the chemical potential lies close to the Dirac point, the ground state exhibits locally both the Neel and ferromagnetic orders, even for weak Hubbard interaction. Whereas the Neel order parameter remains of the same sign in the entire system, the magnetization at the boundary takes the opposite sign from the bulk. The total magnetization vanishes this way, and the magnetic ground state is globally only an antiferromagnet. This peculiar ordering stems from the nature of the strain-induced single-particle zero-energy states, which have support on one sublattice of the honeycomb lattice in the bulk, and on the other sublattice near the boundary of a finite system. We support our claim with the self-consistent numerical calculation of the order parameters, as well as by the Monte Carlo simulations of the Hubbard model in both uniformly and nonuniformly strained honeycomb lattice. The present result is contrasted with the magnetic ground state of the same Hubbard model in the presence of a true magnetic field (and for vanishing Zeeman coupling), which is exclusively Neel ordered, with zero local magnetization everywhere in the system.},
  language  = {en}
}
@phdthesis{Bercx2014,
  author    = {Bercx, Martin Helmut},
  title     = {Numerical studies of heavy-fermion systems and correlated topological insulators},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-116138},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {In this thesis, we investigate aspects of the physics of heavy-fermion systems and correlated topological insulators. We numerically solve the interacting Hamiltonians that model the physical systems using quantum Monte Carlo algorithms to access both ground-state and finite-temperature observables. Initially, we focus on the metamagnetic transition in the Kondo lattice model for heavy fermions. On the basis of the dynamical mean-field theory and the dynamical cluster approximation, our calculations point towards a continuous transition, where the signatures of metamagnetism are linked to a Lifshitz transition of heavy-fermion bands. In the second part of the thesis, we study various aspects of magnetic pi fluxes in the Kane-Mele-Hubbard model of a correlated topological insulator. We describe a numerical measurement of the topological index, based on the localized mid-gap states that are provided by pi flux insertions. Furthermore, we take advantage of the intrinsic spin degree of freedom of a pi flux to devise instances of interacting quantum spin systems. In the third part of the thesis, we introduce and characterize the Kane-Mele-Hubbard model on the pi flux honeycomb lattice. We place particular emphasis on the correlations effects along the one-dimensional boundary of the lattice and compare results from a bosonization study with finite-size quantum Monte Carlo simulations.},
  subject      = {Schwere-Fermionen-System},
  language  = {en}
}
@phdthesis{Winkler2015,
  author    = {Winkler, Marco},
  title     = {On the Role of Triadic Substructures in Complex Networks},
  publisher = {epubli GmbH},
  address   = {Berlin},
  isbn      = {978-3-7375-5654-5},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-116022},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {In the course of the growth of the Internet and due to increasing availability of data, over the last two decades, the field of network science has established itself as an own area of research. With quantitative scientists from computer science, mathematics, and physics working on datasets from biology, economics, sociology, political sciences, and many others, network science serves as a paradigm for interdisciplinary research. One of the major goals in network science is to unravel the relationship between topological graph structure and a network's function. As evidence suggests, systems from the same fields, i.e. with similar function, tend to exhibit similar structure. However, it is still vague whether a similar graph structure automatically implies likewise function. This dissertation aims at helping to bridge this gap, while particularly focusing on the role of triadic structures. After a general introduction to the main concepts of network science, existing work devoted to the relevance of triadic substructures is reviewed. A major challenge in modeling triadic structure is the fact that not all three-node subgraphs can be specified independently of each other, as pairs of nodes may participate in multiple of those triadic subgraphs. In order to overcome this obstacle, we suggest a novel class of generative network models based on so called Steiner triple systems. The latter are partitions of a graph's vertices into pair-disjoint triples (Steiner triples). Thus, the configurations on Steiner triples can be specified independently of each other without overdetermining the network's link structure. Subsequently, we investigate the most basic realization of this new class of models. We call it the triadic random graph model (TRGM). The TRGM is parametrized by a probability distribution over all possible triadic subgraph patterns. In order to generate a network instantiation of the model, for all Steiner triples in the system, a pattern is drawn from the distribution and adjusted randomly on the Steiner triple. We calculate the degree distribution of the TRGM analytically and find it to be similar to a Poissonian distribution. Furthermore, it is shown that TRGMs possess non-trivial triadic structure. We discover inevitable correlations in the abundance of certain triadic subgraph patterns which should be taken into account when attributing functional relevance to particular motifs - patterns which occur significantly more frequently than expected at random. Beyond, the strong impact of the probability distributions on the Steiner triples on the occurrence of triadic subgraphs over the whole network is demonstrated. This interdependence allows us to design ensembles of networks with predefined triadic substructure. Hence, TRGMs help to overcome the lack of generative models needed for assessing the relevance of triadic structure. We further investigate whether motifs occur homogeneously or heterogeneously distributed over a graph. Therefore, we study triadic subgraph structures in each node's neighborhood individually. In order to quantitatively measure structure from an individual node's perspective, we introduce an algorithm for node-specific pattern mining for both directed unsigned, and undirected signed networks. Analyzing real-world datasets, we find that there are networks in which motifs are distributed highly heterogeneously, bound to the proximity of only very few nodes. Moreover, we observe indication for the potential sensitivity of biological systems to a targeted removal of these critical vertices. In addition, we study whole graphs with respect to the homogeneity and homophily of their node-specific triadic structure. The former describes the similarity of subgraph distributions in the neighborhoods of individual vertices. The latter quantifies whether connected vertices are structurally more similar than non-connected ones. We discover these features to be characteristic for the networks' origins. Moreover, clustering the vertices of graphs regarding their triadic structure, we investigate structural groups in the neural network of C. elegans, the international airport-connection network, and the global network of diplomatic sentiments between countries. For the latter we find evidence for the instability of triangles considered socially unbalanced according to sociological theories. Finally, we utilize our TRGM to explore ensembles of networks with similar triadic substructure in terms of the evolution of dynamical processes acting on their nodes. Focusing on oscillators, coupled along the graphs' edges, we observe that certain triad motifs impose a clear signature on the systems' dynamics, even when embedded in a larger network structure.},
  subject      = {Netzwerk},
  language  = {en}
}
@article{SchottdorfEglenWolfetal.2014,
  author    = {Schottdorf, Manuel and Eglen, Stephen J. and Wolf, Fred and Keil, Wolfgang},
  title     = {Can Retinal Ganglion Cell Dipoles Seed Iso-Orientation Domains in the Visual Cortex?},
  series = {PLOS ONE},
  volume    = {9},
  journal   = {PLOS ONE},
  number    = {1},
  issn      = {1932-6203},
  doi       = {10.1371/journal.pone.0086139},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117558},
  pages     = {e86139},
  year      = {2014},
  abstract  = {It has been argued that the emergence of roughly periodic orientation preference maps (OPMs) in the primary visual cortex (V1) of carnivores and primates can be explained by a so-called statistical connectivity model. This model assumes that input to V1 neurons is dominated by feed-forward projections originating from a small set of retinal ganglion cells (RGCs). The typical spacing between adjacent cortical orientation columns preferring the same orientation then arises via Moire 'Interference between hexagonal ON/OFF RGC mosaics. While this Moire-Interference critically depends on long-range hexagonal order within the RGC mosaics, a recent statistical analysis of RGC receptive field positions found no evidence for such long-range positional order. Hexagonal order may be only one of several ways to obtain spatially repetitive OPMs in the statistical connectivity model. Here, we investigate a more general requirement on the spatial structure of RGC mosaics that can seed the emergence of spatially repetitive cortical OPMs, namely that angular correlations between so-called RGC dipoles exhibit a spatial structure similar to that of OPM autocorrelation functions. Both in cat beta cell mosaics as well as primate parasol receptive field mosaics we find that RGC dipole angles are spatially uncorrelated. To help assess the level of these correlations, we introduce a novel point process that generates mosaics with realistic nearest neighbor statistics and a tunable degree of spatial correlations of dipole angles. Using this process, we show that given the size of available data sets, the presence of even weak angular correlations in the data is very unlikely. We conclude that the layout of ON/OFF ganglion cell mosaics lacks the spatial structure necessary to seed iso-orientation domains in the primary visual cortex.},
  language  = {en}
}
@article{HansmannParraghToschietal.2014,
  author    = {Hansmann, P. and Parragh, N. and Toschi, A. and Sangiovanni, G. and Held, K.},
  title     = {Importance of d-p Coulomb interaction for high T-C cuprates and other oxides},
  series = {New Journal of Physics},
  volume    = {16},
  journal   = {New Journal of Physics},
  number    = {33009},
  issn      = {1367-2630},
  doi       = {10.1088/1367-2630/16/3/033009},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117165},
  year      = {2014},
  abstract  = {Current theoretical studies of electronic correlations in transition metal oxides typically only account for the local repulsion between d-electrons even if oxygen ligand p-states are an explicit part of the effective Hamiltonian. Interatomic interactions such as U-pd between d- and (ligand) p-electrons, as well as the local interaction between p-electrons, are neglected. Often, the relative d-p orbital splitting has to be adjusted 'ad hoc' on the basis of the experimental evidence. By applying the merger of local density approximation and dynamical mean field theory to the prototypical case of the three-band Emery dp model for the cuprates, we demonstrate that, without any 'ad hoc' adjustment of the orbital splitting, the charge transfer insulating state is stabilized by the interatomic interaction U-pd. Our study hence shows how to improve realistic material calculations that explicitly include the p-orbitals.},
  language  = {en}
}
@article{BrueneThienelStuiberetal.2014,
  author    = {Br{\"u}ne, Christoph and Thienel, Cornelius and Stuiber, Michael and B{\"o}ttcher, Jan and Buhmann, Hartmut and Novik, Elena G. and Liu, Chao-Xing and Hankiewicz, Ewelina M. and Molenkamp, Laurens W.},
  title     = {Dirac-Screening Stabilized Surface-State Transport in a Topological Insulator},
  series = {Physical Review X},
  volume    = {4},
  journal   = {Physical Review X},
  number    = {4},
  issn      = {2160-3308},
  doi       = {10.1103/PhysRevX.4.041045},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-118091},
  pages     = {041045},
  year      = {2014},
  abstract  = {We report magnetotransport studies on a gated strained HgTe device. This material is a three-dimensional topological insulator and exclusively shows surface-state transport. Remarkably, the Landau-level dispersion and the accuracy of the Hall quantization remain unchanged over a wide density range (3×1011  cm-2<n<2×1012  cm-2). These observations imply that even at large carrier densities, the transport is surface-state dominated, where bulk transport would have been expected to coexist already. Moreover, the density dependence of the Dirac-type quantum Hall effect allows us to identify the contributions from the individual surfaces. A k⋅p model can describe the experiments but only when assuming a steep band bending across the regions where the topological surface states are contained. This steep potential originates from the specific screening properties of Dirac systems and causes the gate voltage to influence the position of the Dirac points rather than that of the Fermi level.},
  language  = {en}
}