TY - JOUR A1 - Skryabin, D.V. A1 - Kartashov, Y.V. A1 - Egorov, O.A. A1 - Sich, M. A1 - Chana, J.K. A1 - Tapia Rodriguez, L.E. A1 - Walker, P.M. A1 - Clarke, E. A1 - Royall, B. A1 - Skolnick, M.S. A1 - Krizhanovskii, D.N. T1 - Backward Cherenkov radiation emitted by polariton solitons in a microcavity wire JF - Nature Communications N2 - Exciton-polaritons in semiconductor microcavities form a highly nonlinear platform to study a variety of effects interfacing optical, condensed matter, quantum and statistical physics. We show that the complex polariton patterns generated by picosecond pulses in microcavity wire waveguides can be understood as the Cherenkov radiation emitted by bright polariton solitons, which is enabled by the unique microcavity polariton dispersion, which has momentum intervals with positive and negative group velocities. Unlike in optical fibres and semiconductor waveguides, we observe that the microcavity wire Cherenkov radiation is predominantly emitted with negative group velocity and therefore propagates backwards relative to the propagation direction of the emitting soliton. We have developed a theory of the microcavity wire polariton solitons and of their Cherenkov radiation and conducted a series of experiments, where we have measured polariton-soliton pulse compression, pulse breaking and emission of the backward Cherenkov radiation. KW - physics KW - polaritons KW - solitons Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-173046 VL - 8 ER - TY - JOUR A1 - Rudno-Rudziński, W. A1 - Syperek, M. A1 - Andrezejewski, J. A1 - Maryński, A. A1 - Misiewicz, J. A1 - Somers, A. A1 - Höfling, S. A1 - Reithmaier, J. P. A1 - Sęk, G. T1 - Carrier delocalization in InAs/InGaAlAs/InP quantum-dash-based tunnel injection system for 1.55 μm emission JF - AIP Advances N2 - We have investigated optical properties of hybrid two-dimensional-zero-dimensional (2D-0D) tunnel structures containing strongly elongated InAs/InP(001) quantum dots (called quantum dashes), emitting at 1.55 μm. These quantum dashes (QDashes) are separated by a 2.3 nm-width barrier from an InGaAs quantum well (QW), lattice matched to InP. We have tailored quantum-mechanical coupling between the states confined in QDashes and a QW by changing the QW thickness. By combining modulation spectroscopy and photoluminescence excitation, we have determined the energies of all relevant optical transitions in the system and proven the carrier transfer from the QW to the QDashes, which is the fundamental requirement for the tunnel injection scheme. A transformation between 0D and mixed-type 2D-0D character of an electron and a hole confinement in the ground state of the hybrid system have been probed by time-resolved photoluminescence that revealed considerable changes in PL decay time with the QW width changes. The experimental discoveries have been explained by band structure calculations in the framework of the eight-band k·p model showing that they are driven by delocalization of the lowest energy hole state. The hole delocalization process from the 0D QDash confinement is unfavorable for optical devices based on such tunnel injection structures. KW - physics KW - surface collisions KW - electronic coupling KW - transition radiation KW - time-resolved photoluminescence KW - photoluminescence excitation KW - modulation spectroscopy KW - quantum dots KW - quantum wells KW - delocalization Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-181787 VL - 7 IS - 1 ER - TY - JOUR A1 - Cerna-Velazco, Nhell A1 - Faber, Thomas A1 - Jones-Pérez, Joel A1 - Porod, Werner T1 - Constraining sleptons at the LHC in a supersymmetric low-scale seesaw scenario JF - European Physical Journal C N2 - We consider a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. We extend the Minimal Supersymmetric Standard Model by adding light right-handed neutrinos and their superpartners, the R-sneutrinos, and consider the lightest neutralinos to be higgsino-like. We consider the possibilities of having either an R-sneutrino or a higgsino as lightest supersymmetric particle. Assuming that squarks and gauginos are heavy, we systematically evaluate the bounds on slepton masses due to existing LHC data. KW - physics KW - particle physics KW - neutrino KW - R-sneutrino KW - supersymmetry (SUSY) KW - supersymmetric model KW - standard seesaw KW - inverse seesaw KW - minimal supersymmetric standard model (MSSM) Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-173809 VL - 77 ER - TY - THES A1 - Posske, Thore Hagen T1 - Dressed Topological Insulators: Rashba Impurity, Kondo Effect, Magnetic Impurities, Proximity-Induced Superconductivity, Hybrid Systems T1 - Topologische Isolatoren mit Zusätzen: Rashba Störstelle, Kondo Effekt, Magnetische Störstellen, Induzierte Supraleitung, Hybridsysteme N2 - 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. N2 - Topologische Isolatoren sind elektronische Phasen, welche im Inneren isolieren, jedoch auf ihren Oberflächen über besondere, metallische Randkanäle mit einer spinabhängigen Dispersion verfügen. Diesen Phasen wird eine große Bedeutung hinsichtlich zukünftiger Realisationen von Spintronik und topologischem Quantenrechnen zugeordnet. Neben der Bestimmung intrinsischer Eigenschaften dieser neuartigen Systeme kann die Betrachtung von Kombinationen mit wohlbekannten physikalischen Systemen originelle, neue Physik generieren. Diese Dissertation befasst sich mit eben solchen Kombinationen. Insbesondere werden die folgenden Systeme analysiert: Ein lokaler Rashba-Rückstreuer, ein Kondo-Quantenpunkt im Zweikanalregime, im Gitter geordnete, magnetische Adatome auf einem starken, dreidimensionalen topologischen Isolator, die näheinduzierte Supraleitung in letzteren Systemen und Hybridverbindungen bestehend aus einem topologischen Isolator und einem Halbmetall. Die primären Resultate sind die analytische Beschreibung der Kondowolke und die Beschreibung ihrer möglichen Detektion in Stromkorrelationen weit entfernt von der Kondo-Region. Dabei wird die Methode der refermionisierbaren Parameterkonfigurationen verwendet und erweitert. Des Weiteren wird die Entdeckung einer Klasse von bandlückenfreien topologischen Phasen beschrieben, deren Randkanäle sich fast wie die von konventionellen topologischen Isolatoren verhalten. Die dargestellte Forschung wird voraussichtlich in der zukünftigen Klassifizierung und Anwendung von Systemen, die als Komponente mindestens einen topologischen Isolator enthalten, hilfreich sein. Dafür werden einige Beispiele gegeben. KW - Topologischer Isolator KW - physics KW - topological insulators KW - Emery Kivelson line KW - Toulouse point KW - Kondo cloud KW - helical liquid KW - Kondo effect KW - Rashba impurity KW - self-organized magnetization KW - hybrid systems KW - Festkörperphysik Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-131249 ER - TY - JOUR A1 - Elsässer, S. A1 - Schiebl, M. A1 - Mukhin, A. A. A1 - Balbashov, A. M. A1 - Pimenov, A. A1 - Geurts, J. T1 - Impact of temperature-dependent local and global spin order in \(R\)MnO\(_3\) compounds for spin-phonon coupling and electromagnon activity JF - New Journal of Physics N2 - The orthorhombic rare-earth manganite compounds \(R\)MnO\(_3\) show a global magnetic order for \(T\) < \(T\)\(_N\), and several representatives are multiferroic with a cycloidal spin ground state order for \(T\) < \(T\)\(_c\)\(_y\)\(_c\)\(_l\) < \(T\)\(_N\) \(\approx\) 40 K. We deduce from the temperature dependence of spin–phonon coupling in Raman spectroscopy for a series of \(R\)MnO\(_3\) compounds that their spin order locally persists up to about twice \(T\)\(_N\). Along the same line, our observation of the persistence of the electromagnon in GdMnO\(_3\) up to \(T\) \(\approx\) 100 K is attributed to a local cycloidal spin order for \(T\) > \(T\)\(_c\)\(_y\)\(_c\)\(_l\), in contrast to the hitherto assumed incommensurate sinusoidal phase in the intermediate temperature range. The development of the magnetization pattern can be described in terms of an order–disorder transition at \(T\)\(_c\)\(_y\)\(_c\)\(_l\) within a pseudospin model of localized spin cycloids with opposite chirality. KW - physics KW - RMnO3 KW - multiferroics KW - electromagnon KW - Raman spectroscopy KW - spin-phonon coupling Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-171978 VL - 19 ER - TY - JOUR A1 - Winter, Walter T1 - Long-baseline sensitivity studies and comparison (discussion session) JF - Journal of Physics: Conference Series N2 - 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 \(θ_{13}\). KW - physics Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129440 N1 - Published under licence by IOP Publishing Ltd VL - 408 IS - 012020 ER - TY - JOUR A1 - Tuan, Dinh Van A1 - Scharf, Benedikt A1 - Žutič, Igor A1 - Dery, Hanan T1 - Marrying excitons and plasmons in monolayer transition-metal dichalcogenides JF - Physical Review X N2 - Just as photons are the quanta of light, plasmons are the quanta of orchestrated charge-density oscillations in conducting media. Plasmon phenomena in normal metals, superconductors, and doped semiconductors are often driven by long-wavelength Coulomb interactions. However, in crystals whose Fermi surface is comprised of disconnected pockets in the Brillouin zone, collective electron excitations can also attain a shortwave component when electrons transition between these pockets. In this work, we show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical sideband that is observed repeatedly in monolayers of WSe\(_2\) and WS\(_2\) but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers. KW - physics KW - excitons KW - plasmons KW - semiconductors KW - spintronics KW - valleytronics Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-173030 VL - 7 IS - 4 ER - TY - JOUR T1 - Measurement of colour flow with the jet pull angle in t\(\overline{t}\) events using the ATLAS detector at \(\sqrt {s}\)=8 TeV JF - Physics Letters B N2 - The distribution and orientation of energy inside jets is predicted to be an experimental handle on colour connections between the hard-scatter quarks and gluons initiating the jets. This Letter presents a measurement of the distribution of one such variable, the jet pull angle. The pull angle is measured for jets produced in t\(\overline{t}\) events with one W boson decaying leptonically and the other decaying to jets using 20.3 fb\(^{−1}\) of data recorded with the ATLAS detector at a centre-of-mass energy of \(\sqrt {s}\)=8 TeV at the LHC. The jet pull angle distribution is corrected for detector resolution and acceptance effects and is compared to various models. KW - physics KW - colour flow Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-144229 VL - 750 ER - TY - JOUR T1 - Measurement of exclusive γγ→ℓ\(^{+}\)ℓ\(^{−}\) production in proton–proton collisions at \(\sqrt {s}\)=7 TeV with the ATLAS detector JF - Physics Letters B N2 - This Letter reports a measurement of the exclusive γγ→ℓ\(^{+}\)ℓ\(^{−}\) (ℓ=e, μℓ=e, μ) cross-section in proton–proton collisions at a centre-of-mass energy of 7 TeV by the ATLAS experiment at the LHC, based on an integrated luminosity of 4.6 fb\(^{−1}\). For the electron or muon pairs satisfying exclusive selection criteria, a fit to the dilepton acoplanarity distribution is used to extract the fiducial cross-sections. The cross-section in the electron channel is determined to be \(^{excl.}_{γγ→e^{+}e^{-}}\)=0.428 ± 0.035 (stat.) ± 0.018 (syst.) pb for a phase–space region with invariant mass of the electron pairs greater than 24 GeV, in which both electrons have transverse momentum p\(_{T}\)>12 GeV and pseudorapidity |η|<2.4. For muon pairs with invariant mass greater than 20 GeV, muon transverse momentum p\(_{T}\)>10 GeV and pseudorapidity |η|<2.4, the cross-section is determined to be \(^{excl.}_{γγ→μ^{+}μ^{-}}\) =0.628 ± 0.032 (stat.) ± 0.021 (syst.) pb. When proton absorptive effects due to the finite size of the proton are taken into account in the theory calculation, the measured cross-sections are found to be consistent with the theory prediction. KW - physics Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-144247 VL - 749 ER - TY - JOUR T1 - Measurement of the branching ratio Γ(Λ\(^0_b\)→ψ(2S)Λ\(^0\))/Γ(Λ\(^0_b\)→J/ψΛ\(^0\)) with the ATLAS detector JF - Physics Letters B N2 - An observation of the View the Λ\(^0_b\)→ψ(2S)Λ\(^0\) decay and a comparison of its branching fraction with that of the Λ\(^0_b\)→J/ψΛ\(^0\) decay has been made with the ATLAS detector in proton–proton collisions at \(\sqrt {s}\)=8 TeV at the LHC using an integrated luminosity of 20.6 fb\(^{-1}\). The J/ψJ/ψ and ψ(2S) mesons are reconstructed in their decays to a muon pair, while the Λ\(^0\)→pπ\(^-\) decay is exploited for the Λ\(^0\) baryon reconstruction. The Λ\(^0_b\) baryons are reconstructed with transverse momentum p\(_T\)>10 GeV pT>10 GeV and pseudorapidity |η|<2.1. The measured branching ratio of the Λ\(^0_b\)→ψ(2S)Λ\(^0\) and Λ\(^0_b\)→J/ψΛ\(^0\) decays is Γ(Λ\(^0_b\)→ψ(2S)Λ\(^0\))/Γ(Λ\(^0_b\)→J/ψΛ\(^0\))=0.501±0.033(stat)±0.019(syst), lower than the expectation from the covariant quark model. KW - physics KW - proton–proton collisions KW - Large Hadron Collider KW - decay Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-143375 VL - 751 ER - TY - JOUR T1 - Measurement of the production of neighbouring jets in lead–lead collisions at =2.76 TeV with the ATLAS detector JF - Physics Letters B N2 - This Letter presents measurements of correlated production of nearby jets in Pb+Pb collisions at \(\sqrt S_{NN}\)=2.76 TeV using the ATLAS detector at the Large Hadron Collider. The measurement was performed using 0.14 nb\(^{-1}\) of data recorded in 2011. The production of correlated jet pairs was quantified using the rate, R\(_{ΔR}\), of “neighbouring” jets that accompany “test” jets within a given range of angular distance, ΔR , in the pseudorapidity–azimuthal angle plane. The jets were measured in the ATLAS calorimeter and were reconstructed using the anti-k\(_t\) algorithm with radius parameters d=0.2, 0.3, and 0.4. R\(_{ΔR}\) was measured in different Pb+Pb collision centrality bins, characterized by the total transverse energy measured in the forward calorimeters. A centrality dependence of R\(_{ΔR}\) is observed for all three jet radii with R\(_{ΔR}\) found to be lower in central collisions than in peripheral collisions. The ratios formed by the R\(_{ΔR}\) values in different centrality bins and the values in the 40–80% centrality bin are presented. KW - physics Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-150448 VL - 751 SP - 376 EP - 395 ER - TY - JOUR T1 - Measurement of transverse energy–energy correlations in multi-jet events in pp collisions at \(\sqrt {s}\)=7 TeV using the ATLAS detector and determination of the strong coupling constant αs(m\(_{Z}\)) JF - Physics Letters B N2 - High transverse momentum jets produced in pp collisions at a centre of mass energy of 7 TeV are used to measure the transverse energy–energy correlation function and its associated azimuthal asymmetry. The data were recorded with the ATLAS detector at the LHC in the year 2011 and correspond to an integrated luminosity of 158 pb\(^{-1}\). The selection criteria demand the average transverse momentum of the two leading jets in an event to be larger than 250 GeV. The data at detector level are well described by Monte Carlo event generators. They are unfolded to the particle level and compared with theoretical calculations at next-to-leading-order accuracy. The agreement between data and theory is good and provides a precision test of perturbative Quantum Chromodynamics at large momentum transfers. From this comparison, the strong coupling constant given at the Z   boson mass is determined to be αs(m\(_{Z}\))=0.1173±0.0010 (exp.) \(^{+0.0065}_{−0.0026}\) (theo.). KW - physics KW - high transverse momentum jets KW - transverse energy–energy correlation function Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-143398 VL - 750 ER - TY - JOUR A1 - Pollinger, Florian A1 - Schmitt, Stefan A1 - Sander, Dirk A1 - Tian, Zhen A1 - Kirschner, Jürgen A1 - Vrdoljak, Pavo A1 - Stadler, Christoph A1 - Maier, Florian A1 - Marchetto, Helder A1 - Schmidt, Thomas A1 - Schöll, Achim A1 - Umbach, Eberhard T1 - Nanoscale patterning, macroscopic reconstruction, and enhanced surface stress by organic adsorption on vicinal surfaces JF - New Journal of Physics N2 - Self-organization is a promising method within the framework of bottom-up architectures to generate nanostructures in an efficient way. The present work demonstrates that self- organization on the length scale of a few to several tens of nanometers can be achieved by a proper combination of a large (organic) molecule and a vicinal metal surface if the local bonding of the molecule on steps is significantly stronger than that on low-index surfaces. In this case thermal annealing may lead to large mass transport of the subjacent substrate atoms such that nanometer-wide and micrometer-long molecular stripes or other patterns are being formed on high-index planes. The formation of these patterns can be controlled by the initial surface orientation and adsorbate coverage. The patterns arrange self-organized in regular arrays by repulsive mechanical interactions over long distances accompanied by a significant enhancement of surface stress. We demonstrate this effect using the planar organic molecule PTCDA as adsorbate and Ag(10 8 7) and Ag(775)surfaces as substrate. The patterns are directly observed by STM, the formation of vicinal surfaces is monitored by highresolution electron diffraction, the microscopic surface morphology changes are followed by spectromicroscopy, and the macroscopic changes of surface stress are measured by a cantilever bending method. The in situ combination of these complementary techniques provides compelling evidence for elastic interaction and a significant stress contribution to long-range order and nanopattern formation. KW - physics KW - patterning KW - reconstruction KW - surface stress KW - STM KW - SPA-LEED KW - vicinal surfaces KW - adsoption Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-171947 VL - 19 ER - TY - JOUR A1 - Biedermann, Benedikt A1 - Denner, Ansgar A1 - Dittmaier, Stefan A1 - Hofer, Lars A1 - Jäger, Barbara T1 - Next-to-leading-order electroweak corrections to the production of four charged leptons at the LHC JF - Journal of High Energy Physics N2 - We present a state-of-the-art calculation of the next-to leading-order electroweak corrections to ZZ production, including the leptonic decays of the Z bosons into μ\(^+\)μ\(^ −\)e\(^+\)e\(^−\) or μ\(^+\)μ\(^−\)μ\(^+\)μ\(^−\) final states. We use complete leading-order and next-to-leading-order matrix elements for four-lepton production, including contributions of virtual photons and all off-shell effects of Z bosons, where the finite Z-boson width is taken into account using the complex-mass scheme. The matrix elements are implemented into Monte Carlo programs allowing for the evaluation of arbitrary differential distributions. We present integrated and differential cross sections for the LHC at 13 TeV both for an inclusive setup where only lepton identification cuts are applied, and for a setup motivated by Higgs-boson analyses in the four-lepton decay channel. The electroweak corrections are divided into photonic and purely weak contributions. The former show the well-known pronounced tails near kinematical thresholds and resonances; the latter are generically at the level of ∼ −5% and reach several −10% in the high-energy tails of distributions. Comparing the results for μ\(^+\)μ\(^−\)e\(^+\)e\(^−\) and μ\(^+\)μ\(^−\)μ\(^+\)μ\(^−\) final states, we find significant differences mainly in distributions that are sensitive to the μ\(^+\)μ\(^−\) pairing in the μ\(^+\)μ\(^−\)μ\(^+\)μ\(^−\) final state. Differences between μ\(^+\)μ\(^−\)e\(^+\)e\(^−\) and μ\(^+\)μ\(^−\)μ\(^+\)μ\(^−\) channels due to interferences of equal-flavour leptons in the final state can reach up to 10% in off-shell-sensitive regions. Contributions induced by incoming photons, i.e. photon-photon and quark-photon channels, are included, but turn out to be phenomenologically unimportant. KW - physics KW - NLO Computations Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-171966 IS - 1 ER - TY - THES A1 - Fuchs, Franziska T1 - Optical spectroscopy on silicon vacancy defects in silicon carbide T1 - Optische Spektroskopie an Silizium-Fehlstellen in Siliziumkarbid N2 - This work sheds light on different aspects of the silicon vacancy in SiC: (1) Defect creation via irradiation is shown both with electrons and neutrons. Optical properties have been determined: the excitation of the vacancy is most efficient at excitation wavelengths between 720nm and 800nm. The PL decay yields a characteristic excited state lifetime of (6.3±0.6)ns. (2) Defect engineering, meaning the controlled creation of vacancies in SiC with varying neutron fluence. The defect density could be engineered over eight orders of magnitude. On the one hand, in the sample with highest emitter density, the huge PL signal could even be enhanced by factor of five via annealing mechanisms. On the other hand, in the low defect density samples, single defects with photostable room temperature NIR emission were doubtlessly proven. Their lifetime of around 7ns confirmed the value of the transient measurement. (3) Also electrical excitation of the defects has been demonstrated in a SiC LED structure. (4) The investigations revealed for the first time that silicon vacancies can even exist SiC nanocrystals down to sizes of about 60 nm. The defects in the nanocrystals show stable PL emission in the NIR and even magnetic resonance in the 600nm fraction. In conclusion, this work ascertains on the one hand basic properties of the silicon vacancy in silicon carbide. On the other hand, proof-of-principle measurements test the potential for various defect-based applications of the vacancy in SiC, and confirm the feasibility of e.g. electrically driven single photon sources or nanosensing applications in the near future. N2 - In dieser Arbeit werden verschiedene Aspekte der Silizium-Fehlstelle in SiC beleuchtet: (1) Die Erzeugung der Defekte durch Bestrahlung, sowohl mit Elektronen als auch Neutronen. Einige optische Eigenschaften wurden ermittelt: die Anregung der Fehlstelle ist im Bereich von 720nm bis 800nm am effizientesten. Das Abklingen der PL zeigt eine charakteristische Lebensdauer des angeregten Zustands von (6.3±0.6)ns. (2) Maßschneidern der Defektdichte meint die kontrollierte Erzeugung von Defekten durch variablen Neutronenfluss. Hier konnte die Defektdichte gezielt über acht Größenordnungen verändert werden. Auf der einen Seite, in der Probe mit der höchsten Defektdichte, konnte das ohnehin schon große PL Signal noch um den Faktor fünf durch Temperprozesse erhöht werden. Auf der anderen Seite konnten in den Proben mit geringer Defektdichte einzelne Defekte mit stabiler nahinfrarot Emission bei Raumtemperatur zweifelsfrei nachgewiesen werden. Ihre Lebensdauer von etwa 7ns bestätigt den Wert aus den transienten Messungen. (3) Auch die elektrische Anregung der Defekte in einer SiC LED Struktur konnte gezeigt werden. (4) Die Untersuchung zeigte zum ersten Mal, dass Silizium-Fehlstellen in SiC Nanokristallen bis hinunter zu einer Größe von ca. 60 nm existieren können. Die Defekte zeigen stabile PL Emission im Nahinfraroten und sogar Magnetresonanz in der 600 nm Fraktion. Zusammenfassend werden in dieser Arbeit zum Einen grundlegende Eigenschaften der Silizium-Fehlstelle in Siliziumkarbid herausgefunden. Zum Anderen können Messungen zur Machbarkeit von verschiedenen Anwendungen sowohl das Potenzial der Fehlstelle in SiC für defektbasierte Anwendungen aufzeigen, als auch die Umsetzbarkeit von z.B. elektrisch betriebenen Einzelphotonenquellen oder Nanosensoren in naher Zukunft bestätigen. KW - Siliciumcarbid KW - Gitterbaufehler KW - Optische Spektroskopie KW - Silicon carbide KW - Silicon vacancy KW - Optical spectroscopy KW - Magnetic resonance KW - Spin defect KW - physics KW - vacancy KW - spin KW - semiconductor Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-124071 ER - TY - JOUR A1 - Ryczko, K. A1 - Misiewicz, J. A1 - Hofling, S. A1 - Kamp, M. A1 - Sęk, G. T1 - Optimizing the active region of interband cascade lasers for passive mode-locking JF - AIP Advances N2 - The work proposes possible designs of active regions for a mode-locked interband cascade laser emitting in the mid infrared. For that purpose we investigated the electronic structure properties of respectively modified GaSb-based type II W-shaped quantum wells, including the effect of external bias in order to simultaneously fulfil the requirements for both the absorber as well as the gain sections of a device. The results show that introducing multiple InAs layers in type II InAs/GaInSb quantum wells or introducing a tensely-strained GaAsSb layer into “W-shaped” type II QWs offers significant difference in optical transitions’ oscillator strengths (characteristic lifetimes) of the two oppositely polarized parts of such a laser, being promising for utilization in mode-locked devices. KW - physics KW - electrostatics KW - transition radiation KW - oscillator strengths KW - laser spectroscopy KW - optical spectroscopy KW - atomic and molecular spectroscopy, KW - frequency combs KW - quantum wells KW - laser physics Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-181790 VL - 7 IS - 1 ER - TY - JOUR A1 - Tymoshenko, Y. V. A1 - Onykiienko, Y. A. A1 - Müller, T. A1 - Thomale, R. A1 - Rachel, S. A1 - Cameron, A. S. A1 - Portnichenko, P. Y. A1 - Efremov, D. V. A1 - Tsurkan, V. A1 - Abernathy, D. L. A1 - Ollivier, J. A1 - Schneidewind, A. A1 - Piovano, A. A1 - Felea, V. A1 - Loidl, A. A1 - Inosov, D. S. T1 - Pseudo-Goldstone magnons in the frustrated \(S=3/2\) Heisenberg helimagnet \(ZnCr_2Se_4\) with a pyrochlore magnetic sublattice JF - Physical Review X N2 - Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. Here we employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet \({ZnCr_2Se_4}\) with a cubic spinel structure, in which spin\(-3/2\) magnetic \({Cr^{3+}}\) ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the \((0~ 0~ {q_h})\) ordering vector, low-energy magnetic excitations in the single-domain proper-screw spiral phase show soft helimagnon modes with a small energy gap of \({∼0.17~ meV}\), emerging from two orthogonal wave vectors \(({q_h}~ 0~ 0)\) and \({(0~ {q_h}~ 0)}\) where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spinwave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. Our results are likely universal for a broad class of symmetric helimagnets, opening up a new way of studying weak magnon-magnon interactions with accessible spectroscopic methods. KW - physics KW - spin waves KW - helimagnets KW - inelastic neutron scattering Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-172770 VL - 7 IS - 4 ER - TY - THES A1 - Förtig, Alexander T1 - Recombination Dynamics in Organic Solar Cells T1 - Rekombinationsdynamiken in organischen Solarzellen N2 - Neben herkömmlichen, konventionellen anorganischen Solarzellen — hauptsächlich auf Silizium basierend — ist die Organische Photovoltaik (OPV) auf dem besten Wege in naher Zukunft eine kostengünstige, umweltfreundliche, komplementäre Technolgie darzustellen. Die Produktionskosten, die Lebenszeit der Solarzellen sowie deren Wirkungsgrad müssen dabei weiter optimiert werden, um einen Markteintritt der OPV zu ermöglichen. Die vorliegende Arbeit befasst sich mit der Effizienz organischer Solarzellen und deren Limitierung durch die Rekombination von Ladungsträgern. Um funktionsfähige Zellen zu untersuchen, werden zeitaufgelöste Experimente wie die Messung der transienten Photospannung (TPV), des transienten Photostroms (TPC), die Ladungsextraktion (CE) sowie die time delayed collection field (TDCF) Methode angewandt. Untersucht werden sowohl flüssig prozessierte als auch aufgedampfte Proben, unterschiedliche Materialzusammensetzungen und verschiedene Probengeometrien. Das Standardmaterialsystem der OPV, P3HT:PC61BM, wird bei verschiedenen emperaturen und Beleuchtungsstärken auf die Lebenszeit und Dichte der photogenerierten Ladungsträger überprüft. Für den Fall spannungsunabhängiger Generation von Ladungsträgern zeigt sich die Anwendbarkeit der Shockley-Gleichung auf organische Solarzellen. Des Weiteren wird ein konsistentes Modell erläutert, welches den Idealtitätsfaktor direkt mit der Rekombination von freien mit gefangenen, exponentiell verteilten Ladungsträgern verknüpft. Ein Ansatz, bekannt unter der Bezeichung j=V Rekonstruktion, ermöglicht es, den leistungslimitierenden Verlustmechanismus in unbehandelten und thermisch geheizten P3HT:PC61BM Solarzellen zu identifizieren. Dieses Verf ahren, welches TPV, CE und TDCF Messungen beinhaltet, wird auf Proben basierend auf dem neuartigen, low-band gap Polymer PTB7 in Verbindung mit dem Fulleren PC71BM ausgeweitet. Während in der Zelle hergestellt aus reinem Chlorbenzol beträchtliche geminale wie nichtgeminale Verluste zu beobachten sind, erleichtert die Zugabe eines Lösungsmittelzusatzes die Polaronenpaartrennung, was zu einer starken Reduktion geminaler Verluste führt. In einer Kooperation mit dem IMEC Institut in Leuven, werden abschließend die beiden bedeutensten Probenarchitekturen organischer Solarzellen, die planare und die Mischübergang Struktur, jeweils basierend auf CuPC und C60, bezüglich nichtgeminaler Rekombination und Ladungsträgerverteilung miteinander verglichen. Neben den beiden experimentellen Techniken um TPV und CE werden makroskopische Simulationen herangezogen, um den Ursprung unterschiedlichen Voc vs. Lichtintensität–Verhaltens zu erklären. N2 - Besides established, conventional inorganic photovoltaics—mainly based on silicon—organic photovoltaics (OPV) are well on the way to represent a lowcost, environment friendly, complementary technology in near future. Production costs, solar cell lifetime and performance are the relevant factors which need to be optimized to enable a market launch of OPV. In this work, the efficiency of organic solar cells and their limitation due to charge carrier recombination are investigated. To analyze solar cells under operating conditions, time-resolved techniques such as transient photovoltage (TPV), transient photocurrent (TPC) and charge extraction (CE) are applied in combination with time delayed collection field (TDCF) measurements. Solution processed and evaporated samples of different material composition and varying device architectures are studied. The standard OPV reference system, P3HT:PC61BM, is analyzed for various temperatures in terms of charge carrier lifetime and charge carrier density for a range of illumination intensities. The applicability of the Shockley Equation for organic solar cells is validated in case of field-independent charge photogeneration. In addition, a consistent model is presented, directly relating the ideality factor to the recombination of free with trapped charge carriers in an exponential density of states. An approach known as j=V reconstruction enables to identify the performance limiting loss mechanism of as-prepared and thermally treated P3HT:PC61BM solar cells. This procedure, involving TPV, CE and TDCF measurements, is extended to samples based on the rather new, low-band gap polymer PTB7 in combination with PC71BM. While in the devices processed from pure chlorobenzene solution considerable geminate and nongeminate losses are observed, the use of a solvent additive facilitates efficient polaron pair dissociation minimizing geminate recombination. Finally, in collaboration with the IMEC institute in Leuven, the two main organic solar cell device architectures, planar and bulk heterojunction—both based on CuPc and C60—are directly compared in terms of nongeminate recombination and charge carrier distribution. Two experimental techniques, TPV and CE, as well as a macroscopic device simulation are applied to reveal the origin of different Voc vs. light intensity dependence. KW - Organische Solarzelle KW - Fotovoltaik KW - Organischer Halbleiter KW - physics KW - solar cell KW - recombination KW - organic semiconductors KW - Physik KW - Solartechnik KW - Energietechnik KW - Solarzelle KW - Rekombination Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-83895 ER - TY - JOUR T1 - Search for the \(X_b\) and other hidden-beauty states in the \(π^+π^−ϒ\)(1S) channel at ATLAS JF - Physics Letters B N2 - This Letter presents a search for a hidden-beauty counterpart of the X(3872) in the mass ranges of 10.05–10.31 GeV and 10.40–11.00 GeV, in the channel X\(_b\)→π\(^+\)π\(^−\)ϒ(1S)(→μ\(^+\)μ\(^−\)), using 16.2 fb\(^{−1}\) of s=8 TeV \(pp\) collision data collected by the ATLAS detector at the LHC. No evidence for new narrow states is found, and upper limits are set on the product of the X\(_b\) cross section and branching fraction, relative to those of the ϒ(2S), at the 95% confidence level using the CLSCLS approach. These limits range from 0.8% to 4.0%, depending on mass. For masses above 10.1 GeV, the expected upper limits from this analysis are the most restrictive to date. Searches for production of the ϒ(1\(^3\)D\(_J\)), ϒ(10860), and ϒ(11020) states also reveal no significant signals. KW - physics KW - hidden-beauty states Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-138273 VL - 740 ER - TY - JOUR T1 - Search for the associated production of the Higgs boson with a top quark pair in multilepton final states with the ATLAS detector JF - Physics Letters B N2 - A search for the associated production of the Higgs boson with a top quark pair is performed in multilepton final states using 20.3 fb\(^{−1}\) of proton–proton collision data recorded by the ATLAS experiment at \(\sqrt {s}\)=8 TeV at the Large Hadron Collider. Five final states, targeting the decays H→WW\(^{*}\), ττ, and ZZ\(^{*}\), are examined for the presence of the Standard Model (SM) Higgs boson: two same-charge light leptons (e or μ) without a hadronically decaying τ lepton; three light leptons; two same-charge light leptons with a hadronically decaying τ lepton; four light leptons; and one light lepton and two hadronically decaying τ leptons. No significant excess of events is observed above the background expectation. The best fit for the t\(\overline{t}\)H production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1\(^{+1.4}_{-1.2}\) times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The p-value for compatibility with the background-only hypothesis is 1.8σ; the expectation in the presence of a Standard Model signal is 0.9σ. KW - physics KW - associated production KW - Higgs boson Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-144231 VL - 749 ER - TY - JOUR A1 - Sánchez, Rafael A1 - Thierschmann, Holger A1 - Molenkamp, Laurens W. T1 - Single-electron thermal devices coupled to a mesoscopic gate JF - New Journal of Physics N2 - We theoretically investigate the propagation of heat currents in a three-terminal quantum dot engine. Electron–electron interactions introduce state-dependent processes which can be resolved by energy-dependent tunneling rates. We identify the relevant transitions which define the operation of the system as a thermal transistor or a thermal diode. In the former case, thermal-induced charge fluctuations in the gate dot modify the thermal currents in the conductor with suppressed heat injection, resulting in huge amplification factors and the possible gating with arbitrarily low energy cost. In the latter case, enhanced correlations of the state-selective tunneling transitions redistribute heat flows giving high rectification coefficients and the unexpected cooling of one conductor terminal by heating the other one. We propose quantum dot arrays as a possible way to achieve the extreme tunneling asymmetries required for the different operations. KW - physics KW - quantum dot KW - heat currents KW - thermal devices KW - single-electron tunneling Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-172982 VL - 19 ER - TY - JOUR A1 - Scholz, M. A1 - Sauer, C. A1 - Wiessner, M. A1 - Nguyen, N. A1 - Scholl, A. A1 - Reinert, F. T1 - Structure formation in organic thin films observed in real time by energy dispersive near-edge x-ray absorption fine-structure spectroscopy JF - New Journal of Physics N2 - We study the structure formation of 1,4,5,8-naphthalenetetracarboxylicacid- dianhydride (NTCDA) multilayer films on Ag(111) surfaces by energy dispersive near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) and photoelectron spectroscopy. The time resolution of seconds of the method allows us to identify several sub-processes, which occur during the post-growth three-dimensional structural ordering, as well as their characteristic time scales. After deposition at low temperature the NTCDA molecules are preferentially flat lying and the films exhibit no long-range order. Upon annealing the molecules flip into an upright orientation followed by an aggregation in a transient phase which exists for several minutes. Finally, threedimensional islands are established with bulk-crystalline structure involving substantial mass transport on the surface and morphological roughening. By applying the Kolmogorov–Johnson–Mehl–Avrami model the activation energies of the temperature-driven sub-processes can be derived from the time evolution of the NEXAFS signal. KW - physics Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129749 VL - 15 IS - 083052 ER - TY - JOUR A1 - Thierschmann, H A1 - Arnold, F A1 - Mittermüller, M A1 - Maier, L A1 - Heyn, C A1 - Hansen, W A1 - Buhmann, H A1 - Molenkamp, L W T1 - Thermal gating of charge currents with Coulomb coupled quantum dots JF - New Journal of Physics N2 - We have observed thermal gating, i.e. electrostatic gating induced by hot electrons. The effect occurs in a device consisting of two capacitively coupled quantum dots. The double dot system is coupled to a hot electron reservoir on one side (QD1), while the conductance of the second dot (QD2) is monitored. When a bias across QD2 is applied we observe a current which is strongly dependent on the temperature of the heat reservoir. This current can be either enhanced or suppressed, depending on the relative energetic alignment of the QD levels. Thus, the system can be used to control a charge current by hot electrons. KW - oscillations KW - physics KW - quantum dot systems KW - Coulomb interaction KW - thermal gating KW - three terminal device KW - thermoelectrics KW - energy KW - thermopower Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-145196 VL - 17 IS - 113003 ER - TY - THES A1 - Then, Patrick T1 - Waveguide-based single molecule detection in flow T1 - Wellenleiter-basierte Einzelmoleküldetektion in Strömungen N2 - In this work fluorescence-based single molecule detection at low concetration is investigated, with an emphasis on the usage of active transport and waveguides. Active transport allows to overcome the limits of diffusion-based systems in terms of the lowest detectable threshold of concentration. The effect of flow in single molecule experiments is investigated and a theoretical model is derived for laminar flow. Waveguides on the other hand promise compact detection schemes and show great potential for their possible integration into lab-on-a-chip applications. Their properties in single molecule experiments are analyzed with help of a method based on the reciprocity theorem of electromagnetic theory. N2 - Diese Arbeit untersucht fluoreszenzbasierte Einzelmoleküldetektion bei niedrigen Konzentrationen, mit einem Fokus auf den Einsatz von aktivem Transport und Wellenleitern. Aktiver Transport ermöglicht es, Limitierungen von diffusionsbasierten Systemen im Hinblick auf die niedrigste erreichbare Konzentration zu überwinden. Der Einfluss von Strömungen auf Einzelmolekülexperimente wird untersucht und ein theoretisches Modell für laminare Strömungen hergeleitet. Wellenleiter hingegen versprechen kompakte Detektorsysteme und zeigen beträchtliches Potential für eine mögliche Integration in lab-on-a-chip Anwendungen. Ihre Eigenschaften in Einzelmolekülexperimenten werden mithilfe einer auf dem Reziprozitätstheorem aus der elektromagnetischen Theorie basierenden Methode analysiert. KW - Optik KW - physics KW - optics KW - waveguides Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-140548 ER - TY - JOUR A1 - Bisti, F. A1 - Rogalev, V. A. A1 - Karolak, M. A1 - Paul, S. A1 - Gupta, A. A1 - Schmitt, T. A1 - Güntherodt, G. A1 - Eyert, V. A1 - Sangiovanni, G. A1 - Profeta, G. A1 - Strocov, V. N. T1 - Weakly-correlated nature of ferromagnetism in nonsymmorphic CrO\(_2\) revealed by bulk-sensitive soft-X-ray ARPES JF - Physical Review X N2 - Chromium dioxide CrO\(_2\) belongs to a class of materials called ferromagnetic half-metals, whose peculiar aspect is that they act as a metal in one spin orientation and as a semiconductor or insulator in the opposite one. Despite numerous experimental and theoretical studies motivated by technologically important applications of this material in spintronics, its fundamental properties such as momentumresolved electron dispersions and the Fermi surface have so far remained experimentally inaccessible because of metastability of its surface, which instantly reduces to amorphous Cr\(_2\)O\(_3\). In this work, we demonstrate that direct access to the native electronic structure of CrO\(_2\) can be achieved with soft-x-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr\(_2\)O\(_3\) layer. For the first time, the electronic dispersions and Fermi surface of CrO\(_2\) are measured, which are fundamental prerequisites to solve the long debate on the nature of electronic correlations in this material. Since density functional theory augmented by a relatively weak local Coulomb repulsion gives an exhaustive description of our spectroscopic data, we rule out strong-coupling theories of CrO\(_2\). Crucial for the correct interpretation of our experimental data in terms of the valence-band dispersions is the understanding of a nontrivial spectral response of CrO\(_2\) caused by interference effects in the photoemission process originating from the nonsymmorphic space group of the rutile crystal structure of CrO\(_2\). KW - physics KW - electronic structure KW - half-metals KW - angle-resolved photoemission spectroscopy KW - band structure methods KW - DFT+U KW - condensed matter physics Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-172251 VL - 7 IS - 4 ER -