@article{OPUS4-13609, title = {Search for pair-produced long-lived neutral particles decaying to jets in the ATLAS hadronic calorimeter in pp collisions at \(\sqrt{2}\)=8 TeV}, series = {Physics Letters B}, volume = {743}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.02.015}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-136099}, pages = {15-34}, year = {2015}, abstract = {The ATLAS detector at the Large Hadron Collider at CERN is used to search for the decay of a scalar boson to a pair of long-lived particles, neutral under the Standard Model gauge group, in 20.3 fb-120.3 fb\(^{-1}\) of data collected in proton-proton collisions at \(\sqrt{2}\)=8 TeV. This search is sensitive to long-lived particles that decay to Standard Model particles producing jets at the outer edge of the ATLAS electromagnetic calorimeter or inside the hadronic calorimeter. No significant excess of events is observed. Limits are reported on the product of the scalar boson production cross section times branching ratio into long-lived neutral particles as a function of the proper lifetime of the particles. Limits are reported for boson masses from 100 GeV to 900 GeV, and a long-lived neutral particle mass from 10 GeV to 150 GeV.}, language = {en} } @article{OPUS4-13610, title = {Search for H→γγH→γγ produced in association with top quarks and constraints on the Yukawa coupling between the top quark and the Higgs boson using data taken at 7 TeV and 8 TeV with the ATLAS detector}, series = {Physics Letters B}, volume = {740}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2014.11.049}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-136100}, pages = {222-242}, year = {2015}, abstract = {A search is performed for Higgs bosons produced in association with top quarks using the diphoton decay mode of the Higgs boson. Selection requirements are optimized separately for leptonic and fully hadronic final states from the top quark decays. The dataset used corresponds to an integrated luminosity of 4.5 fb\(^{-1}\) of proton-proton collisions at a center-of-mass energy of 7 TeV and 20.3 fb-120.3 fb\(^{-1}\) at 8 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess over the background prediction is observed and upper limits are set on the \(t\overline{t}H\) production cross section. The observed exclusion upper limit at 95\% confidence level is 6.7 times the predicted Standard Model cross section value. In addition, limits are set on the strength of the Yukawa coupling between the top quark and the Higgs boson, taking into account the dependence of the \(t\overline{t}H\) and tH   cross sections as well as the H→γγH→γγ branching fraction on the Yukawa coupling. Lower and upper limits at 95\% confidence level are set at -1.3 and +8.0 times the Yukawa coupling strength in the Standard Model.}, language = {en} } @phdthesis{Bachschmidt2015, author = {Bachschmidt, Theresa}, title = {Magnetic Resonance Imaging in Proximity to Metal Implants at 3 Tesla}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-135690}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {Magnetic resonance imaging is derogated by the presence of metal implants and image quality is impaired. Artifacts are categorized according to their sources, the differences in susceptibility between metal and tissue and the modulation of the magnetic radiofrequency (RF) transmit field. Generally, these artifacts are intensified at higher field strength. The purpose of this work is to analyze the efficiency of current methods used for metal artifact reduction at 3T and to investigate improvements. The impact of high-bandwidth RF pulses on susceptibility-induced artifacts is tested. In addition, the benefit of a two-channel transmit system with respect to shading close to total hip replacements and other elongated metal structures in parallel to the magnetic field is analyzed. Local transmit/receive coils feature a higher peak B1 amplitude than conventional body coils and thus enable high-bandwidth RF pulses. Susceptibility-induced through-plane distortion relates reciprocally to the RF bandwidth, which is evaluated in vitro for a total knee arthroplasty. Clinically relevant sequences (TSE and SEMAC) with conventional and high RF pulse bandwidths and different contrasts are tested on eight patients with different types of knee implants. Distortion is rated by two radiologists. An additional analysis assesses the capability of a local spine transmit coil. Furthermore, B1 effects close to elongated metal structures are described by an analytical model comprising a water cylinder and a metal rod, which is verified numerically and experimentally. The dependence of the optimal polarization of the transmit B1 field, creating minimum shading, on the position of the metal is analyzed. In addition, the optimal polarization is determined for two patients; its benefit compared to circular polarization is assessed. Phantom experiments confirm the relation of the RF bandwidth and the through-plane distortion, which can be reduced by up to 79\% by exploitation of a commercial local transmit/receive knee coil at 3T. On average, artifacts are rated "hardly visible" for patients with joint arthroplasties, when high-bandwidth RF pulses and SEMAC are used, and for patients with titanium fixtures, when high-bandwidth RF pulses are used in combination with TSE. The benefits of the local spine transmit coil are less compared to the knee coil, but enable a bandwidth 3.9 times as high as the body coil. The modulation of B1 due to metal is approximated well by the model presented and the position of the metal has strong influence on this effect. The optimal polarization can mitigate shading substantially. In conclusion, through-plane distortion and related artifacts can be reduced significantly by the application of high-bandwidth RF pulses by local transmit coils at 3T. Parallel transmission offers an option to substantially reduce shading close to long metal structures aligned with the magnetic field. Effective techniques dedicated for metal implant imaging at 3T are introduced in this work.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{Praetorius2015, author = {Praetorius, Christian Michael}, title = {Ce M4,5 XAS and XMCD as Local Probes for Kondo and Heavy Fermion Materials - A Study of CePt5/Pt(111) Surface Intermetallics -}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-132504}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {The aim of the present thesis is to explore the potential of X-ray magnetic circular dichroism(XMCD) experiments on gaining new insights into Kondo and heavy fermion materials. XMCD, which is derived from X-ray absorption spectroscopy (XAS), allows probing magnetic polarization specific to the different elements in a material and to their atomic orbitals. In particular, at the Ce M4,5 edges the method is sensitive to the localized 4f level, which provides the magnetic impurity moment responsible for Kondo physics in Ce compounds. Hence, Ce M4,5 XMCD is ideally suited to investigate local magnetism in the presence of interaction of impurity and conduction electrons in such materials. As a model material, CePt5/Pt(111) surface intermetallics were chosen for the present study. This thin-film material can be prepared by well-defined procedures involving molecular beam epitaxy. Crystalline Ordered samples are obtained by exploiting the single-crystallinity of the Pt(111) substrate. The surface character of thin films ideally matches the probing depth of soft X-ray spectroscopy in the total electron yield mode. The XMCD and XAS experiments, taking into account dependence on temperature, angle of incidence, sample thickness and external magnetic field, revealed the presence of four relevant energy scales that influence the magnetic response: 1. The 4f level in CePt5/Pt(111) is subject to significant crystal field (CF) splitting, which leads to reorganization of the six j = 5/2 sublevels. The hexagonal symmetry of the crystal structure conserves mj as a good quantum number. The proposed CF scheme, which is derived from measurements of the paramagnetic susceptibility by XMCD as well as linear dichroism in XAS, consists of nearly degenerate |1/2> and |3/2> doublets with the |5/2> doublet excited by E5/2 = 15 ... 25 meV. 2. Single impurity Kondo interaction significantly couples the magnetic moments of the impurity and conduction electrons. A signature thereof is the f0 -> f1 contribution to Ce M4,5 XAS, the strength of which can be tuned by control of the sample thickness. This finding is in line with the observation of reduced effective 4f moments as detected by XMCD. 3. Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction induces ferromagnetic correlations on the impurity lattice, which induces a positive Curie-Weiss temperature in the temperature-dependent inverse susceptibility. 4. Indications for the transition to a coherent heavy fermion state are found in the inverse susceptibility at T ~ 20 K; the ferromagnetic ground state is not observed. The fielddependence of the magnetic moment in the coherent state can be interpreted in terms of a metamagnetic transition. This allows studying basic characteristics of the renormalized band structure of a heavy fermion system by XMCD. The disentanglement of these different contributions to the 4f magnetism not only required extensive Ce M4,5 XAS and XMCD data, but also a thorough structural characterization of the material, a fundamental study of the Ce M4,5 line shape in relation to the degree of 4f hybridization and the development of a model for the paramagnetic susceptibility. The unit cell dimensions and sample morphology of CePt5/Pt(111) intermetallics were studied by low-energy electron diffraction (LEED) and scanning transmission electron microscopy (STEM). These experiments showed that well-defined intermetallic films form on top of the substrate. This lead to introduction of the film thickness t, measured in unit cells (u.c.), as a key feature to characterize the samples. Systematic LEED measurements in the thickness range t ~ 1 ... 15 u.c. allowed identification of six different phases, which could be interpreted as resulting from the same crystal structure with different rotational alignments and lattice constants. An accurate determination of the surface lattice constant at t ~ 3 u.c. could be achieved by interpretation of additional superstructure spots as arising from a well-defined combination of substrate and film lattices. The thicknessdependence of the lateral lattice constant could be explained in terms of lattice relaxation. Confirmation of the CePt5 stoichiometry and structure was performed by use of thicknessdependent XAS and a representative LEED-IV study. The results of this study indicate that the intermetallic films exhibit hexagonal CaCu5 structure over the entire range of thicknesses that were studied. The terminating layer consists purely of Pt with one additional Pt atom per unit cell compared to the bulk structure. The line shape of Ce M4,5 spectra was analyzed with the help of full multiplet calculations. Experimentally, characteristic variations of the line shape were observed with increasing f0 -> f1 contribution. The calculations show that these variations are not due to an admixture of j = 7/2 character to the ground state, as often stated in the literature. As alternatives, this observation can be explained by either considering an additional contribution to the spectrum or by assumption of an asymmetric lifetime profile. The model that was developed for the inverse paramagnetic susceptibility contains the hexagonal crystal field, magnetic coupling of the impurity moments in a mean field scheme and Kondo screening. The latter is included phenomenologically by screening factors for the effective moment. Assumption of doublet-specific screening factors, which means that the degree of Kondo interaction depends on the mj character of the 4f sublevels, allows satisfactory reproduction of the experimental data.}, subject = {Magnetischer R{\"o}ntgenzirkulardichroismus}, language = {en} } @phdthesis{Weber2015, author = {Weber, Christian}, title = {Electrochemical Energy Storage: Carbon Xerogel-Manganese Oxide Composites as Supercapacitor Electrode Materials}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-130748}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {Electrochemical double layer capacitors (EDLC), most commonly referred to as "supercapacitors", have gained increasing scientific and commercial interest in recent years. Purely electrostatic charge storage processes allow charge- and discharge cycles in the second-time scale, exhibiting a theoretical capacitance in the order of 100 F per gram of electrode material, thereby providing efficient recuperation devices for electromechanical processes, for example. Introducing electrochemically active materials such as manganese oxides into the supercapacitor electrode, allows to combine the double-layer storage with a battery-like storage process, leading to capacitance that can be up to two orders of magnitude larger than those in EDLC. In the present work, an electroless deposition approach of manganese oxide on a carbon scaffold is adapted and further investigated. The carbon material is derived from an organic xerogel, which in turn is prepared via a sol-gel process, allowing tailoring of the structural properties of the carbon, making it an ideal model system to study the relation between morphology and electrochemical performance in the carbon-manganese oxide hybrid electrode. In the first part of this thesis, a variation of manganese oxide deposition time at a low concentration of precursor solution is analyzed. Mass uptakes reach up to 58 wt.\%, leading to an increase of volumetric capacitance by a factor 5, however reducing the dynamic performance of the electrode. The structural characterization gives hints on the deposition location of the active material either in the intra-particular pores of the carbon backbone or on the enveloping surface area of the particles forming the backbone. In order to comprehensively answer the question of the location of the active material within the hybrid electrode, the particle size of the carbon backbone and therefore the enveloping surface area of the carbon particles was varied. For samples with high mass uptakes, scanning electron microscopy (SEM) images show a layer thickness of 27 nm of active material around the carbon particles. In order to quantitatively investigate this layer morphology, even for low mass uptakes where no layer is visible in SEM images, a model interpreting data from anomalous small angle X-ray scattering (ASAXS) measurements was developed. The results confirm the presence of a layer around the carbon particles, exhibiting a layer thickness ranging from 3 to 26 nm. From an electrochemical point of view, carbon backbones with a large enveloping surface area will lead to high mass uptakes in the electroless deposition process and therefore lead to high capacitance of the electrode. However, for future application, electrodeposition approaches should be investigated in detail, since no deposits will form on the interface between carbon backbone and current collector, leading to a better dynamic performance of the hybrid electrode. Furthermore, the ASAXS-method should be promoted and applied on other material systems, since this technique allows to draw important conclusions and allows to deduce integral and quantitative information towards a rational design of high performance electrodes.}, subject = {Superkondensator}, language = {en} } @phdthesis{Bieker2015, author = {Bieker, Steffen}, title = {Time and Spatially Resolved Photoluminescence Spectroscopy of Hot Excitons in Gallium Arsenide}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-134419}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {The present thesis investigates the impact of hot exciton effects on the low-temperature time and spatially resolved photoluminescence (PL) response of free excitons in high-purity gallium arsenide (GaAs). The work at hand extends available studies of hot carrier effects, which in bulk GaAs have up to now focused on hot electron populations. In crucial distinction from previous work, we extensively study the free exciton second LO-phonon replica. The benefit of this approach is twofold. First, the two LO phonon-assisted radiative recombination allows to circumvent the inherent interpretation ambiguities of the previously investigated free exciton zero-phonon line. Second, the recombination line shape of the second LO-phonon replica provides direct experimental access to the exciton temperature, thereby enabling the quantitative assessment of hot exciton effects. In the first part of the thesis, we address the influence of transient cooling on the time evolution of an initially hot photocarrier ensemble. To this end, we investigate time-resolved photoluminescence (TRPL) signals detected on the free exciton second LO-phonon replica. Settling a long-standing question, we show by comparison with TRPL transients of the free exciton zero-phonon line that the slow free exciton photoluminescence rise following pulsed optical excitation is dominated by the slow buildup of a free exciton population and not by the relaxation of large K-vector excitons to the Brillouin zone center. To establish a quantitative picture of the delayed photoluminescence onset, we determine the cooling dynamics of the initially hot photocarrier cloud from a time-resolved line shape analysis of the second LO-phonon replica. We demonstrate that the Saha equation, which fundamentally describes the thermodynamic population balance between free excitons and the uncorrelated electron-hole plasma, directly translates the experimentally derived cooling curves into the time-dependent conversion of unbound electron-hole pairs into free excitons. In the second part of the thesis, we establish the impact of hot exciton effects on low-temperature spatially resolved photoluminescence (SRPL) studies. Such experiments are widely used to investigate charge carrier and free exciton diffusion in semiconductors and semiconductor nanostructures. By SRPL spectroscopy of the second LO-phonon replica, we show that above-band gap focused laser excitation inevitably causes local heating in the carrier system, which crucially affects the diffusive expansion of a locally excited exciton packet. Undistorted free exciton diffusion profiles, which are correctly described by the commonly used formulation of the photocarrier diffusion equation, are only observed in the absence of spatial temperature gradients. At low sample temperatures, the reliable determination of free exciton diffusion coefficients from both continuous-wave and time-resolved SRPL spectroscopy requires strictly resonant optical excitation. Using resonant laser excitation, we observe the dimensional crossover of free exciton diffusion in etched wire structures of a thin, effectively two-dimensional GaAs epilayer. When the lateral wire width falls below the diffusion length, the sample geometry becomes effectively one-dimensional. The exciton diffusion profile along the wire stripe is then consistently reproduced by the steady-state solution to the one-dimensional diffusion equation. Finally, we demonstrate the formation of macroscopic free and bound exciton photoluminescence rings in bulk GaAs around a focused laser excitation spot. Both ring formation effects are due to pump-induced local heating in the exciton system. For a quantitative assessment of the mechanism underlying the free exciton ring formation, we directly determine the exciton temperature gradient from a spatially resolved line shape analysis of the free exciton second LO-phonon replica. We demonstrate that a pump-induced hot spot locally modifies the thermodynamic population balance between free excitons and unbound electron-hole pairs described by the Saha equation, which naturally explains the emergence of macroscopic free exciton ring structures. In summary, we demonstrate that quantitative consideration of hot exciton effects provides a coherent picture both of the time-domain free exciton luminescence kinetics and of the distinct spatially resolved photoluminescence patterns developing under the influence of spatial photocarrier diffusion.}, subject = {Exziton}, language = {en} } @article{HeStolteBurschkaetal.2015, author = {He, Tao and Stolte, Matthias and Burschka, Christian and Hansen, Nis Hauke and Musiol, Thomas and K{\"a}lblein, Daniel and Pflaum, Jens and Tao, Xutang and Brill, Jochen and W{\"u}rthner, Frank}, title = {Single-crystal field-effect transistors of new Cl\(_{2}\)-NDI polymorph processed by sublimation in air}, series = {Nature Communications}, volume = {6}, journal = {Nature Communications}, number = {5954}, doi = {10.1038/ncomms6954}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149255}, year = {2015}, abstract = {Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (\(\beta\)-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution (\(\alpha\)-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm\(^{2}\)V\(^{-1}\)s\(^{-1}\) (\(\alpha\)-phase) and up to 3.5 cm\(^{2}\)V\(^{-1}\)s\(^{-1}\) (\(\beta\)-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on \(\beta\)-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm\(^{2}\)V\(^{-1}\)s\(^{-1}\). The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps.}, language = {en} } @article{OPUS4-15016, title = {Search for production of WW/WZ resonances decaying to a lepton, neutrino and jets in pp collisions at \(\sqrt {s}\)= 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {5}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3425-6}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150164}, pages = {209}, year = {2015}, abstract = {A search is presented for narrow diboson resonances decaying to WW or WZ in the final state where one W boson decays leptonically (to an electron or a muon plus a neutrino) and the other W/Z boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb\(^{-1}\) of pp collisions at \(\sqrt {s}\) = 8 TeV collected by the ATLAS detector at the large hadron collider. No evidence for resonant diboson production is observed, and resonance masses below 700 and 1490 GeV are excluded at 95 \% confidence level for the spin-2 Randall-Sundrum bulk graviton G\(^{*}\) with coupling constant of 1.0 and the extended gauge model W′ boson respectively.}, language = {en} } @article{OPUS4-14990, title = {Measurement of the top-quark mass in the fully hadronic decay channel from ATLAS data at \(\sqrt {s}\)=7TeV}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {4}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3373-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149905}, pages = {158}, year = {2015}, abstract = {The mass of the top quark is measured in a data set corresponding to 4.6  fb\(^{-1}\) of proton-proton collisions with centre-of-mass energy \(\sqrt {s}\)=7 TeV collected by the ATLAS detector at the LHC. Events consistent with hadronic decays of top-antitop quark pairs with at least six jets in the final state are selected. The substantial background from multijet production is modelled with data-driven methods that utilise the number of identified b-quark jets and the transverse momentum of the sixth leading jet, which have minimal correlation. The top-quark mass is obtained from template fits to the ratio of three-jet to dijet mass. The three-jet mass is calculated from the three jets produced in a top-quark decay. Using these three jets the dijet mass is obtained from the two jets produced in the W boson decay. The top-quark mass obtained from this fit is thus less sensitive to the uncertainty in the energy measurement of the jets. A binned likelihood fit yields a top-quark mass of m\(_{t}\)= 175.1 ± 1.4 (stat.) ± 1.2 (syst.) GeV.}, language = {en} } @article{LeeSongHanetal.2015, author = {Lee, Eun-Hye and Song, Jin-Dong and Han, Il-Ki and Chang, Soo-Kyung and Langer, Fabian and H{\"o}fling, Sven and Forchel, Alfred and Kamp, Martin and Kim, Jong-Su}, title = {Structural and optical properties of position-retrievable low-density GaAs droplet epitaxial quantum dots for application to single photon sources with plasmonic optical coupling}, series = {Nanoscale Research Letters}, volume = {10}, journal = {Nanoscale Research Letters}, number = {114}, doi = {10.1186/s11671-015-0826-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143692}, year = {2015}, abstract = {The position of a single GaAs quantum dot (QD), which is optically active, grown by low-density droplet epitaxy (DE) (approximately 4 QDs/μm\(^{2}\)), was directly observed on the surface of a 45-nm-thick Al\(_{0.3}\)Ga\(_{0.7}\)As capping layer. The thin thickness of AlGaAs capping layer is useful for single photon sources with plasmonic optical coupling. A micro-photoluminescence for GaAs DE QDs has shown exciton/biexciton behavior in the range of 1.654 to 1.657 eV. The direct observation of positions of low-density GaAs DE QDs would be advantageous for mass fabrication of devices that use a single QD, such as single photon sources.}, language = {en} } @article{OPUS4-14992, title = {Measurements of the W production cross sections in association with jets with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {2}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3262-7}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149925}, pages = {82}, year = {2015}, abstract = {This paper presents cross sections for the production of a W boson in association with jets, measured in proton-proton collisions at \(\sqrt {s}\) = 7 TeV with the ATLAS experiment at the large hadron collider. With an integrated luminosity of 4.6 fb\(^{-1}\), this data set allows for an exploration of a large kinematic range, including jet production up to a transverse momentum of 1 TeV and multiplicities up to seven associated jets. The production cross sections for W bosons are measured in both the electron and muon decay channels. Differential cross sections for many observables are also presented including measurements of the jet observables such as the rapidities and the transverse momenta as well as measurements of event observables such as the scalar sums of the transverse momenta of the jets. The measurements are compared to numerous QCD predictions including next-to-leading-order perturbative calculations, resummation calculations and Monte Carlo generators.}, language = {en} } @article{OPUS4-14961, title = {Constraints on the off-shell Higgs boson signal strength in the high-mass ZZ and WW final states with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3542-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149615}, pages = {335}, year = {2015}, abstract = {Measurements of the ZZ and WW final states in the mass range above the 2m\(_Z\) and 2m\(_W\) thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the ZZ→4ℓ, ZZ→2ℓ2ν and WW→eνμν final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb\(^{-1}\) at a collision energy of \(\sqrt {s}\)=8 TeV. Using the CL\(_S\) method, the observed 95 \% confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1-8.6, with an expected range of 6.7-11.0. In each case the range is determined by varying the unknown gg→ZZ and gg→WW background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95 \% CL upper limit on Γ\(_H\)/Γ\(^{SM}_{H}\) in the range 4.5-7.5 (6.5-11.2) using the same variations of the background K-factor. Assuming that the unknown gg→VV background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95 \% CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.}, language = {en} } @article{OPUS4-14339, title = {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\(_{Z}\))}, series = {Physics Letters B}, volume = {750}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.09.050}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143398}, pages = {427-447}, year = {2015}, abstract = {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.).}, language = {en} } @article{OPUS4-14991, title = {Measurement of three-jet production cross-sections in pp collisions at 7 TeV centre-of-mass energy using the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {5}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3363-3}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149918}, pages = {228}, year = {2015}, abstract = {Double-differential three-jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of \(\sqrt {s}\) = 7 TeV using the ATLAS detector at the large hadron collider. The measurements are presented as a function of the three-jet mass (m\(_{jjj}\)), in bins of the sum of the absolute rapidity separations between the three leading jets (|Y\(^{*}\)|). Invariant masses extending up to 5 TeV are reached for 8 < |Y\(^{*}\)| < 10. These measurements use a sample of data recorded using the ATLAS detector in 2011, which corresponds to an integrated luminosity of 4.51 fb\(^{-1}\). Jets are identified using the anti-k\(_{t}\) algorithm with two different jet radius parameters, R = 0.4 and R = 0.6. The dominant uncertainty in these measurements comes from the jet energy scale. Next-to-leading-order QCD calculations corrected to account for non-perturbative effects are compared to the measurements. Good agreement is found between the data and the theoretical predictions based on most of the available sets of parton distribution functions, over the full kinematic range, covering almost seven orders of magnitude in the measured cross-section values.}, language = {en} } @article{OPUS4-14989, title = {Measurement of the top quark mass in the t\(\overline{t}\)→ lepton+jets and t\(\overline{t}\)→ dilepton channels using \(\sqrt {s}\)=7 TeV ATLAS data}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3544-0}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149890}, pages = {330}, year = {2015}, abstract = {The top quark mass was measured in the channels t\(\overline{t}\) → lepton+jets and t\(\overline{t}\) → dilepton (lepton = e,μ) based on ATLAS data recorded in 2011. The data were taken at the LHC with a proton-proton centre-of-mass energy of \(\sqrt {s}\) = 7 TeV and correspond to an integrated luminosity of 4.6 fb\(^{-1}\). The t\(\overline{t}\) → lepton+jets analysis uses a three-dimensional template technique which determines the top quark mass together with a global jet energy scale factor (JSF), and a relative b-to-light-jet energy scale factor(bJSF), where the terms b-jets and light-jets refer to jets originating from b-quarks and u, d, c, s-quarks or gluons, respectively. The analysis of the t\(\overline{t}\) → dilepton channel exploits a one-dimensional template method using the m\(_{lb}\) observable, defined as the average invariant mass of the two lepton+b-jet pairs in each event. The top quark mass is measured to be 172.33±0.75(stat + JSF + bJSF)±1.02(syst) GeV, and 173.79 ± 0.54(stat) ± 1.30(syst) GeV in the t\(\overline{t}\) → lepton+jets and t\(\overline{t}\) → dilepton channels, respectively. The combination of the two results yields m\(_{top}\) =172.99 ± 0.48(stat) ± 0.78(syst) GeV, with a total uncertainty of 0.91 GeV.}, language = {en} } @article{OPUS4-15018, title = {Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in \(\sqrt {s}\) = 8 TeV pp collisions with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3518-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150184}, pages = {318}, year = {2015}, abstract = {Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy \(\sqrt {s}\) = 8 TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb\(^{-1}\). Two leptonic production mechanisms are considered: decays of squarks and gluinos with Z bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the Z-boson mass; and decays of neutralinos (e.g. \(\tilde{χ}\)\(^{0}_{2}\) → ℓ\(^{+}\)ℓ\(^{-}\)\(\tilde{χ}\)\(^{0}_{1}\)), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of three standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.}, language = {en} } @article{OPUS4-15022, title = {Two-particle Bose-Einstein correlations in pp collisions at \(\sqrt {s}\) = 0.9 and 7 TeV measured with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {10}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3644-x}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150222}, pages = {466}, year = {2015}, abstract = {The paper presents studies of Bose-Einstein Correlations (BEC) for pairs of like-sign charged particles measured in the kinematic range p\(_{T}\) > 100 MeV and |η| <  2.5 in proton collisions at centre-of-mass energies of 0.9 and 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. The integrated luminosities are approximately 7 μb\(^{-1}\), 190 μb\(^{-1}\) and 12.4 nb\(^{-1}\) for 0.9 TeV, 7 TeV minimum-bias and 7 TeV high-multiplicity data samples, respectively. The multiplicity dependence of the BEC parameters characterizing the correlation strength and the correlation source size are investigated for charged-particle multiplicities of up to 240. A saturation effect in the multiplicity dependence of the correlation source size parameter is observed using the high-multiplicity 7 TeV data sample. The dependence of the BEC parameters on the average transverse momentum of the particle pair is also investigated.}, language = {en} } @article{OPUS4-15007, title = {Search for a new resonance decaying to a W or Z boson and a Higgs boson in the ℓℓ/ℓν/νν+b\(\overline{b}\) final states with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {6}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3474-x}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150075}, pages = {263}, year = {2015}, abstract = {A search for a new resonance decaying to a W or Z boson and a Higgs boson in the ℓℓ/ℓν/νν+b\(\overline{b}\) final states is performed using 20.3 fb\(^{-1}\) of pp collision data recorded at \(\sqrt {s}\) = 8 TeV with the ATLAS detector at the Large Hadron Collider. The search is conducted by examining the WH / ZH invariant mass distribution for a localized excess. No significant deviation from the Standard Model background prediction is observed. The results are interpreted in terms of constraints on the Minimal Walking Technicolor model and on a simplified approach based on a phenomenological Lagrangian of Heavy Vector Triplets.}, language = {en} } @article{OPUS4-14337, title = {Measurement of the branching ratio Γ(Λ\(^0_b\)→ψ(2S)Λ\(^0\))/Γ(Λ\(^0_b\)→J/ψΛ\(^0\)) with the ATLAS detector}, series = {Physics Letters B}, volume = {751}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.10.009}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143375}, pages = {63-80}, year = {2015}, abstract = {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.}, language = {en} } @article{OPUS4-14423, title = {Search for the associated production of the Higgs boson with a top quark pair in multilepton final states with the ATLAS detector}, series = {Physics Letters B}, volume = {749}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.07.079}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-144231}, pages = {519-541}, year = {2015}, abstract = {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.}, language = {en} } @article{OPUS4-14424, title = {Measurement of exclusive γγ→ℓ\(^{+}\)ℓ\(^{-}\) production in proton-proton collisions at \(\sqrt {s}\)=7 TeV with the ATLAS detector}, series = {Physics Letters B}, volume = {749}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.07.069}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-144247}, pages = {242-261}, year = {2015}, abstract = {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.}, language = {en} } @article{OPUS4-14422, title = {Measurement of colour flow with the jet pull angle in t\(\overline{t}\) events using the ATLAS detector at \(\sqrt {s}\)=8 TeV}, series = {Physics Letters B}, volume = {750}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.09.051}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-144229}, pages = {475-493}, year = {2015}, abstract = {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.}, language = {en} } @article{LeeLimSchneideretal.2015, author = {Lee, Chang-Min and Lim, Hee-Jin and Schneider, Christian and Maier, Sebastian and H{\"o}fling, Sven and Kamp, Martin and Lee, Yong-Hee}, title = {Efficient single photon source based on \(\mu\)-fibre-coupled tunable microcavity}, series = {Scientific Reports}, volume = {5}, journal = {Scientific Reports}, number = {14309}, doi = {10.1038/srep14309}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-145835}, year = {2015}, abstract = {Efficient and fast on-demand single photon sources have been sought after as critical components of quantum information science. We report an efficient and tunable single photon source based on an InAs quantum dot (QD) embedded in a photonic crystal cavity coupled with a highly curved \(\mu\)-fibre. Exploiting evanescent coupling between the \(\mu\)-fibre and the cavity, a high collection efficiency of 23\% and Purcell-enhanced spontaneous emissions are observed. In our scheme, the spectral position of a resonance can be tuned by as much as 1.5 nm by adjusting the contact position of the \(\mu\)-fibre, which increases the spectral coupling probability between the QD and the cavity mode. Taking advantage of the high photon count rate and the tunability, the collection efficiencies and the decay rates are systematically investigated as a function of the QD-cavity detuning.}, language = {en} } @article{JobstWielpuetzTriphanetal.2015, author = {Jobst, Bertram J. and Wielp{\"u}tz, Mark O. and Triphan, Simon M.F. and Anjorin, Angela and Ley-Zaporozhan, Julia and Kauczor, Hans-Ulrich and Biederer, J{\"u}rgen and Ley, Sebastian and Sedlaczek, Oliver}, title = {Morpho-Functional 1H-MRI of the Lung in COPD: Short-Term Test-Retest Reliability}, series = {PLOS ONE}, volume = {10}, journal = {PLOS ONE}, number = {9}, doi = {10.1371/journal.pone.0137282}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-151365}, pages = {e0137282}, year = {2015}, abstract = {Purpose Non-invasive end-points for interventional trials and tailored treatment regimes in chronic obstructive pulmonary disease (COPD) for monitoring regionally different manifestations of lung disease instead of global assessment of lung function with spirometry would be valuable. Proton nuclear magnetic resonance imaging (1H-MRI) allows for a radiation-free assessment of regional structure and function. The aim of this study was to evaluate the short-term reproducibility of a comprehensive morpho-functional lungMRI protocol in COPD. Materials and Methods 20 prospectively enrolled COPD patients (GOLD I-IV) underwent 1H-MRI of the lung at 1.5T on two consecutive days, including sequences for morphology, 4D contrast-enhanced perfusion, and respiratory mechanics. Image quality and COPD-related morphological and functional changes were evaluated in consensus by three chest radiologists using a dedicated MRI-based visual scoring system. Test-retest reliability was calculated per each individual lung lobe for the extent of large airway (bronchiectasis, wall thickening, mucus plugging) and small airway abnormalities (tree in bud, peripheral bronchiectasis, mucus plugging), consolidations, nodules, parenchymal defects and perfusion defects. The presence of tracheal narrowing, dystelectasis, pleural effusion, pulmonary trunk ectasia, right ventricular enlargement and, finally, motion patterns of diaphragma and chest wall were addressed. Results Median global scores [10(Q1:8.00; Q3:16.00) vs. 11(Q1:6.00; Q3:15.00)] as well as category subscores were similar between both timepoints, and kappa statistics indicated "almost perfect" global agreement (\(\kappa\)= 0.86, 95\%CI = 0.81-0.91). Most subscores showed at least "substantial" agreement of MRI1 and MRI2 (\(\kappa\)= 0.64-1.00), whereas the agreement for the diagnosis of dystelectasis/effusion (\(\kappa\)= 0.42, 95\%CI = 0.00-0.93) was "moderate" and of tracheal abnormalities (\(\kappa\)= 0.21, 95\%CI = 0.00-0.75) "fair". Most MRI acquisitions showed at least diagnostic quality at MRI1 (276 of 278) and MRI2 (259 of 264). Conclusion Morpho-functional 1H-MRI can be obtained with reproducible image quality and high short-term test-retest reliability for COPD-related morphological and functional changes of the lung. This underlines its potential value for the monitoring of regional lung characteristics in COPD trials.}, language = {en} } @article{OPUS4-14985, title = {Jet energy measurement and its systematic uncertainty in proton-proton collisions at \(\sqrt {s}\)=7 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {1}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-014-3190-y}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149854}, pages = {17}, year = {2015}, abstract = {The jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector using proton-proton collision data with a centre-of-mass energy of \(\sqrt {s}\)=7 TeV corresponding to an integrated luminosity of 4.7 fb\(^{-1}\). Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells using the anti-k\(_t\) algorithm with distance parameters R=0.4 or R=0.6, and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a reference object such as a photon or a Z boson, for 20≤p\(^{jet}_{T}\)<1000 GeV and pseudorapidities |η|<4.5. The effect of multiple proton-proton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1 \% is found in the central calorimeter region (|η|<1.2) for jets with 55≤p\(^{jet}_{T}\)<500 GeV. For central jets at lower p\(_{T}\), the uncertainty is about 3 \%. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in proton-proton collisions and test-beam data, which also provide the estimate for p\(^{jet}_{T}\)>1 TeV. The calibration of forward jets is derived from dijet p\(_{T}\) balance measurements. The resulting uncertainty reaches its largest value of 6 \% for low-p\(_{T}\) jets at |η|=4.5. Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.5-3 \%.}, language = {en} } @article{OPUS4-15009, title = {Search for direct pair production of a chargino and a neutralino decaying to the 125 GeV Higgs boson in \(\sqrt {s}\) = 8 TeV pp collisions with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {5}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3408-7}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150099}, pages = {208}, year = {2015}, abstract = {A search is presented for the direct pair production of a chargino and a neutralino pp → \(\tilde{χ}\)\(^{±}_{1}\)\(\tilde{χ}\)\(^{0}_{2}\), where the chargino decays to the lightest neutralino and the W boson, \(\tilde{χ}\)\(^{±}_{1}\)→\(\tilde{χ}\)\(^{0}_{1}\)(W\(^{±}\)→ℓ\(^{±}\)ν), while the neutralino decays to the lightest neutralino and the 125 GeV Higgs boson, \(\tilde{χ}\)\(^{0}_{2}\)→\(\tilde{χ}\)\(^{0}_{1}\)(h→bb/γγ/ℓ\(^{±}\)νqq). The final states considered for the search have large missing transverse momentum, an isolated electron or muon, and one of the following: either two jets identified as originating from bottom quarks, or two photons, or a second electron or muon with the same electric charge. The analysis is based on 20.3 fb\(^{-1}\) of \(\sqrt {s}\) = 8 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations, and limits are set in the context of a simplified supersymmetric model.}, language = {en} } @article{OPUS4-15019, title = {Search for the Standard Model Higgs boson produced in association with top quarks and decaying into b\(\overline{b}\) in pp collisions at \(\sqrt {s}\) = 8 TeV with the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3543-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150197}, pages = {349}, year = {2015}, abstract = {A search for the Standard Model Higgs boson produced in association with a top-quark pair, t\(\overline{t}\)H, is presented. The analysis uses 20.3 fb\(^{-1}\) of pp collision data at \(\sqrt {s}\) = 8 TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H→b\(\overline{b}\) decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by t\(\overline{t}\)+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible t\(\overline{t}\)+b\(\overline{b}\) background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 \% confidence level. The ratio of the measured t\(\overline{t}\)H signal cross section to the Standard Model expectation is found to be μ = 1.5 ± 1.1 assuming a Higgs boson mass of 125 GeV.}, language = {en} } @article{OPUS4-14982, title = {Identification and energy calibration of hadronically decaying tau leptons with the ATLAS experiment in pp collisions at \(\sqrt {s}\)=8 TeV}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {7}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3500-z}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149829}, pages = {303}, year = {2015}, abstract = {This paper describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from pp collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy \(\sqrt {s}\)=8 TeV. The performance of these algorithms is measured in most cases with Z decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb\(^{-1}\). An uncertainty on the offline reconstructed tau energy scale of 2-4 \%, depending on transverse energy and pseudorapidity, is achieved using two independent methods. The offline tau identification efficiency is measured with a precision of 2.5 \% for hadronically decaying tau leptons with one associated track, and of 4 \% for the case of three associated tracks, inclusive in pseudorapidity and for a visible transverse energy greater than 20 GeV. For hadronic tau lepton decays selected by offline algorithms, the tau trigger identification efficiency is measured with a precision of 2-8 \%, depending on the transverse energy. The performance of the tau algorithms, both offline and at the trigger level, is found to be stable with respect to the number of concurrent proton-proton interactions and has supported a variety of physics results using hadronically decaying tau leptons at ATLAS.}, language = {en} } @article{OPUS4-15010, title = {Search for heavy long-lived multi-charged particles in pp collisions at \(\sqrt {s}\) = 8 TeV using the ATLAS detector}, series = {European Physical Journal C: Particles and Fields}, volume = {75}, journal = {European Physical Journal C: Particles and Fields}, number = {8}, organization = {ATLAS Collaboration}, doi = {10.1140/epjc/s10052-015-3534-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150100}, pages = {362}, year = {2015}, abstract = {A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2012 at \(\sqrt {s}\) = 8 TeV from pp collisions corresponding to an integrated luminosity of 20.3 fb\(^{-1}\) are examined. Particles producing anomalously high ionisation, consistent with long-lived massive particles with electric charges from |q| = 2e to |q| = 6e are searched for. No signal candidate events are observed, and 95 \% confidence level cross-section upper limits are interpreted as lower mass limits for a Drell-Yan production model. The mass limits range between 660 and 785 GeV.}, language = {en} } @article{FuchsStenderTrupkeetal.2015, author = {Fuchs, F. and Stender, B. and Trupke, M. and Simin, D. and Pflaum, J. and Dyakonov, V. and Astakhov, G.V.}, title = {Engineering near-infrared single-photon emitters with optically active spins in ultrapure silicon carbide}, series = {Nature Communications}, volume = {6}, journal = {Nature Communications}, number = {7578}, doi = {10.1038/ncomms8578}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-148502}, year = {2015}, abstract = {Vacancy-related centres in silicon carbide are attracting growing attention because of their appealing optical and spin properties. These atomic-scale defects can be created using electron or neutron irradiation; however, their precise engineering has not been demonstrated yet. Here, silicon vacancies are generated in a nuclear reactor and their density is controlled over eight orders of magnitude within an accuracy down to a single vacancy level. An isolated silicon vacancy serves as a near-infrared photostable single-photon emitter, operating even at room temperature. The vacancy spins can be manipulated using an optically detected magnetic resonance technique, and we determine the transition rates and absorption cross-section, describing the intensity-dependent photophysics of these emitters. The on-demand engineering of optically active spins in technologically friendly materials is a crucial step toward implementation of both maser amplifiers, requiring high-density spin ensembles, and qubits based on single spins.}, language = {en} } @article{ZienerKurzBuschleetal.2015, author = {Ziener, Christian H. and Kurz, Felix T. and Buschle, Lukas R. and Kampf, Thomas}, title = {Orthogonality, Lommel integrals and cross product zeros of linear combinations of Bessel functions}, series = {SpringerPlus}, volume = {4}, journal = {SpringerPlus}, number = {390}, doi = {10.1186/s40064-015-1142-0}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-151432}, year = {2015}, abstract = {The cylindrical Bessel differential equation and the spherical Bessel differential equation in the interval R\(\leq\)r\(\leq\)\(\gamma\)R with Neumann boundary conditions are considered. The eigenfunctions are linear combinations of the Bessel function \(\Phi\)\(_{n,ν}\)(r) = Y'\(_{ν}\) (\(\lambda\)\(_{n,ν}\))J\(_{ν}\)(\(\lambda\)\(_{n,ν}\) r/R) - J'\(_{ν}\)(\(\lambda\)\(_{n,ν}\))Y\(_{ν}\)(\(\lambda\)\(_{n,ν}\)r/R) or linear combinations of the spherical Bessel functions \(\psi\)\(_{m,ν}\)(r) = y'\(_{ν}\)(\(\lambda\)\(_{m,ν}\))j\(_{ν}\)(\(\lambda\)\(_{m,ν}\)r/R) - j'\(_{ν}\)(\(\lambda\)\(_{m,ν}\))y\(_{ν}\)(\(\lambda\)\(_{m,ν}\)r/R). The orthogonality relations with analytical expressions for the normalization constant are given. Explicit expressions for the Lommel integrals in terms of Lommel functions are derived. The cross product zeros Y'\(_{ν}\)\(\lambda\)\(_{n,ν}\))J'\(_{ν}\)(\(\gamma\)\(\lambda\)\(_{n,ν}\))- J'\(_{ν}\)(\(\lambda\)\(_{n,ν}\))Y'\(_{ν}\)(\(\gamma\)\(\lambda\)\(_{n,ν}\)) = 0 and y'\(_{ν}\)(\(\lambda\)\(_{m,ν}\))j'\(_{ν}\)(\(\gamma\)\(\lambda\)\(_{m,ν}\)) - j'\(_{ν}\)(\(\lambda\)\(_{m,ν}\))y'\(_{ν}\)(\(\gamma\)\(\lambda\)\(_{m,ν}\)) = 0 are considered in the complex plane for real as well as complex values of the index ν and approximations for the exceptional zero \(\lambda\)\(_{1,ν}\) are obtained. A numerical scheme based on the discretization of the twodimensional and three-dimensional Laplace operator with Neumann boundary conditions is presented. Explicit representations of the radial part of the Laplace operator in form of a tridiagonal matrix allow the simple computation of the cross product zeros.}, language = {en} } @article{OPUS4-14305, title = {Search for a CP-odd Higgs boson decaying to Zh in pp collisions at \(\sqrt{s}\)=8 TeV with the ATLAS detector}, series = {Physics Letters B}, volume = {744}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2015.03.054}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143050}, pages = {163-183}, year = {2015}, abstract = {A search for a heavy, CP-odd Higgs boson, A, decaying into a Z boson and a 125 GeV Higgs boson, h, with the ATLAS detector at the LHC is presented. The search uses proton-proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb\(^{-1}\). Decays of CP-even h bosons to ττ or bb pairs with the Z boson decaying to electron or muon pairs are considered, as well as h→bb decays with the Z boson decaying to neutrinos. No evidence for the production of an A boson in these channels is found and the 95\% confidence level upper limits derived for σ(gg→A)×BR(A→Zh)×BR(h→f\(\bar{f}\)) are 0.098-0.013 pb for f=τ and 0.57-0.014 pb for f=b in a range of m\(_{A}\)=220-1000 GeVmA=220-1000 GeV. The results are combined and interpreted in the context of two-Higgs-doublet models.}, language = {en} } @article{MaCalvoWangetal.2015, author = {Ma, Eric Yue and Calvo, M. Reyes and Wang, Jing and Lian, Biao and M{\"u}hlbauer, Mathias and Br{\"u}ne, Christoph and Cui, Yong-Tao and Lai, Keji and Kundhikanjana, Worasom and Yang, Yongliang and Baenninger, Matthias and K{\"o}nig, Markus and Ames, Christopher and Buhmann, Hartmut and Leubner, Philipp and Molenkamp, Laurens W. and Zhang, Shou-Cheng and Goldhaber-Gordon, David and Kelly, Michael A. and Shen, Zhi-Xun}, title = {Unexpected edge conduction in mercury telluride quantum wells under broken time-reversal symmetry}, series = {Nature Communications}, volume = {6}, journal = {Nature Communications}, number = {7252}, doi = {10.1038/ncomms8252}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143185}, year = {2015}, abstract = {The realization of quantum spin Hall effect in HgTe quantum wells is considered a milestone in the discovery of topological insulators. Quantum spin Hall states are predicted to allow current flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction yet to be experimentally verified is the breakdown of the edge conduction under broken time-reversal symmetry. Here we first establish a systematic framework for the magnetic field dependence of electrostatically gated quantum spin Hall devices. We then study edge conduction of an inverted quantum well device under broken time-reversal symmetry using microwave impedance microscopy, and compare our findings to a noninverted device. At zero magnetic field, only the inverted device shows clear edge conduction in its local conductivity profile, consistent with theory. Surprisingly, the edge conduction persists up to 9 T with little change. This indicates physics beyond simple quantum spin Hall model, including material-specific properties and possibly many-body effects.}, language = {en} } @article{CharnukhaThirupathaiahZabolotnyyetal.2015, author = {Charnukha, A. and Thirupathaiah, S. and Zabolotnyy, V. B. and B{\"u}chner, B. and Zhigadlo, N. D. and Batlogg, B. and Yaresko, A. N. and Borisenko, S. V.}, title = {Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor}, series = {Scientific Reports}, volume = {5}, journal = {Scientific Reports}, number = {10392}, doi = {10.1038/srep10392}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-151987}, year = {2015}, abstract = {In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature T\(_{c}\)\(\approx\)55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe\(_{0.92}\)Co\(_{0.08}\)AsO, is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors.}, language = {en} } @article{CzerniukBrueggemannTepperetal.2014, author = {Czerniuk, T. and Br{\"u}ggemann, C. and Tepper, J. and Brodbeck, S. and Schneider, C. and Kamp, M. and H{\"o}fling, S. and Glavin, B. A. and Yakovlev, D. R. and Akimov, A. V. and Bayer, M.}, title = {Lasing from active optomechanical resonators}, series = {Nature Communications}, volume = {5}, journal = {Nature Communications}, doi = {10.1038/ncomms5038}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-121559}, pages = {4038}, year = {2014}, abstract = {Planar microcavities with distributed Bragg reflectors (DBRs) host, besides confined optical modes, also mechanical resonances due to stop bands in the phonon dispersion relation of the DBRs. These resonances have frequencies in the 10- to 100-GHz range, depending on the resonator's optical wavelength, with quality factors exceeding 1,000. The interaction of photons and phonons in such optomechanical systems can be drastically enhanced, opening a new route towards the manipulation of light. Here we implemented active semiconducting layers into the microcavity to obtain a vertical-cavity surface-emitting laser (VCSEL). Thereby, three resonant excitations--photons, phonons and electrons--can interact strongly with each other providing modulation of the VCSEL laser emission: a picosecond strain pulse injected into the VCSEL excites long-living mechanical resonances therein. As a result, modulation of the lasing intensity at frequencies up to 40 GHz is observed. From these findings, prospective applications of active optomechanical resonators integrated into nanophotonic circuits may emerge.}, language = {en} } @phdthesis{Gerhard2014, author = {Gerhard, Felicitas Irene Veronika}, title = {Controlling structural and magnetic properties of epitaxial NiMnSb for application in spin torque devices}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-111690}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {This thesis describes the epitaxial growth of the Half-Heusler alloy NiMnSb by molecular beam epitaxy. Its structural and magnetic properties are controlled by tuning the composition and the resulting small deviation from stoichiometry. The magnetic in-plane anisotropy depends on the Mn concentration of the sample and can be controlled in both strength and orientation. This control of the magnetic anisotropy allows for growing NiMnSb layers of a given thickness and magnetic properties as requested for the design of NiMnSb-based devices. The growth and characterization of NiMnSb-ZnTe-NiMnSb heterostructures is presented - such heterostructures form an all-NiMnSb based spin-valve and are a promising basis for spin torque devices.}, subject = {Nickelverbindungen}, language = {en} } @phdthesis{TranGia2014, author = {Tran-Gia, Johannes}, title = {Model-Based Reconstruction Methods for MR Relaxometry}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-109774}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {In this work, a model-based acceleration of parameter mapping (MAP) for the determination of the tissue parameter T1 using magnetic resonance imaging (MRI) is introduced. The iterative reconstruction uses prior knowledge about the relaxation behavior of the longitudinal magnetization after a suitable magnetization preparation to generate a series of fully sampled k-spaces from a strongly undersampled acquisition. A Fourier transform results in a spatially resolved time course of the longitudinal relaxation process, or equivalently, a spatially resolved map of the longitudinal relaxation time T1. In its fastest implementation, the MAP algorithm enables the reconstruction of a T1 map from a radial gradient echo dataset acquired within only a few seconds after magnetization preparation, while the acquisition time of conventional T1 mapping techniques typically lies in the range of a few minutes. After validation of the MAP algorithm for two different types of magnetization preparation (saturation recovery \& inversion recovery), the developed algorithm was applied in different areas of preclinical and clinical MRI and possible advantages and disadvantages were evaluated.}, subject = {Kernspintomographie}, language = {en} } @article{AmthorWeissenseelFischeretal.2014, author = {Amthor, Matthias and Weißenseel, Sebastian and Fischer, Julian and Kamp, Martin and Schneider, Christian and H{\"o}fling, Sven}, title = {Electro-optical switching between polariton and cavity lasing in an InGaAs quantum well microcavity}, doi = {10.1364/OE.22.031146}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-111130}, year = {2014}, abstract = {We report on the condensation of microcavity exciton polaritons under optical excitation in a microcavity with four embedded InGaAs quantum wells. The polariton laser is characterized by a distinct nonlinearity in the input-output-characteristics, which is accompanied by a drop of the emission linewidth indicating temporal coherence and a characteristic persisting emission blueshift with increased particle density. The temporal coherence of the device at threshold is underlined by a characteristic drop of the second order coherence function to a value close to 1. Furthermore an external electric field is used to switch between polariton regime, polariton condensate and photon lasing.}, language = {en} } @article{FiedlerElKarehEremeevetal.2014, author = {Fiedler, Sebastian and El-Kareh, Lydia and Eremeev, Sergey V. and Tereshchenko, Oleg E. and Seibel, Christoph and Lutz, Peter and Kokh, Konstantin A. and Chulkov, Evgueni V. and Kuznetsova, Tatyana V. and Grebennikov, Vladimir I. and Bentmann, Hendrik and Bode, Matthias and Reinert, Friedrich}, title = {Defect and structural imperfection effects on the electronic properties of BiTeI surfaces}, series = {New Journal of Physics}, volume = {16}, journal = {New Journal of Physics}, number = {075013}, issn = {1367-2630}, doi = {10.1088/1367-2630/16/7/075013}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119467}, year = {2014}, abstract = {The surface electronic structure of the narrow-gap seminconductor BiTeI exhibits a large Rashba-splitting which strongly depends on the surface termination. Here we report on a detailed investigation of the surface morphology and electronic properties of cleaved BiTeI single crystals by scanning tunneling microscopy, photoelectron spectroscopy (ARPES, XPS), electron diffraction (SPA-LEED) and density functional theory calculations. Our measurements confirm a previously reported coexistence of Te- and I-terminated surface areas originating from bulk stacking faults and find a characteristic length scale of ~100 nm for these areas. We show that the two terminations exhibit distinct types of atomic defects in the surface and subsurface layers. For electronic states resided on the I terminations we observe an energy shift depending on the time after cleavage. This aging effect is successfully mimicked by depositon of Cs adatoms found to accumulate on top of the I terminations. As shown theoretically on a microscopic scale, this preferential adsorbing behaviour results from considerably different energetics and surface diffusion lengths at the two terminations. Our investigations provide insight into the importance of structural imperfections as well as intrinsic and extrinsic defects on the electronic properties of BiTeI surfaces and their temporal stability.}, language = {en} } @article{LueftnerMilkoHuppmannetal.2014, author = {L{\"u}ftner, Daniel and Milko, Matus and Huppmann, Sophia and Scholz, Markus and Ngyuen, Nam and Wießner, Michael and Sch{\"o}ll, Achim and Reinert, Friedrich and Puschnig, Peter}, title = {CuPc/Au(1 1 0): Determination of the azimuthal alignment by a combination of angle-resolved photoemission and density functional theory}, series = {Journal of Electron Spectroscopy and Related Phenomena}, volume = {195}, journal = {Journal of Electron Spectroscopy and Related Phenomena}, issn = {0368-2048}, doi = {10.1016/j.elspec.2014.06.002}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-120986}, pages = {293-300}, year = {2014}, abstract = {Here we report on a combined experimental and theoretical study on the structural and electronic properties of a monolayer of Copper-Phthalocyanine (CuPc) on the Au(1 1 0) surface. Low-energy electron diffraction reveals a commensurate overlayer unit cell containing one adsorbate species. The azimuthal alignment of the CuPc molecule is revealed by comparing experimental constant binding energy (kxky)-maps using angle-resolved photoelectron spectroscopy with theoretical momentum maps of the free molecule's highest occupied molecular orbital (HOMO). This structural information is confirmed by total energy calculations within the framework of van-der-Waals corrected density functional theory. The electronic structure is further analyzed by computing the molecule-projected density of states, using both a semi-local and a hybrid exchange-correlation functional. In agreement with experiment, the HOMO is located about 1.2 eV below the Fermi-level, while there is no significant charge transfer into the molecule and the CuPc LUMO remains unoccupied on the Au(1 1 0) surface.}, language = {en} } @article{TvingstedtMalinkiewiczBaumannetal.2014, author = {Tvingstedt, Kristofer and Malinkiewicz, Olga and Baumann, Andreas and Deibel, Carsten and Snaith, Henry J. and Dyakonov, Vladimir and Bolink, Henk J.}, title = {Radiative efficiency of lead iodide based perovskite solar cells}, series = {Scientific Reports}, volume = {4}, journal = {Scientific Reports}, doi = {10.1038/srep06071}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119360}, pages = {6071}, year = {2014}, abstract = {The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ~0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate.}, language = {en} } @article{BaumannTvingstedtHeiberetal.2014, author = {Baumann, A. and Tvingstedt, K. and Heiber, M. C. and V{\"a}th, S. and Momblona, C. and Bolink, H. J. and Dyakonov, V.}, title = {Persistent photovoltage in methylammonium lead iodide perovskite solar cells}, series = {APL Materials}, volume = {2}, journal = {APL Materials}, number = {8}, doi = {10.1063/1.4885255}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119397}, pages = {081501}, year = {2014}, abstract = {We herein perform open circuit voltage decay (OCVD) measurements on methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells to increase the understanding of the charge carrier recombination dynamics in this emerging technology. Optically pulsed OCVD measurements are conducted on CH3NH3PbI3 solar cells and compared to results from another type of thin-film photovoltaics, namely, the two reference polymer-fullerene bulk heterojunction solar cell devices based on P3HT:PC60BM and PTB7:PC70BM blends. We observe two very different time domains of the voltage transient in the perovskite solar cell with a first drop on a short time scale that is similar to the decay in the studied organic solar cells. However, 65\%-70\% of the maximum photovoltage persists on much longer timescales in the perovskite solar cell than in the organic devices. In addition, we find that the recombination dynamics in all time regimes are dependent on the starting illumination intensity, which is also not observed in the organic devices. We then discuss the potential origins of these unique behaviors.}, language = {en} } @article{MaierGoldForcheletal.2014, author = {Maier, Sebastian and Gold, Peter and Forchel, Alfred and Gregersen, Niels and Mork, Jesper and H{\"o}fling, Sven and Schneider, Christian and Kamp, Martin}, title = {Bright single photon source based on self-aligned quantum dot-cavity systems}, series = {Optics Express}, volume = {22}, journal = {Optics Express}, number = {7}, issn = {1094-4087}, doi = {10.1364/OE.22.008136}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119801}, pages = {8136-42}, year = {2014}, abstract = {We report on a quasi-planar quantum-dot-based single-photon source that shows an unprecedented high extraction efficiency of 42\% without complex photonic resonator geometries or post-growth nanofabrication. This very high efficiency originates from the coupling of the photons emitted by a quantum dot to a Gaussian shaped nanohill defect that naturally arises during epitaxial growth in a self-aligned manner. We investigate the morphology of these defects and characterize the photonic operation mechanism. Our results show that these naturally arising coupled quantum dot-defects provide a new avenue for efficient (up to 42\% demonstrated) and pure (g(2)(0) value of 0.023) single-photon emission.}, language = {en} } @article{OPUS4-13827, title = {Search for the \(X_b\) and other hidden-beauty states in the \(π^+π^-ϒ\)(1S) channel at ATLAS}, series = {Physics Letters B}, volume = {740}, journal = {Physics Letters B}, organization = {ATLAS Collaboration}, doi = {10.1016/j.physletb.2014.11.055}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-138273}, pages = {199-217}, year = {2014}, abstract = {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.}, language = {en} } @phdthesis{Thierschmann2014, author = {Thierschmann, Holger}, title = {Heat Conversion in Quantum Dot Systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-133348}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {This thesis treats the thermopower and other thermal effects in single quantum dots (QD) and quantum dot systems. It contributes new experimental results to the broad and active field of research on thermoelectrics in low dimensional systems. The thermopower experiments discussed in this work focus on QDs which exhibit a net spin and on tunnel-coupled double QDs (DQD). Furthermore, experiments are presented which address the realization of a QD device which extracts thermal energy from a heat reservoir and converts it into a directed charge current in a novel way. The samples used for these investigations have been fabricated from GaAs/AlGaAs heterostructures which contain a two dimensional electron gas. Using optical and electron beam lithography, the devices have been realized by means of the top-gate technology. All experiments have been performed at low temperature. In order to create a controllable temperature difference in the samples the current heating technique has been used. These experimental basics as well as fundamentals of electric and thermoelectric transport are introduced in Part I of this thesis. The experiments on the thermopower of a single QD are described in Part II. Essentially, they deal with the problem of how a single spin situated on a QD influences the thermoelectric properties of the system. In this context, the Kondo-effect plays a crucial role. Generally, the Kondo effect is the result of a many-body state which arises from an antiferromagnetic coupling of a magnetic impurity with the surrounding conduction electrons. Here, the magnetic impurity is represented by a QD which is occupied with an odd number of electrons so that it exhibits a net spin. For the first time the thermopower of a Kondo-QD has been studied systematically as a function of two parameters, namely the QD coupling energy and the sample temperature. Both parameters are crucial quantities for Kondo-physics to be observed. Based on these data, it is shown that the thermopower line shape as a function of QD energy is mainly determined by two competing contributions: On the one hand by the enhanced density of states around the Fermi level due to Kondo-correlations and on the other hand by thermopower contributions from the Coulomb resonances. Furthermore, the experiments confirm theoretical predictions which claim that the spectral DOS arising from Kondo-correlations shifts away from the Fermi level for those QD level configurations which are not electron-hole symmetric. Comparison with model calculations by T. Costi and V. Zlatic [Phys. Rev. B 81, 235127 (2010)] shows qualitative and partly even quantitative agreement. A finite thermovoltage at the center of the Kondo-region, which occurred in previous investigations, is also observed in the experiments presented here. It is not covered by the current theory of the Kondo effect. The dependence of this signal on temperature, coupling energy and magnetic field, which differ from non-Kondo regions, is analyzed. In order to clarify the physics behind this phenomenon further studies are desirable. Furthermore, it is shown by variation of the QD coupling energy over a wide range that Kondo-correlations can be detected in the thermopower even in the regime of very weak coupling. In contrast, no Kondo signatures are visible in the conductance in this energy range. It is found that in the limit of weak coupling the Kondo effect causes the thermopower to exhibit a diminished amplitude in close vicinity of a conductance resonance. Subsequent filling of spin-degenerate states then leads to a thermopower amplitude modulation (odd-even-effect). Although this effect had been observed in previous studies, no connection to Kondo physics had been established in order to explain the observations. Hence, the experiments on a single QD presented in this thesis provide unique insight into the complex interplay of different transport mechanisms in a spin-correlated QD. Moreover, the results confirm the potential of thermopower measurements as a highly sensitive tool to probe Kondo-correlations. In Part III thermal effects are investigated in systems which contain two coupled QDs. Such QD-systems are particularly interesting with respect to thermoelectric applications: Many proposals utilize the extremely sharp energy filtering properties of such coupled QDs and also different kinds of inter dot coupling to construct novel and highly efficient thermoelectric devices. In the present work, thermopower characterizations are performed on a tunnel-coupled DQD for the first time. The key result of these investigations is the thermopower stability diagram. Here it is found, that in such a system maximal thermopower is generated in the vicinity of the so-called triple points (TP) at which three charge states of the DQD are degenerate. Along the axis of total energy, which connects two adjacent TP, a typical thermopower line shape is observed. It is explained and modeled within an intuitive picture that assumes two transport channels across the DQD, representing the TP. For those regions which are far away from the TP, the thermopower turns out to be very sensitive to the relative configuration of the QD energies. The conductance and thermopower data are well reproduced within a model that assumes transport via molecular states. Integration of both models into one then allows model calculations for a complete stability cell in conductance and thermopower to be done. Furthermore, experiments on two capacitively coupled QDs are presented. In these studies the focus lies on testing the feasibility of such systems for the manipulation and generation of charge currents from thermal energy. In a series of experiments it is shown that such a system of QDs can be utilized to increase or decrease a current flowing between two electron reservoirs by varying the temperature in a third reservoir. This effect is based on the cross-correlation of occupation fluctuations of the individual QDs. These are positive for certain QD energy level configurations and negative for others, which increases or decreases the charge current in the experiments, respectively. In the stability diagram this is manifested in a characteristic clover leaf shaped structure of positive and negative current changes in vicinity of the TP. All main experimental results are reproduced qualitatively in simple model calculations. Due to the close analogy between electrical and thermal conductance of a QD, this effect of thermal switching can, in principle, also be used to built a thermal transistor. Finally, it is shown that a system consisting of two Coulomb-coupled QDs, which couple a hot electron reservoir electrostatically to two cold electron reservoirs, can be utilized as a novel device which extracts heat from its environment and converts it into a directed charge current. The idea of this heat-to-current converter (HCC) was first proposed by R. S{\´a}nchez and M. B{\"u}ttiker [Phys. Rev. B 83, 085428 (2011)]. It is not only characterized by the novelty of its working principle but also by the fact, that it decouples the directions of charge current and energy flow. In the experiments presented here, such HCC-currents are identified unambiguously: For certain QD-level configurations an electric current between the two cold reservoirs is observed if the temperature in the third reservoir is increased. The direction of this current is shown to be independent of an external voltage. In contrast, the direction of the current exhibits a characteristic dependence on the tunneling coefficients of the QDs, as predicted by theory: By adjusting the thickness and the shape of the respective tunnel junctions, a charge current can be generated between two cold reservoirs, and it can even be inverted. The experimental observations are quantitatively reproduced by model calculations by R. S{\´a}nchez and B. Sothmann. Thus, the results represent direct evidence for the existence of HCC-currents. Due to the novelty of the working principle of the HCC and its relevance from a fundamental scientific point of view, the results presented here are an important step towards energy harvesting devices at the nano scale.}, subject = {Quantenpunkt}, language = {en} } @phdthesis{Gorenflot2014, author = {Gorenflot, Julien Fran{\c{c}}ois}, title = {Optical study of the excited states in the semiconducting polymer poly(3-hexylthiophene) for photovoltaic applications}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-116730}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {In the course of this dissertation, we have presented the interest of using spectroscopic methods to unravel the physics of polymer semiconductors in photovoltaic applications. Applying photoluminescence and photoinduced absorption spectroscopy to the reference system P3HT:PCBM has enabled us to study the major steps of photocurrent generation in organic bulk heterojunctions, from excitons generation to charges extraction and loss mechanisms and thus to improve the understanding of those mechanisms. The exciton binding energy, is the first obstacle to overcome for photocurrent generation in organic solar cell and the reason for the use of two materials, whose heterojunction act as a driving force for charge separation. We developed an original photoluminescence-detected field-induced exciton quenching method to investigate this energy. Absorption and photoluminescence spectra of pure P3HT show that, while both amorphous and crystalline domains participate in absorption, the energy is then transferred to the crystalline domains, from where the photoluminescence is exclusively originating. The field dependence of this photoluminescence showed that an energy of no less than 420 meV is necessary to split excitons into non photon-emitting species. Comparing those results with energy levels obtained by absorption and photoelectron spectroscopies, confirmed that the formation of those species is only a first step toward dissociation into free charges. Indeed, photoemission spectroscopy and the onset of photocurrent upon increasing the photon energy in a pure P3HT solar cell, concomitantly show that the energy level of a pair of free polarons is located 0.7 eV above the one of the exciton. The comprehensive analysis of those results originating from those different method enable us to draw a global picture of the states and energies involved in free polarons generation in pure material. This work has been widely acknowledged by the scientific community, published in Physical Review B in 2010 [1] and presented in national [2] and international [3] conferences. The spectroscopy of excited states is used to detect the presence of wanted species (charges) and potentially unwanted neutral species upon photoexcitation. As such, it offers us the possibility to qualify the efficiency of charge generation and, if any, identify the competing processes and the generation of unwanted species. In the frame of the European Marie Curie Research Network SolarNType,[4] this possibility was used - in combination with morphological, charge transport and devices characterizationsn - to study a number of new donor:acceptor blends. Thanks to those techniques, we were able to not only quantify the potential of those blends, but also to provide the chemist laboratories with a precious and detailed feedback on the strengths and weakness of the molecules, regarding charge generation, transport and extraction. The detailed study of terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for solar cells application was published in the peer review journal Synthetic Metals and was chosen to illustrate the cover page of the issue [5]. Finally, in the last chapter, we have used time resolved photoinduced absorption to improve the understanding of the charge carrier loss mechanisms in P3HT:PCBM active layers. This comprehension is of prime importance because, the fact that this recombination is far weaker than expected from the Langevin theory, enable polarons to travel further without recombining and thus to build thicker and more efficient devices. A comprehensive analysis of steady-state PIA spectra of pure P3HT, indicates that probing at 980 nm at a temperature between 140 and 250 K enables to monitor specifically polaron densities in both neat P3HT and P3HT:PCBM. Applying this finding to transient absorption enabled us to monitor, for the first time, the bimolecular recombination in pure P3HT, and to discover that - in sharp contrast with the blend - this recombination was in agreement with the Langevin theory. Moreover, it enables us to pinpoint the important role played by the existence of two materials and of energetical traps in the slow recombination and high recombination orders observed in the blend. This work has been published in the Journal of Applied Physics.[6] Those new insights in the photophysics of polymer:fullerene photoactive layers could have a strong impact on the future developement of those materials. Consistent measurements of the binding energy of excitons and intermediate species, would enable to clarify the role played by excess thermal energy in interfacial states dissociation. Better understanding of blends morphology and its influence on solar cells parameters and in particular on recombination could enable to reproduce the conditions of limited recombination on material systems offering some promising performances but with only limited active layer thicknesses. However, due to the number of parameters involved, further experimentation is required, before we can reach a quantitative modeling of bimolecular recombination. [1] Deibel et al., Phys. Rev. B, 81:085202, 2010 [2] Gorenflot et al., Deutsche Physikalische Gesellschaft Fr{\"u}hjahrstagung 2010, CPP20:10, Regensburg, Germany, 2010 [3] Gorenflot et al., International Conference of Synthetic Metals, 7Ax:05, Kyoto, Japan, 2010 [4] Marie-Curie RTN "SolarNTyp" Contract No. MRTN-CT-2006-035533 [5] Gorenflot et al., Synth. Met., 161(23{24):2669-2676, 2012 [6] Gorenflot et al., J. Appl. Phys., 115(14):144502, 2014}, subject = {Organische Solarzelle}, language = {en} } @article{RauHeindelUnsleberetal.2014, author = {Rau, Markus and Heindel, Tobias and Unsleber, Sebastian and Braun, Tristan and Fischer, Julian and Frick, Stefan and Nauerth, Sebastian and Schneider, Christian and Vest, Gwenaelle and Reitzenstein, Stephan and Kamp, Martin and Forchel, Alfred and H{\"o}fling, Sven and Weinfurter, Harald}, title = {Free space quantum key distribution over 500 meters using electrically driven quantum dot single-photon sources-a proof of principle experiment}, series = {New Journal of Physics}, volume = {16}, journal = {New Journal of Physics}, number = {043003}, doi = {10.1088/1367-2630/16/4/043003}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-116760}, year = {2014}, abstract = {Highly efficient single-photon sources (SPS) can increase the secure key rate of quantum key distribution (QKD) systems compared to conventional attenuated laser systems. Here we report on a free space QKD test using an electrically driven quantum dot single-photon source (QD SPS) that does not require a separate laser setup for optical pumping and thus allows for a simple and compact SPS QKD system. We describe its implementation in our 500 m free space QKD system in downtown Munich. Emulating a BB84 protocol operating at a repetition rate of 125 MHz, we could achieve sifted key rates of 5-17 kHz with error ratios of 6-9\% and g((2))(0)-values of 0.39-0.76.}, language = {en} } @article{BraunSchneiderMaieretal.2014, author = {Braun, T. and Schneider, C. and Maier, S. and Igusa, R. and Iwamoto, S. and Forchel, A. and H{\"o}fling, S. and Arakawa, Y. and Kamp, M.}, title = {Temperature dependency of the emission properties from positioned In(Ga)As/GaAs quantum dots}, series = {AIP Advances}, volume = {4}, journal = {AIP Advances}, number = {9}, issn = {2158-3226}, doi = {10.1063/1.4896284}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-115448}, year = {2014}, abstract = {In this letter we study the influence of temperature and excitation power on the emission linewidth from site-controlled InGaAs/GaAs quantum dots grown on nanoholes defined by electron beam lithography and wet chemical etching. We identify thermal electron activation as well as direct exciton loss as the dominant intensity quenching channels. Additionally, we carefully analyze the effects of optical and acoustic phonons as well as close-by defects on the emission linewidth by means of temperature and power dependent micro-photoluminescence on single quantum dots with large pitches. (C) 2014 Author(s).}, language = {en} } @phdthesis{Kern2014, author = {Kern, Johannes}, title = {Optical and electrical excitation of nanoantennas with atomic-scale gaps}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-115492}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {Nano-antennas are an emerging concept for the manipulation and control of optical fields at the sub-wavelength scale. In analogy to their radio- and micro-wave counterparts they provide an efficient link between propagating and localized fields. Antennas operating at optical frequencies are typically on the order of a few hundred nanometer in size and are fabricated from noble metals. Upon excitation with an external field the electron gas inside the antenna can respond resonantly, if the dimensions of the antenna are chosen appropriate. Consequently, the resonance wavelength depends on the antenna dimensions. The electron-density oscillation is a hybrid state of electron and photon and is called a localized plasmon resonance. The oscillating currents within the antenna constitute a source for enhanced optical near-fields, which are strongly localized at the metal surface. A particular interesting type of antennas are pairs of metal particles separated by a small insulating gap. For anti-symmetric gap modes charges of opposite sign reside across the gap. The dominating field-components are normal to the metal surface and due to the boundary conditions they are sizable only inside the gap. The attractive Coulomb interaction increases the surface-charge accumulation at the gap and enhanced optical fields occur within the insulating gap. The Coulomb interaction increases with decreasing gap size and extreme localization and strongest intensity enhancement is expected for small gap sizes. In this thesis optical antennas with extremely small gaps, just slightly larger than inter-atomic distances, are investigated by means of optical and electrical excitation. In the case of electrical excitation electron tunneling across the antenna gap is exploited. At the beginning of this thesis little was known about the optical properties of antennas with atomic scale gaps. Standard measurement techniques of field confinement and enhancement involving well-separated source, sample and detector are not applicable at atomic length-scales due to the interaction of the respective elements. Here, an elegant approach has been found. It is based on the fact that for closely-spaced metallic particles the energy splitting of a hybridized mode pair, consisting of symmetric and anti-symmetric mode, provides a direct measure for the Coulomb interaction over the gap. Gap antennas therefore possess an internal ruler which sensitively reports the size of the gap. Upon self-assembly side-by-side aligned nanorods with gap sizes ranging from 2 to 0.5nm could be obtained. These antennas exhibit various symmetric and anti-symmetric modes in the visible range. In order to reveal optical modes of all symmetries a novel scattering setup has been developed and is successfully applied. Careful analysis of the optical spectra and comparison to numerical simulations suggests that extreme field confinement and localization can occur in gaps down to 0.5 nm. This is possibly the limit of plasmonic enhancement since for smaller gaps electron tunneling as well as non-locality of the dielectric function affect plasmonic resonances. The strongly confined and intense optical fields provided by atomic-scale gaps are ideally suited for enhanced light-matter interaction. The interplay of intense optical-frequency fields and static electric fields or currents is of great interest for opto-electronic applications. In this thesis a concept has been developed, which allows for the electrical connection of optical antennas. By means of numerical simulations the concept was first verified for antennas with gap sizes on the order of 25 nm. It could be shown, that by attaching the leads at positions of a field minimum the resonant properties are nearly undisturbed. The resonance wavelengths shift only by a small amount with respect to isolated antennas and the numerically calculated near-field intensity enhancement is about 1000, which is just slightly lower than for an unconnected antenna. The antennas are fabricated from single-crystalline gold and exhibit superior optical and electrical properties. In particular, the conductivity is a factor of 4 larger with respect to multi-crystalline material, the resistance of the gap is as large as 1 TOhm and electric fields of at least 10^8 V/m can be continuously applied without damage. Optical scattering spectra reveal well-pronounced and tunable antenna resonances, which demonstrates the concept of electrically-connected antennas also experimentally. By combining atomic-scale gaps and electrically-connected optical antennas a novel sub-wavelength photon source has been realized. To this end an antenna featuring an atomic scale gap is electrically driven by quantum tunneling across the antenna gap. The optical frequency components of this fluctuating current are efficiently converted to photons by the antenna. Consequently, light generation and control are integrated into a planar single-material nano-structure. Tunneling junctions are realized by positioning gold nanoparticles into the antenna gap, using an atomic force microscope. The presence of a stable tunneling junction between antenna and particle is demonstrated by measuring its distinct current-voltage characteristic. A DC voltage is applied to the junction and photons are generated by inelastically tunneling electrons via the enhanced local density of photonic states provided by the antenna resonance. The polarization of the emitted light is found to be along the antenna axis and the directivity is given by the dipolar antenna mode. By comparing electroluminescence and scattering spectra of different antennas, it has been shown that the spectrum of the generated light is determined by the geometry of the antenna. Moreover, the light generation process is enhanced by two orders of magnitude with respect to a non-resonant structure. The controlled fabrication of the presented single-crystalline structures has not only pushed the frontiers of nano-technology, but the extreme confinement and enhancement of optical fields as well as the light generation by tunneling electrons lays a groundwork for a variety of fundamental studies and applications. Field localization down to the (sub-)nanometer scale is a prerequisite for optical spectroscopy with near-atomic resolution. Indeed, recently first pioneering experiments have achieved molecular resolution exploiting plasmon-enhanced Raman scattering. The small modal volume of antennas with atomic-scale gaps can lead to light-matter interaction in the strong coupling regime. Quantum electro-dynamical effects such as Rabi splitting or oscillations are likely when a single emitter is placed into resonant structures with atomic-scale gaps. The concept of electrically-connected optical antennas is expected to be widely applied within the emerging field of electro-plasmonics. The sub-wavelength photon source developed during this thesis will likely gain attention for future plasmonic nanocircuits. It is envisioned that in such a circuit the optical signal provided by the source is processed at ultrafast speed and nanometer-scales on the chip and is finally converted back into an electronic signal. An integrated optical transistor could be realized by means of photon-assisted tunneling. Moreover, it would be interesting to investigate, if it is possible to imprint the fermionic nature of electrons onto photons in order to realize an electrically-driven source of single photons. Non-classical light sources with the potential for on-chip integration could be built from electrically-connected antennas and are of great interest for quantum communication. To this end single emitters could be placed in the antenna gap or single electron tunneling could be achieved by means of a single-channel quantum point contact or the Coulomb-blockade effect.}, subject = {Nanooptik}, language = {en} } @article{BareilleFortunaRoedeletal.2014, author = {Bareille, C. and Fortuna, F. and R{\"o}del, T. C. and Bertran, F. and Gabay, M. and Hijano Cubelos, O. and Taleb-Ibrahimi, A. and Le F{\`e}vre, P. and Bibes, M. and Barthelemy, A. and Maroutian, T. and Lecoeur, P. and Rozenberg, M. J. and Santander-Syro, A. F.}, title = {Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3}, series = {Scientific Reports}, volume = {4}, journal = {Scientific Reports}, issn = {2045-2322}, doi = {10.1038/srep03586}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117703}, pages = {3586}, year = {2014}, abstract = {Two-dimensional electron gases (2DEGs) at transition-metal oxide (TMO) interfaces, and boundary states in topological insulators, are being intensively investigated. The former system harbors superconductivity, large magneto-resistance, and ferromagnetism. In the latter, honeycomb-lattice geometry plus bulk spin-orbit interactions lead to topologically protected spin-polarized bands. 2DEGs in TMOs with a honeycomb-like structure could yield new states of matter, but they had not been experimentally realized, yet. We successfully created a 2DEG at the (111) surface of KTaO3, a strong insulator with large spin-orbit coupling. Its confined states form a network of weakly-dispersing electronic gutters with 6-fold symmetry, a topology novel to all known oxide-based 2DEGs. If those pertain to just one Ta-(111) bilayer, model calculations predict that it can be a topological metal. Our findings demonstrate that completely new electronic states, with symmetries not realized in the bulk, can be tailored in oxide surfaces, promising for TMO-based devices.}, language = {en} } @phdthesis{Zusan2014, author = {Zusan, Andreas}, title = {The Effect of Morphology on the Photocurrent Generation in Organic Solar Cells}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117852}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {Organic solar cells have great potential to become a low-cost and clean alternative to conventional photovoltaic technologies based on the inorganic bulk material silicon. As a highly promising concept in the field of organic photovoltaics, bulk heterojunction (BHJ) solar cells consist of a mixture of an electron donating and an electron withdrawing component. Their degree of intermixing crucially affects the generation of photocurrent. In this work, the effect of an altered blend morphology on polaron pair dissociation, charge carrier transport, and nongeminate recombination is analyzed by the charge extraction techniques time delayed collection field (TDCF) and open circuit corrected transient charge extraction (OTRACE). Different comparative studies cover a broad range of material systems, including polymer and small-molecule donors in combination with different fullerene acceptors. The field dependence of polaron pair dissociation is analyzed in blends based on the polymer pBTTT-C16, allowing a systematic tuning of the blend morphology by varying the acceptor type and fraction. The effect of both excess photon energy and intercalated phases are minor compared to the influence of excess fullerene, which reduces the field dependence of photogeneration. The study demonstrates that the presence of neat fullerene domains is the major driving force for efficient polaron pair dissociation that is linked to the delocalization of charge carriers. Furthermore, the influence of the processing additive diiodooctane (DIO) is analyzed using the photovoltaic blends PBDTTT-C:PC71BM and PTB7:PC71BM. The study reveals amulti-tiered alteration of the blend morphology of PBDTTT-C based blends upon a systematic increase of the amount of DIO. Domains on the hundred nanometers length scale in the DIO-free blend are identified as neat fullerene agglomerates embedded in an intermixed matrix. With the addition of the additive, 0.6\% and 1\% DIO already substantially reduces the size of these domains until reaching the optimum 3\% DIO mixture, where a 7.1\% power conversion efficiency is obtained. It is brought into connection with the formation of interpenetrating polymer and fullerene phases. Similar to PBDTTT-C, the morphology of DIO-free PTB7:PC71BM blends is characterized by large fullerene domains being decreased in size upon the addition of 3\% DIO. OTRACE measurements reveal a reduced Langevin-type, super-second order recombination in both blends. It is demonstrated that the deviation from bimolecular recombination kinetics cannot be fully attributed to the carrier density dependence of the mobility but is rather related to trapping in segregated PC71BM domains. Finally, with regard to small-molecule donors, a higher yield of photogeneration and balanced transport properties are identified as the dominant factors enhancing the efficiency of vacuum deposited MD376:C60 relative to its solution processed counterpart MD376:PC61BM. The finding is explained by a higher degree of dimerization of the merocyanine dye MD376 and a stronger donor-acceptor interaction at the interface in the case of the vacuum deposited blend.}, subject = {Organische Solarzelle}, language = {en} } @article{BillerCholiBlaimeretal.2014, author = {Biller, Armin and Choli, Morwan and Blaimer, Martin and Breuer, Felix A. and Jakob, Peter M. and Bartsch, Andreas J.}, title = {Combined Acquisition Technique (CAT) for Neuroimaging of Multiple Sclerosis at Low Specific Absorption Rates (SAR)}, series = {PLOS ONE}, volume = {9}, journal = {PLOS ONE}, number = {3}, issn = {1932-6203}, doi = {10.1371/journal.pone.0091030}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-117179}, pages = {e91030}, year = {2014}, abstract = {Purpose: To compare a novel combined acquisition technique (CAT) of turbo-spin-echo (TSE) and echo-planar-imaging (EPI) with conventional TSE. CAT reduces the electromagnetic energy load transmitted for spin excitation. This radiofrequency (RF) burden is limited by the specific absorption rate (SAR) for patient safety. SAR limits restrict high-field MRI applications, in particular. Material and Methods: The study was approved by the local Medical Ethics Committee. Written informed consent was obtained from all participants. T2- and PD-weighted brain images of n = 40 Multiple Sclerosis (MS) patients were acquired by CAT and TSE at 3 Tesla. Lesions were recorded by two blinded, board-certificated neuroradiologists. Diagnostic equivalence of CAT and TSE to detect MS lesions was evaluated along with their SAR, sound pressure level (SPL) and sensations of acoustic noise, heating, vibration and peripheral nerve stimulation. Results: Every MS lesion revealed on TSE was detected by CAT according to both raters (Cohen's kappa of within-rater/across-CAT/TSE lesion detection kappa(CAT) = 1.00, at an inter-rater lesion detection agreement of kappa(LES) = 0.82). CAT reduced the SAR burden significantly compared to TSE (p<0.001). Mean SAR differences between TSE and CAT were 29.0 (+/- 5.7) \% for the T2-contrast and 32.7 (+/- 21.9) \% for the PD-contrast (expressed as percentages of the effective SAR limit of 3.2 W/kg for head examinations). Average SPL of CAT was no louder than during TSE. Sensations of CAT-vs. TSE-induced heating, noise and scanning vibrations did not differ. Conclusion: T2-/PD-CAT is diagnostically equivalent to TSE for MS lesion detection yet substantially reduces the RF exposure. Such SAR reduction facilitates high-field MRI applications at 3 Tesla or above and corresponding protocol standardizations but CAT can also be used to scan faster, at higher resolution or with more slices. According to our data, CAT is no more uncomfortable than TSE scanning.}, language = {en} } @article{KuemmelLindenberger2014, author = {K{\"u}mmel, Reiner and Lindenberger, Dietmar}, title = {How energy conversion drives economic growth far from the equilibrium of neoclassical economics}, series = {New Journal of Physics}, volume = {16}, journal = {New Journal of Physics}, number = {125008}, issn = {1367-2630}, doi = {10.1088/1367-2630/16/12/125008}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-118102}, year = {2014}, abstract = {Energy conversion in the machines and information processors of the capital stock drives the growth of modern economies. This is exemplified for Germany, Japan, and the USA during the second half of the 20th century: econometric analyses reveal that the output elasticity, i.e. the economic weight, of energy is much larger than energyʼs share in total factor cost, while for labor just the opposite is true. This is at variance with mainstream economic theory according to which an economy should operate in the neoclassical equilibrium, where output elasticities equal factor cost shares. The standard derivation of the neoclassical equilibrium from the maximization of profit or of time-integrated utility disregards technological constraints. We show that the inclusion of these constraints in our nonlinear-optimization calculus results in equilibrium conditions, where generalized shadow prices destroy the equality of output elasticities and cost shares. Consequently, at the prices of capital, labor, and energy we have known so far, industrial economies have evolved far from the neoclassical equilibrium. This is illustrated by the example of the German industrial sector evolving on the mountain of factor costs before and during the first and the second oil price explosion. It indicates the influence of the 'virtually binding' technological constraints on entrepreneurial decisions, and the existence of 'soft constraints' as well. Implications for employment and future economic growth are discussed.}, language = {en} } @article{SchwenderKoenigKlapperstuecketal.2014, author = {Schwender, Joerg and Koenig, Christina and Klapperstueck, Matthias and Heinzel, Nicolas and Munz, Eberhard and Hebbelmann, Inga and Hay, Jordan O. and Denolf, Peter and De Bodt, Stefanie and Redestig, Henning and Caestecker, Evelyne and Jakob, Peter M. and Borisjuk, Ljudmilla and Rolletschek, Hardy}, title = {Transcript abundance on its own cannot be used to infer fluxes in central metabolism}, series = {Frontiers in Plant Science}, volume = {5}, journal = {Frontiers in Plant Science}, issn = {1664-462X}, doi = {10.3389/fpls.2014.00668}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-114586}, year = {2014}, abstract = {An attempt has been made to define the extent to which metabolic flux in central plant metabolism is reflected by changes in the transcriptome and metabolome, based on an analysis of in vitro cultured immature embryos of two oilseed rape (Brassica napus) accessions which contrast for seed lipid accumulation. Metabolic flux analysis (MFA) was used to constrain a flux balance metabolic model which included 671 biochemical and transport reactions within the central metabolism. This highly confident flux information was eventually used for comparative analysis of flux vs. transcript (metabolite). Metabolite profiling succeeded in identifying 79 intermediates within the central metabolism, some of which differed quantitatively between the two accessions and displayed a significant shift corresponding to flux. An RNA-Seq based transcriptome analysis revealed a large number of genes which were differentially transcribed in the two accessions, including some enzymes/proteins active in major metabolic pathways. With a few exceptions, differential activity in the major pathways (glycolysis, TCA cycle, amino acid, and fatty acid synthesis) was not reflected in contrasting abundances of the relevant transcripts. The conclusion was that transcript abundance on its own cannot be used to infer metabolic activity/fluxes in central plant metabolism. This limitation needs to be borne in mind in evaluating transcriptome data and designing metabolic engineering experiments.}, language = {en} } @article{DauthWiessnerFeyeretal.2014, author = {Dauth, M. and Wiessner, M. and Feyer, V. and Sch{\"o}ll, A. and Puschnig, P. and Reinert, F. and Kuemmel, S.}, title = {Angle resolved photoemission from organic semiconductors: orbital imaging beyond the molecular orbital interpretation}, series = {New Journal of Physics}, volume = {16}, journal = {New Journal of Physics}, issn = {1367-2630}, doi = {10.1088/1367-2630/16/10/103005}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-115180}, pages = {103005}, year = {2014}, abstract = {Fascinating pictures that can be interpreted as showing molecular orbitals have been obtained with various imaging techniques. Among these, angle resolved photoemission spectroscopy (ARPES) has emerged as a particularly powerful method. Orbital images have been used to underline the physical credibility of the molecular orbital concept. However, from the theory of the photoemission process it is evident that imaging experiments do not show molecular orbitals, but Dyson orbitals. The latter are not eigenstates of a single-particle Hamiltonian and thus do not fit into the usual simple interpretation of electronic structure in terms of molecular orbitals. In a combined theoretical and experimental study we thus check whether a Dyson-orbital and a molecular-orbital based interpretation of ARPES lead to differences that are relevant on the experimentally observable scale. We discuss a scheme that allows for approximately calculating Dyson orbitals with moderate computational effort. Electronic relaxation is taken into account explicitly. The comparison reveals that while molecular orbitals are frequently good approximations to Dyson orbitals, a detailed understanding of photoemission intensities may require one to go beyond the molecular orbital picture. In particular we clearly observe signatures of the Dyson-orbital character for an adsorbed semiconductor molecule in ARPES spectra when these are recorded over a larger momentum range than in earlier experiments.}, language = {en} } @phdthesis{ElKareh2014, author = {El-Kareh, Lydia}, title = {Rashba-type spin-split surface states: Heavy post transition metals on Ag(111)}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-112722}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {In the framework of this thesis, the structural and electronic properties of bismuth and lead deposited on Ag(111) have been investigated by means of low-temperature scanning tunneling microscopy (LT-STM) and spectroscopy (STS). Prior to spectroscopic investigations the growth characteristics have been investigated by means of STM and low energy electron diffraction (LEED) measurements. Submonolayer coverages as well as thick films have been investigated for both systems. Subsequently the quantum well characteristics of thick Pb films on Ag(111) have been analyzed and the quantum well character could be proved up to layer thicknesses of N ≈ 100 ML. The observed characteristics in STS spectra were explained by a simple cosine Taylor expansion and an in-plane energy dispersion could be detected by means of quasi-particle interferences. The main part of this work investigates the giant Rashba-type spin-split surface alloys of (√3 × √3)Pb/Ag(111)R30◦ and (√3 × √3)Bi/Ag(111)R30◦. With STS experiments the band positions and splitting strengths of the unoccupied (√3 × √3)Pb/Ag(111)R30◦ band dispersions could be resolved, which were unclear so far. The investigation by means of quasi-particle interferences resulted in the observation of several scattering events, which could be assigned as intra- and inter-band transitions. The analysis of scattering channels within a simple spin-conservation-approach turned out to be incomplete and led to contradictions between experiment and theory. In this framework more sophisticated DFT calculations could resolve the apparent deviations by a complete treatment of scattering in spin-orbit-coupled materials, which allows for constructive interferences in spin-flip scattering processes as long as the total momentum J_ is conserved. In a similar way the band dispersion of (√3 × √3)Bi/Ag(111)R30◦ was investigated. The STS spectra confirmed a hybridization gap opening between both Rashba-split bands and several intra- and inter-band scattering events could be observed in the complete energy range. The analysis within a spin-conservation-approach again turned out to be insufficient for explaining the observed scattering events in spin-orbit-coupled materials, which was confi by DFT calculations. Within these calculations an inter-band scattering event that has been identified as spin-conserving in the simple model could be assigned as a spin-flip scattering channel. This illustrates evidently how an incomplete description can lead to completely different indications. The present work shows that different spectroscopic STM modes are able to shed light on Rashba-split surface states. Whereas STS allowed to determine band onsets and splitting strengths, quasi-particle interferences could shed light on the band dispersions. A very important finding of this work is that spin-flip scattering events may result in constructive interferences, an eff which has so far been overlooked in related publications. Additionally it has been found that STM measurements can not distinguish between spin-conserving scattering events or spin-flip scattering events, which prevents to give a definite conclusion on the spin polarization for systems with mixed orbital symmetries just from the observed scattering events.}, subject = {Silber}, language = {en} } @article{BrixnerPawłowskaGoetzetal.2014, author = {Brixner, Tobias and Pawłowska, Monika and Goetz, Sebastian and Dreher, Christian and Wurdack, Matthias and Krauss, Enno and Razinskas, Gary and Geisler, Peter and Hecht, Bert}, title = {Shaping and spatiotemporal characterization of sub-10-fs pulses focused by a high-NA objective}, doi = {10.1364/OE.22.031496}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-111120}, year = {2014}, abstract = {We describe a setup consisting of a 4 f pulse shaper and a microscope with a high-NA objective lens and discuss the spects most relevant for an undistorted spatiotemporal profile of the focused beam. We demonstrate shaper-assisted pulse compression in focus to a sub-10-fs duration using phase-resolved interferometric spectral modulation (PRISM). We introduce a nanostructure-based method for sub-diffraction spatiotemporal characterization of strongly focused pulses. The distortions caused by optical aberrations and space-time coupling from the shaper can be reduced by careful setup design and alignment to about 10 nm in space and 1 fs in time.}, language = {en} } @article{ElKarehBihlmayerBuchteretal.2014, author = {El-Kareh, Lydia and Bihlmayer, Gustav and Buchter, Arne and Bentmann, Hendrik and Bl{\"u}gel, Stefan and Reinert, Friedrich and Bode, Matthias}, title = {A combined experimental and theoretical study of Rashba-split surface states on the ( √3x√3) Pb/Ag (111)R30° surface}, doi = {doi:10.1088/1367-2630/16/4/045017}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-112786}, year = {2014}, abstract = {We report on a combined low-temperature scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) investigation of the ( √3x√3) Pb/Ag (111)R30° surface alloy which provides a giant Rashba-type spin splitting. With STS we observed spectroscopic features that are assigned to two hole-like Rashba-split bands in the unoccupied energy range. By means of STS and quantum interference mapping we determine the band onsets, splitting strengths, and dispersions for both bands. The unambiguous assignment of scattering vectors is achieved by comparison to ARPES measurements. While intra-band scattering is found for both Rashba bands, inter-band scattering is only observed in the occupied energy range. Spin- and orbitally-resolved band structures were obtained by DFT calculations. Considering the scattering between states of different spin- and orbital character, the apparent deviation between experimentally observed scattering events and the theoretically predicted spin polarization could be resolved.}, language = {en} } @article{AstakhovKrausSoltamovetal.2014, author = {Astakhov, Georgy V. and Kraus, Hannes and Soltamov, V. A. and Fuchs, Franziska and Simin, Dimitrij and Sperlich, Andreas and Baranov, P. G. and Dyakonov, Vladimir}, title = {Magnetic field and temperature sensing with atomic-scale spin defects in silicon carbide}, doi = {10.1038/srep05303}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-113025}, year = {2014}, abstract = {Quantum systems can provide outstanding performance in various sensing applications, ranging from bioscience to nanotechnology. Atomic-scale defects in silicon carbide are very attractive in this respect because of the technological advantages of this material and favorable optical and radio frequency spectral ranges to control these defects. We identified several, separately addressable spin-3/2 centers in the same silicon carbide crystal, which are immune to nonaxial strain fluctuations. Some of them are characterized by nearly temperature independent axial crystal fields, making these centers very attractive for vector magnetometry. Contrarily, the zero-field splitting of another center exhibits a giant thermal shift of -1.1 MHz/K at room temperature, which can be used for thermometry applications. We also discuss a synchronized composite clock exploiting spin centers with different thermal response.}, language = {en} } @phdthesis{Henn2014, author = {Henn, Tobias}, title = {Hot spin carriers in cold semiconductors : Time and spatially resolved magneto-optical Kerr effect spectroscopy of optically induced electron spin dynamics in semiconductor heterostructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-110265}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {The present thesis "Hot spin carriers in cold semiconductors" investigates hot carrier effects in low-temperature photoinduced magneto-optical Kerr effect (MOKE) microscopy of electron spins in semiconductor heterostructures. Our studies reveal that the influence of hot photocarriers in magneto-optical pump-probe experiments is twofold. First, it is commonly assumed that a measurement of the local Kerr rotation using an arbitrary probe wavelength maps the local electron spin polarization. This is the fundamental assumption that underlies the widely used two-color MOKE microscopy technique. Our continuous-wave (cw) spectroscopy experiments demonstrate that this assumption is not correct. At low lattice temperatures the nonresonant spin excitation by the focused pump laser inevitably leads to a strong heating of the electron system. This heating, in turn, locally modifies the magneto-optical coefficient which links the experimentally observed Kerr rotation to the electron spin polarization. As a consequence, the spin-induced local Kerr rotation is augmented by spin-unrelated changes in the magneto-optical coefficient. A spatially resolved measurement of the Kerr rotation then does not correctly map the electron spin polarization profile. We demonstrate different ways to overcome this limitation and to correctly measure the electron spin profile. For cw spectroscopy we show how the true local electron spin polarization can be obtained from a quantitative analysis of the full excitonic Kerr rotation spectrum. Alternatively, picosecond MOKE microscopy using a spectrally broad probe laser pulse mitigates hot-carrier effects on the magneto-optical spin detection and allows to directly observe the time-resolved expansion of optically excited electron spin packets in real-space. Second, we show that hot photocarriers strongly modify the spin diffusion process. Owing to their high kinetic energy, hot carriers greatly enhance the electron spin diffusion coefficient with respect to the intrinsic value of the undisturbed system. Therefore, for steady-state excitation the spin diffusivity is strongly enhanced close to the pump spot center where hot electrons are present. Similarly, for short delays following pulsed excitation the high initial temperature of the electrons leads to a very fast initial expansion of the spin packet which gradually slows as the electrons cool down to the lattice temperature. While few previous publications have recognized the possible influence of hot carriers on the electron spin transport properties, the present work is the first to directly observe and quantify such hot carrier contributions. We develop models which for steady-state and pulsed excitation quantitatively describe the experimentally observed electron spin diffusion. These models are capable of separating the intrinsic spin diffusivity from the hot electron contribution, and allow to obtain spin transport parameters of the undisturbed system. We perform extensive cw and time-resolved spectroscopy studies of the lattice temperature dependence of the electron spin diffusion in bulk GaAs. Using our models we obtain a consistent set of parameters for the intrinsic temperature dependence of the electron spin diffusion coefficient and spin relaxation time and the hot carrier contributions which quantitatively describes all experimental observations. Our analysis unequivocally demonstrates that we have, as we believe for the first time, arrived at a coherent understanding of photoinduced low-temperature electron spin diffusion in bulk semiconductors.}, subject = {Galliumarsenid}, language = {en} } @phdthesis{Proppert2014, author = {Proppert, Sven Martin}, title = {Design, implementation and characterization of a microscope capable of three-dimensional two color super-resolution fluorescence imaging}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-107905}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {This thesis reviews the fundamentals of three-dimensional super-resolution localization imaging. In order to infer the axial coordinate of the emission of single fluorophores, the point spread function is engineered following a technique usually referred to as astigmatic imaging by the introduction of a cylindrical lens to the detection path of a microscope. After giving a short introduction to optics and localization microscopy, I outline sources of aberrations as frequently encountered in 3D-localization microscopy and will discuss their respective impact on the precision and accuracy of the localization process. With the knowledge from these considerations, experiments were designed and conducted to verify the validity of the conclusions and to demonstrate the abilities of the proposed microscope to resolve biological structures in the three spatial dimensions. Additionally, it is demonstrated that measurements of huge volumes with virtually no aberrations is in principle feasible. During the course of this thesis, a new method was introduced for inferring axial coordinates. This interpolation method based on cubic B-splines shows superior performance in the calibration of a microscope and the evaluation of subsequent measurement and will therefore be used and explained in this work. Finally, this work is also meant to give future students some guidance for entering the field of 3D localization microscopy and therefore, detailed protocols are provided covering the specific aspects of two color 3D localization imaging.}, subject = {Dimension 3}, language = {en} } @phdthesis{Sauer2014, author = {Sauer, Christoph}, title = {Accessing molecule-metal and hetero-molecular interfaces with direct and resonant photoelectron spectroscopy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-107928}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {This thesis consists of two parts of original experimental work, its evaluation, and in- terpretation. Its final goal is to investigate dynamical charge transfer (CT) at a hetero- molecular interface with resonant photoelectron spectroscopy (RPES). In order to achieve this goal preliminary studies have been necessary. First two hetero-molecular inter- faces that exhibit adequate structural properties as well as an appropriate photoelec- tron spectroscopy (PES) spectrum of the valence regime have been identified. The de- sired CT analysis with RPES of these hetero-molecular systems is then conducted on the basis of the knowledge gained by previous RPES studies of homo-molecular sys- tems. The characterization of hetero-molecular films on single crystal Ag surfaces in the first part of this thesis is performed with high resolution core level PES and valence PES. The reproduction of the core level PES data with reference spectra of homo-molecular films allows me to determine which molecule is in direct contact to the Ag surface and which one is situated in higher layers (not the first one). Due to the direct correspon- dence of core level and valence PES the assignment of features in the spectra of the latter technique can be achieved with the identification of the contributions extracted from the evaluation of the data of the former technique. It is found that the systems PTCDA on one monolayer (ML) of SnPc on Ag(111) and CuPc/1 ML PTCDA/Ag(111) are stable at 300 K which means that no significant layer exchange occurs for these systems. In contrast a vertical exchange of CuPc and PTCDA molecules is observed for PTCDA de- posited on top of 1 ML CuPc/Ag(111). Up to a coverage of approximately 0.5 ML of PTCDA molecules these diffuse into the first layer, replace CuPc molecules, and con- sequently force them into higher layers. Above a coverage of approximately 0.5 ML of PTCDA molecules these are also found in higher layers. The search for a promising system for the intended RPES study then leads to an investigation of hetero-molecular films with a combination of F4TCNQ and PTCDA molecules on Ag(110) within the same approach. Depositing F4TCNQ molecules onto a 1 ML PTCDA/Ag(110) film in the herringbone phase at 300 K results in an instable hetero-organic system which un- dergoes a layer exchange. Hereby PTCDA molecules in the first layer are replaced by F4TCNQ molecules similar to the behavior of the system PTCDA/1 ML CuPc/Ag(111). Switching the order of the preparation steps leads to a stable film of PTCDA/1.0 ML F4TCNQ/Ag(110) at 300 K. Among the stable hetero-molecular films only the system CuPc/1 ML PTCDA/Ag(111) exhibits the required wetting growth of the first two layers at 300 K and a valence PES spectrum with energetically separable molecular orbital signals in the same intensity range. Thus this system is identified to be appropriate for a detailed analysis with RPES. The unexpected findings of vertical exchanges in the hetero-molecular films at 300 K motivate a study of the behavior at elevated temperatures for all systems investigated before. Therein it is revealed that annealing 1.5 ML SnPc/1 ML PTCDA/Ag(111) and 1.0 ML PTCDA/1 ML SnPc/Ag(111) to a temperature above the desorption temperature of molecules not in direct contact to the Ag(111) surface results in a 1 ML SnPc/Ag(111) film in both cases. Hence at elevated temperatures (approximately above 420 K) SnPc molecules replace PTCDA molecules in the first layer on Ag(111). At higher temper- atures (approximately above 470 K) PTCDA molecules and SnPc molecules situated above the first layer then desorb from the 1 ML SnPc/Ag(111) sample. Annealing all hetero-molecular films with CuPc and PTCDA molecules on Ag(111) to 570 K leads to a sample with CuPc and PTCDA molecules in the first and only layer. Depending on the initial CuPc coverage different ratios of both molecules are obtained. With a CuPc coverage of exactly 1 ML, or above, films with PTCDA coverages of approxi- mately 0.1-0.2 ML are produced. So at elevated temperatures CuPc molecules replace PTCDA molecules in the first layer of the system CuPc/1 ML PTCDA/Ag(111). Anal- ogously the layer exchange at 300 K for the system PTCDA/1 ML CuPc/Ag(111) is reversed at elevated temperatures. In the case of SnPc and CuPc coverages below 1 ML annealing vertical hetero-molecular systems with PTCDA on Ag(111) up to 570 K re- sults in a single layer of mixed hetero-molecular films with lateral long range order. In this way the system CuPc + PTCDA/Ag(111) is prepared and then characterized as a proper system for a detailed analysis with RPES. Additional annealing experiments of hetero-organic films consisting of F4TCNQ and PTCDA molecules on Ag(110) with an F4TCNQ coverage of 1.0 ML (and above) end in a submonolayer (sub-ML) film of F4TCNQ/Ag(110) that exhibits a contribution of amorphous carbon. Consequently, it can be concluded that at elevated temperatures part of the F4TCNQ molecules decom- pose. In the second part of this thesis homo-molecular multilayer samples and (sub-)ML films on single crystalline metal surfaces are investigated with RPES in order to enable the final RPES study of vertical and lateral hetero-molecular interface systems. First a pho- ton energy (hν) dependent intensity variation of (groups of) molecular orbital signals of exemplary multilayer films (NTCDA and coronene) is studied and explained on the basis of the local character of the electronic transitions in near edge x-ray absorption fine structure (NEXAFS) spectroscopy in combination with the real space probability den- sity of the contributing molecular orbitals. This simple approach is found to be able to correctly describe relative intensity variations by orders of magnitude while it fails for hν dependent relative intensity changes in the same order of magnitude. After that the hν dependent line-shape evolution of an energetically separated molecular orbital signal of a CuPc multilayer is discussed in relation to small molecules in the gas phase and explained with an effect of electron vibration coupling. Through a comparison of the hν dependent line-shape evolution of the highest occupied molecular orbital (HOMO) of a CuPc with a SnPc multilayer the molecule specific character of this effect is identified. Then the same effect with either two (or more) electronic transitions or multiple coupling vibrational modes is observed for a coronene multilayer. Thereafter the influence of the adsorption on metal surfaces on this effect is studied and discussed with special emphasis on a possible contribution by features which are related to dynamical interface CT. For a sub-ML of SnPc/Au(111) no variation with respect to a SnPc multilayer film is detected while for a sub-ML of CuPc/Au(111) less intensity is distributed into the high binding energy (EB) part of the HOMO signal with respect to the corresponding multilayer film. In the RPES data of a sub-ML of coronene/Ag(111) a resonance specific variation of the hν dependent line-shape evolution of the HOMO signal is found by the revelation of a change of this effect with respect to the coronene multilayer data in only one of the two NEXAFS resonances. All these findings are consistently explained within one effect and a common set of parameters, namely all quantities that characterize the potential energy surfaces involved in the RPES process. Through that an alternative explanation that re- lies on dynamical CT can be excluded which influences the following CT analysis with RPES. Three criteria for such an analysis of dynamical interface CT with RPES are identified. In the system coronene on Ag(111) a low EB feature is related to metal-molecule inter- face CT through the assignment of a particular final state and hence named CT state. In the EB region of the frontier molecular orbital signals of the molecule-metal inter- face systems with a signal from the lowest unoccupied molecular orbital (LUMO) in direct valence PES a broad line-shape is measured in RPES. This finding is related to interface CT by a possible explanation that emerges through the comparison to the line- shape of the CT state. The constant kinetic energy (EK ) features detected for several molecule-metal interfaces constitute the third criterion for a CT analysis with RPES. For the molecule-metal interface systems without a LUMO signal in direct valence PES the energy of these features can be calculated with the assignment of the responsible decay channel in combination with explicitly given simplifying assumptions. Through that the involvement of metal-molecule interface CT in the generation of these constant EK fea- tures is demonstrated. The RPES data of the lateral and the vertical hetero-molecular interface, identified in the first part, is then scanned for these three CT criteria. Thereby neither for the lateral hetero-molecular system CuPc + PTCDA/Ag(111) nor for the verti- cal hetero-molecular system CuPc/1 ML PTCDA/Ag(111) dynamical hetero-molecular interface CT can be confirmed. In the former system the molecule-metal interface in- teraction is found to dominate the physics of the system in RPES while in the latter system no hints for a significant hybridization at the CuPc-PTCDA interface can be revealed}, subject = {Organisches Molek{\"u}l}, language = {en} } @phdthesis{Neumann2014, author = {Neumann, Daniel}, title = {Advances in Fast MRI Experiments}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-108165}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {Magnetic Resonance Imaging (MRI) is a non-invasive medical imaging technique, that is rou- tinely used in clinical practice for detection and diagnosis of a wide range of different diseases. In MRI, no ionizing radiation is used, making even repeated application unproblematic. This is an important advantage over other common imaging methods such as X-rays and Computer To- mography. One major drawback of MRI, however, are long acquisition times and associated high costs of experiments. Since the introduction of MRI, several important technical developments have been made to successfully reduce acquisition times. In this work, novel approaches were developed to increase the efficiency of MRI acquisitions. In Chapter 4, an improved radial turbo spin-echo (TSE) combined acquisition and reconstruction strategy was introduced. Cartesian turbo spin-echo sequences [3] are widely used especially for the detection and diagnosis of neurological pathologies, as they provide high SNR images with both clinically important proton density and T2 contrasts. TSE acquisitions combined with radial sampling are very efficient, since it is possible to obtain a number of ETL images with different contrasts from a single radial TSE measurement [56-58]. Conventionally, images with a particular contrast are obtained from both radial and Cartesian TSE acquisitions by combining data from different echo times into a single image. In the radial case, this can be achieved by employing k-space weighted image contrast (KWIC) reconstruction. In KWIC, the center region of k-space is filled exclusively with data belonging to the desired contrast while outer regions also are assembled with data acquired at other echo times. However, this data sharing leads to mixed contrast contributions to both Cartesian and radial TSE images. This is true especially for proton density weighted images and therefore may reduce their diagnostic value. In the proposed method, an adapted golden angle reordering scheme is introduced for radial TSE acquisitions, that allows a free choice of the echo train length and provides high flexibility in image reconstruction. Unwanted contrast contaminations are greatly reduced by employing a narrow-band KWIC filter, that restricts data sharing to a small temporal window around the de- sired echo time. This corresponds to using fewer data than required for fully sampled images and consequently leads to images exhibiting aliasing artifacts. In a second step, aliasing-free images are obtained using parallel imaging. In the neurological examples presented, the CG-SENSE algorithm [42] was chosen due to its stable convergence properties and its ability to reconstruct arbitrarily sampled data. In simulations as well as in different in vivo neurological applications, no unwanted contrast contributions could be observed in radial TSE images reconstructed with the proposed method. Since this novel approach is easy to implement on today's scanners and requires low computational power, it might be valuable for the clinical breakthrough of radial TSE acquisitions. In Chapter 5, an auto-calibrating method was introduced to correct for stimulated echo contribu- tions to T2 estimates from a mono-exponential fit of multi spin-echo (MSE) data. Quantification of T2 is a useful tool in clinical routine for the detection and diagnosis of diseases as well as for tis- sue characterization. Due to technical imperfections, refocusing flip angles in a MSE acquisition deviate from the ideal value of 180○. This gives rise to significant stimulated echo contributions to the overall signal evolution. Therefore, T2 estimates obtained from MSE acquisitions typically are notably higher than the reference. To obtain accurate T2 estimates from MSE acquisitions, MSE signal amplitudes can be predicted using the extended phase graph (EPG, [23, 24]) algo- rithm. Subsequently, a correction factor can be obtained from the simulated EPG T2 value and applied to the MSE T2 estimates. However, EPG calculations require knowledge about refocus- ing pulse amplitudes, T2 and T1 values and the temporal spacing of subsequent echoes. While the echo spacing is known and, as shown in simulations, an approximate T1 value can be assumed for high ratios of T1/T2 without compromising accuracy of the results, the remaining two parameters are estimated from the data themselves. An estimate for the refocusing flip angle can be obtained from the signal intensity ratio of the second to the first echo using EPG. A conventional mono- exponential fit of the MSE data yields a first estimate for T2. The T2 correction is then obtained iteratively by updating the T2 value used for EPG calculations in each step. For all examples pre- sented, two iterations proved to be sufficient for convergence. In the proposed method, a mean flip angle is extracted across the slice. As shown in simulations, this assumption leads to greatly reduced deviations even for more inhomogeneous slice profiles. The accuracy of corrected T2 values was shown in experiments using a phantom consisting of bottles filled with liquids with a wide range of different T2 values. While T2 MSE estimates were shown to deviate significantly from the spin-echo reference values, this is not the case for corrected T2 values. Furthermore, applicability was demonstrated for in vivo neurological experiments. In Chapter 6, a new auto-calibrating parallel imaging method called iterative GROG was pre- sented for the reconstruction of non-Cartesian data. A wide range of different non-Cartesian schemes have been proposed for data acquisition in MRI, that present various advantages over conventional Cartesian sampling such as faster acquisitions, improved dynamic imaging and in- trinsic motion correction. However, one drawback of non-Cartesian data is the more complicated reconstruction, which is ever more problematic for non-Cartesian parallel imaging techniques. Iterative GROG uses Calibrationless Parallel Imaging by Structured Low-Rank Matrix Completion (CPI) for data reconstruction. Since CPI requires points on a Cartesian grid, it cannot be used to directly reconstruct non-Cartesian data. Instead, Grappa Operator Gridding (GROG) is employed in a first step to move the non-Cartesian points to the nearest Cartesian grid locations. However, GROG requires a fully sampled center region of k-space for calibration. Combining both methods in an iterative scheme, accurate GROG weights can be obtained even from highly undersampled non-Cartesian data. Subsequently, CPI can be used to reconstruct either full k- space or a calibration area of arbitrary size, which can then be employed for data reconstruction with conventional parallel imaging methods. In Chapter 7, a new 2D sampling scheme was introduced consisting of multiple oscillating effi- cient trajectories (MOET), that is optimized for Compressed Sensing (CS) reconstructions. For successful CS reconstruction of a particular data set, some requirements have to be met. First, ev- ery data sample has to carry information about the whole object, which is automatically fulfilled for the Fourier sampling employed in MRI. Additionally, the image to be reconstructed has to be sparse in an arbitrary domain, which is true for a number of different applications. Last, data sam- pling has to be performed in an incoherent fashion. For 2D imaging, this important requirement of CS is difficult to achieve with conventional Cartesian and non-Cartesian sampling schemes. Ra- dial sampling is often used for CS reconstructions of dynamic data despite the streaking present in undersampled images. To obtain incoherent aliasing artifacts in undersampled images while at the same time preserving the advantages of radial sampling for dynamic imaging, MOET com- bines radial spokes with oscillating gradients of varying amplitude and alternating orientation orthogonal to the readout direction. The advantage of MOET over radial sampling in CS re- constructions was demonstrated in simulations and in in vivo cardiac imaging. MOET provides superior results especially when used in CS reconstructions with a sparsity constraint directly in image space. Here, accurate results could be obtained even from few MOET projections, while the coherent streaking artifacts present in the case of radial sampling prevent image recovery even for smaller acceleration factors. For CS reconstructions of dynamic data with sparsity constraint in xf-space, the advantage of MOET is smaller since the temporal reordering is responsible for an important part of incoherency. However, as was shown in simulations of a moving phantom and in the reconstruction of ungated cardiac data, the additional spatial incoherency provided by MOET still leads to improved results with higher accuracy and may allow reconstructions with higher acceleration factors.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{Gieseking2014, author = {Gieseking, Bj{\"o}rn}, title = {Excitation Dynamics and Charge Carrier Generation in Organic Semiconductors}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-101625}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {The transport of optically excited states, called excitons, as well as their conversion into charges define the two major steps allowing for the operation of organic photovoltaic (OPV) devices. Hence, a deep understanding of these processes, the involved mechanisms as well as possible loss channels is crucial for further improving the efficiency of organic solar cells. For studying the aforementioned processes spectroscopic methods like absorption and emission measurements are useful tools. As many of the processes take place on a sub-nanosecond (ns) timescale ultrafast spectroscopic methods are required. Due to this reason two experiments based on a femtosecond laser system were built and employed in this work, namely picosecond (ps) time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy. By analyzing the PL decay dynamics in the prototypical organic semiconductor rubrene, the feasibility of a new approach for improving the efficiency of organic solar cells by harvesting triplet excitons generated by singlet fission was examined. Singlet fission describes a process where two triplet excitons are generated via a photoexcited singlet exciton precursor state if the energy of the two triplets is comparable with the energy of the singlet. For this purpose the influence of characteristic length scales on the exciton dynamics in different rubrene morphologies exhibiting an increasing degree of confinement was analyzed. The results show that the quenching at interfacial states efficiently suppresses the desired fission process if these states are reached by excitons during migration. Since interfacial states are expected to play a significant role in thin film solar cells and are easily accessible for the migrating excitons, the results have to be considered for triplet-based OPV. While the aforementioned approach is only investigated for model systems so far, the efficiency of disordered organic bulk heterojunction (BHJ) solar cells could be significantly enhanced in the last couple of years by employing new and more complex copolymer donor materials. However, little is known about the photophysics and in particular the excitation dynamics of these systems. By carrying out a systematic optical study on the prominent copolymer PCDTBT and its building blocks we were able to identify the nature of the two characteristic absorption bands and the coupling mechanism between these levels. The latter mechanism is based on an intrachain partial charge transfer between two functional subunits and our time-resolved measurements indicate that this coupling governs the photophysical properties of solar cells based on these copolymers. The efficient coupling of functional subunits can be seen as a key aspect that guarantees for the success of the copolymer approach. Another important issue concerns the optimization of the morphology of BHJ solar cells. It arises from the discrepancy between the exciton diffusion length \mbox{(\$\approx\$ 10 nm)} and the absorption length of solar irradiation (\$\approx\$ 100 nm). Due to this reason, even for devices based on new copolymer materials, processing parameters affecting the morphology like annealing or employing processing additives are of major importance. In our combined optical, electrical and morphological study for solar cells based on the high-efficient copolymer PBDTTT-C we find a direct correlation between additive content and intermixing of the active layer. The observed maximum in device efficiency can be attributed to a morphology guaranteeing for an optimized balance between charge generation and transport. Our results highlight the importance of understanding the influence of processing parameters on the morphology of the BHJ and thus on the efficiency of the device.}, subject = {Organische Solarzelle}, language = {en} } @phdthesis{Samiepour2014, author = {Samiepour, Marjan}, title = {Fabrication and characterization of CPP-GMR and spin-transfer torque induced magnetic switching}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-102226}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {Even though the unique magnetic behavior for ferromagnets has been known for thousands of years, explaining this interesting phenomenon only occurred in the 20th century. It was in 1920, with the discovery of electron spin, that a clear explanation of how ferromagnets achieve their unique magnetic properties came to light. The electron carries an intrinsic electric charge and intrinsic angular momentum. Use of this property in a device was achieved in 1998 when Fert and Gru¨nberg independently found that the resistance of FM/NM/FM trilayer depended on the angle between the magnetization of the two layers. This phenomena which is called giant magnetoresistance (GMR) brought spin transfer into mainstream. This new discovery created a brand new research fi called "spintronics" or "spin based electronics" which exploits the intrinsic spin of electron. As expected spintronics delivered a new generation of magnetic devices which are currently used in magnetic disk drives and magnetic random access memories (MRAM). The potential advantages of spintronics devices are non-volatility, higher speed, increased data density and low power consumption. GMR devices are already used in industry as magnetic memories and read heads. The quality of GMR devices can be increased by developing new magnetic materials and also by going down to nanoscale. The desired characteristic properties of these new materials are higher spin polarization, higher curie temperature and better spin filtering. Half-metals are a good candidate for these devices since they are expected to have high polarization. Some examples of half-metals are Half-Heusler alloy, full Heusler alloy and Perovskite or double Perovskite oxides. The devices discussed in this thesis have NiMnSb half-Heusler alloy and permalloy as the ferromagnetic layers separated by Cu as the nonmagnetic layer. This dissertation includes mainly two parts, fabrication and characterization of nan- opillars. The layer stack used for the fabrication is Ru/Py/Cu/NiMnSb which is grown on an InP substrate with an (In,Ga)As buff by molecule beam epitaxy (MBE). A new method of fabrication using metal mask which has a higher yield of working samples over the previous method (using the resist mask) used in our group is discussed in detail. Also, the advantages of this new method and draw backs of the old method are explained thoroughly (in chapter 3). The second part (chapters 4 and 5) is focused on electrical measurements and charac- terization of the nanopillar, specially with regard to GMR and spin-transfer torque (STT) measurements. In chapter 4, the results of current perpendicular the plane giant mag- netoresistance (CPP-GMR) measurements at various temperatures and in-plane magnetic fi are presented. The dependence of CPP-GMR on bias current and shape anisotropy of the device are investigated. Results of these measurements show that the device has strong shape anisotropy. The following chapter deals with spin-transfer torque induced magnetic switching measurements done on the device. Critical current densities are on the order of 106 A/cm2, which is one order of magnitude smaller than the current industry standards. Our results show that the two possible magnetic configurations of the nanopillar (parallel and anti-parallel) have a strong dependence on the applied in-plane magnetic fi Fi- nally, four magnetic fi regimes based on the stability of the magnetic configuration (P stable, AP stable, both P and AP stable, both P and AP unstable) are identified.}, subject = {Riesenmagnetowiderstand}, language = {en} } @phdthesis{Rewitz2014, author = {Rewitz, Christian}, title = {Far-Field Characterization and Control of Propagating Ultrashort Optical Near Fields}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-94887}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {In this work, femtosecond laser pulses are used to launch optical excitations on different nanostructures. The excitations are confined below the diffraction limit and propagate along the nanostructures. Fundamental properties of these ultrashort optical near fields are determined by characterizing the far-field emission after propagation with a setup developed for this task. Furthermore, control of the nanooptical excitations' spatial and temporal evolution is demonstrated for a designed nanostructure.}, subject = {Nahfeldoptik}, language = {en} } @article{MoffetRoedelKellyetal.2013, author = {Moffet, R. C. and R{\"o}del, R. C. and Kelly, S. T. and Yu, X. Y. and Carroll, G. T. and Fast, J. and Zaveri, R. A. and Laskin, A. and Gilles, M. K.}, title = {Spectro-microscopic measurements of carbonaceous aerosol aging in Central California}, series = {Atmospheric Chemistry and Physics}, volume = {13}, journal = {Atmospheric Chemistry and Physics}, number = {20}, issn = {1680-7324}, doi = {10.5194/acp-13-10445-2013}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-121827}, year = {2013}, abstract = {Carbonaceous aerosols are responsible for large uncertainties in climate models, degraded visibility, and adverse health effects. The Carbonaceous Aerosols and Radiative Effects Study (CARES) was designed to study carbonaceous aerosols in the natural environment of the Central Valley, California, and learn more about their atmospheric formation and aging. This paper presents results from spectro-microscopic measurements of carbonaceous particles collected during CARES at the time of a pollution accumulation event (27-29 June 2010), when in situ measurements indicated an increase in the organic carbon content of aerosols as the Sacramento urban plume aged. Computer-controlled scanning electron microscopy coupled with an energy dispersive X-ray detector (CCSEM/EDX) and scanning transmission X-ray microscopy coupled with near-edge X-ray absorption spectroscopy (STXM/NEXAFS) were used to probe the chemical composition and morphology of individual particles. It was found that the mass of organic carbon on individual particles increased through condensation of secondary organic aerosol. STXM/NEXAFS indicated that the number fraction of homogenous organic particles lacking inorganic inclusions (greater than similar to 50 nm equivalent circular diameter) increased with plume age, as did the organic mass per particle. Comparison of the CARES spectro-microscopic dataset with a similar dataset obtained in Mexico City during the MILAGRO campaign showed that fresh particles in Mexico City contained three times as much carbon as those sampled during CARES. The number fraction of soot particles at the Mexico City urban site (ranging from 16.6 to 47.3 \%) was larger than at the CARES urban site (13.4-15.7\%), and the most aged samples from CARES contained fewer carbon-carbon double bonds. Differences between carbonaceous particles in Mexico City and California result from different sources, photochemical conditions, gas phase reactants, and secondary organic aerosol precursors. The detailed results provided by these spectro-microscopic measurements will allow for a comprehensive evaluation of aerosol process models used in climate research.}, language = {en} } @article{KoenigBaenningerGarciaetal.2013, author = {K{\"o}nig, Markus and Baenninger, Matthias and Garcia, Andrei G. F. and Harjee, Nahid and Pruitt, Beth L. and Ames, C. and Leubner, Philipp and Br{\"u}ne, Christoph and Buhmann, Hartmut and Molenkamp, Laurens W. and Goldhaber-Gordon, David}, title = {Spatially Resolved Study of Backscattering in the Quantum Spin Hall State}, series = {Physical Review X}, volume = {3}, journal = {Physical Review X}, number = {2}, issn = {2160-3308}, doi = {10.1103/PhysRevX.3.021003}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-127225}, pages = {21003}, year = {2013}, abstract = {The discovery of the quantum spin Hall (QSH) state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the quantum spin Hall state have confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples, and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using scanning gate microscopy to perturb the QSH edge states on a submicron scale, we identify well-localized scattering sites which likely limit the expected nondissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed, as expected for an ideal QSH system, and are found to be robust against weak induced potential fluctuations.}, language = {en} } @article{DasenbrookLuDonnolaetal.2013, author = {Dasenbrook, Elliot C. and Lu, Luan and Donnola, Shannon and Weaver, David E. and Gulani, Viskas and Jakob, Peter M. and Konstan, Michael W. and Flask, Chris A.}, title = {Normalized T1 Magnetic Resonance Imaging for Assessment of Regional Lung Function in Adult Cystic Fibrosis Patients - A Cross-Sectional Study}, series = {PLOS ONE}, volume = {8}, journal = {PLOS ONE}, number = {9}, issn = {1932-6203}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-128346}, pages = {e73286}, year = {2013}, abstract = {Background: Cystic fibrosis (CF) patients would benefit from a safe and effective tool to detect early-stage, regional lung disease to allow for early intervention. Magnetic Resonance Imaging (MRI) is a safe, non-invasive procedure capable of providing quantitative assessments of disease without ionizing radiation. We developed a rapid normalized T1 MRI technique to detect regional lung disease in early-stage CF patients. Materials and Methods: Conventional multislice, pulmonary T1 relaxation time maps were obtained for 10 adult CF patients with normal spirometry and 5 healthy non-CF control subjects using a rapid Look-Locker MRI acquisition (5 seconds/imaging slice). Each lung absolute T1 map was separated into six regions of interest (ROI) by manually selecting upper, central, and lower lung regions in the left and right lungs. In order to reduce the effects of subject-to-subject variation, normalized T1 maps were calculated by dividing each pixel in the absolute T1 maps by the mean T1 time in the central lung region. The primary outcome was the differences in mean normalized T1 values in the upper lung regions between CF patients with normal spirometry and healthy volunteers. Results: Normalized T1 (nT1) maps showed visibly reduced subject-to-subject variation in comparison to conventional absolute T1 maps for healthy volunteers. An ROI analysis showed that the variation in the nT1 values in all regions was <= 2\% of the mean. The primary outcome, the mean (SD) of the normalized T1 values in the upper right lung regions, was significantly lower in the CF subjects [.914 (.037)] compared to the upper right lung regions of the healthy subjects [.983 (.003)] [difference of .069 (95\% confidence interval .032-.105); p=.001). Similar results were seen in the upper left lung region. Conclusion: Rapid normalized T1 MRI relaxometry obtained in 5 seconds/imaging slice may be used to detect regional early-stage lung disease in CF patients.}, language = {en} } @article{OkadaRotenbergKevanetal.2013, author = {Okada, Michio and Rotenberg, Eli and Kevan, S. D. and Sch{\"a}fer, J. and Ujfalussy, Balazs and Stocks, G. Malcolm and Genatempo, B. and Bruno, E. and Plummer, E. W.}, title = {Evolution of the electronic structure in \(Mo_{1-x}Re_x\) alloys}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {093010}, issn = {1367-2630}, doi = {10.1088/1367-2630/15/9/093010}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-122993}, year = {2013}, abstract = {We report a detailed experimental and theoretical study of the electronic structure of \(Mo_{1-x}Re_x\) random alloys. We have measured electronic band dispersions for clean and hydrogen-covered \(Mo_{1-x}Re_x\) ( 110) with x = 0-0.25 using angle-resolved photoemission spectroscopy. Our results suggest that the bulk and most surface electronic bands shift relative to the Fermi level systematically and approximately rigidly with Re concentration. We distinguish and quantify two contributions to these shifts: a raise of the Fermi energy and an increase of the overall bandwidth. Alloy bands calculated using the first-principles Korringa-Kohn-Rostoker coherent-potential-approximation method accurately predict both of these effects. As derived from the rigid band model, the Fermi energy shift is inversely related to the bulk density of states in this energy region. Using our results, we also characterize an electronic topological transition of the bulk Fermi surface and relate this to bulk transport properties. Finally, we distinguish effects beyond the rigid band approximation: a highly surface-localized state and a composition-dependent impact of the spin-orbit interaction.}, language = {en} } @article{KasprzakSivalertpornAlbertetal.2013, author = {Kasprzak, J. and Sivalertporn, K. and Albert, F. and Schneider, C. and H{\"o}fling, S. and Kamp, M. and Forchel, A. and Muljarov, E. A. and Langbein, W.}, title = {Coherence dynamics and quantum-to-classical crossover in an exciton-cavity system in the quantum strong coupling regime}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {045013}, issn = {1367-2630}, doi = {10.1088/1367-2630/15/4/045013}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123005}, year = {2013}, abstract = {Interaction between light and matter generates optical nonlinearities, which are particularly pronounced in the quantum strong coupling regime. When a single bosonic mode couples to a single fermionic mode, a Jaynes-Cummings (JC) ladder is formed, which we realize here using cavity photons and quantum dot excitons. We measure and model the coherent anharmonic response of this strongly coupled exciton-cavity system at resonance. Injecting two photons into the cavity, we demonstrate a \(\sqrt 2\) larger polariton splitting with respect to the vacuum Rabi splitting. This is achieved using coherent nonlinear spectroscopy, specifically four-wave mixing, where the coherence between the ground state and the first (second) rung of the JC ladder can be interrogated for positive (negative) delays. With increasing excitation intensity and thus rising average number of injected photons, we observe spectral signatures of the quantum-to-classical crossover of the strong coupling regime.}, language = {en} } @article{KimKusudoLoeffleretal.2013, author = {Kim, N. Y. and Kusudo, K. and L{\"o}ffler, A. and H{\"o}fling, S. and Forchel, A. and Yamamoto, Y.}, title = {Exciton-polariton condensates near the Dirac point in a triangular lattice}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {035032}, issn = {1367-2630}, doi = {10.1088/1367-2630/15/3/035032}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123103}, year = {2013}, abstract = {Dirac particles, massless relativistic entities, obey linear energy dispersions and hold important implications in particle physics. The recent discovery of Dirac fermions in condensed matter systems including graphene and topological insulators has generated a great deal of interest in exploring the relativistic properties associated with Dirac physics in solid-state materials. In addition, there are stimulating research activities to engineer Dirac particles, elucidating their exotic physical properties in a controllable setting. One of the successful platforms is the ultracold atom-optical lattice system, whose dynamics can be manipulated and probed in a clean environment. A microcavity exciton-polariton-lattice system offers the advantage of forming high-orbital condensation in non-equilibrium conditions, which enables one to explore novel quantum orbital order in two dimensions. In this paper, we experimentally construct the band structures near Dirac points, the vertices of the first hexagonal Brillouin zone with exciton-polariton condensates trapped in a triangular lattice. Due to the finite spectral linewidth, the direct map of band structures at Dirac points is elusive; however, we identify the linear part above Dirac points and its associated velocity value is similar to ~0.9-2 x \(10^8 cm s^{-1}\), consistent with the theoretical estimate \(1 x 10^8 cm s^{-1}\) with a \(2 \mu m\) lattice constant. We envision that the exciton-polariton condensates in lattices would be a promising solid-state platform, where the system order parameter can be accessed in both real and momentum spaces.}, language = {en} } @article{HopfmannAlbertSchneideretal.2013, author = {Hopfmann, C. and Albert, F. and Schneider, C. and H{\"o}fling, S. and Kamp, M. and Forchel, A. and Kanter, I. and Reizenstein, S.}, title = {Nonlinear emission characteristics of quantum dot-micropillar lasers in the presence of polarized optical feedback}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {025030}, issn = {1367-2630}, doi = {10.1088/1367-2630/15/2/025030}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123127}, year = {2013}, abstract = {We report on electrically pumped quantum dot-microlasers in the presence of polarized self-feedback. The high-\(\beta\) microlasers show two orthogonal, linearly polarized emission modes which are coupled via the common gain medium. This coupling is explained in terms of gain competition between the two lasing modes and leads to distinct differences in their input-output characteristics. By applying polarized self-feedback via an external mirror, we are able to control the laser characteristics of the emission modes in terms of the output power, the coherence time and the photon statistics. We find that linearly polarized self-feedback stabilizes the lasing of a given mode, while cross-polarized feedback between the two modes reduces strongly the intensity of the other emission mode showing particular high-intensity fluctuations and even super-thermal values of the photon autocorrelation function \(g^{(2)} (\tau)\) at zero delay. Measurements of \(g^{(2)} (\tau)\) under external feedback also allow us to detect revival peaks associated with the round trip time of the external cavity. Analyzing the damping and shape of the \(g^{(2)} (\tau)\) revival peaks by a phenomenological model provides us insight into the underlying physics such as the effective exciton lifetime and gain characteristics of the quantum dots in the active region of these microlasers.}, language = {en} } @phdthesis{Schmitt2013, author = {Schmitt, Peter}, title = {MR imaging of tumors: Approaches for functional and fast morphological characterization}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-135967}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {The subject of this work was to develop, implement, optimize and apply methods for quantitative MR imaging of tumors. In the context of functional and physiological characterization, this implied transferring techniques established in tumor model research to human subjects and assessing their feasibility for use in patients. In the context of the morphologic assessment and parameter imaging of tumors, novel concepts and techniques were developed, which facilitated the simultaneous quantification of multiple MR parameters, the generation of "synthetic" MR images with various contrasts, and the fast single-shot acquisition of purely T2-weighted images.}, subject = {Kernspintomografie}, language = {en} } @article{ArcaTeddeSrameketal.2013, author = {Arca, Francesco and Tedde, Sandro F. and Sramek, Maria and Rauh, Julia and Lugli, Paolo and Hayden, Oliver}, title = {Interface Trap States in Organic Photodiodes}, series = {Scientific Reports}, volume = {3}, journal = {Scientific Reports}, doi = {10.1038/srep01324}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131507}, pages = {1324}, year = {2013}, abstract = {Organic semiconductors are attractive for optical sensing applications due to the effortless processing on large active area of several \(cm^2\), which is difficult to achieve with solid-state devices. However, compared to silicon photodiodes, sensitivity and dynamic behavior remain a major challenge with organic sensors. Here, we show that charge trapping phenomena deteriorate the bandwidth of organic photodiodes (OPDs) to a few Hz at low-light levels. We demonstrate that, despite the large OPD capacitances of similar to 10 nF \(cm^{-2}\), a frequency response in the kHz regime can be achieved at light levels as low as 20 nW \(cm^{-2}\) by appropriate interface engineering, which corresponds to a 1000-fold increase compared to state-of-the-art OPDs. Such device characteristics indicate that large active area OPDs are suitable for industrial sensing and even match medical requirements for single X-ray pulse detection in the millisecond range.}, language = {en} } @article{OPUS4-13153, title = {Measurement of hard double-parton interactions in W(→ℓν) + 2-jet events at \(\sqrt{s}=7\) TeV with the ATLAS detector}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {033038}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/3/033038}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131531}, year = {2013}, abstract = {The production of W bosons in association with two jets in proton-proton collisions at a centre-of-mass energy of \(\sqrt{s}=7\),TeV has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of \(36 pb^{-1}\), collected with the ATLAS detector at the Large Hadron Collider. The fraction of events arising from double-parton interactions, \(f^D_{DP}\), has been measured through the pT balance between the two jets and amounts to \(f^D_{DP}\) = 0.08 ± 0.01 (stat.) ± 0.02 (sys.) for jets with transverse momentum \(p_T\) > 20 GeV and rapidity |y| < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of \(\sigma_{eff} = 15 ± 3 (stat.)^{+5}_{-3} (sys.)\) mb.}, language = {en} } @article{OPUS4-13154, title = {Search for extra dimensions in diphoton events from proton-proton collisions at \(\sqrt {s} = 7\)  TeV in the ATLAS detector at the LHC}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/4/043007}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131548}, pages = {043007}, year = {2013}, abstract = {The large difference between the Planck scale and the electroweak scale, known as the hierarchy problem, is addressed in certain models through the postulate of extra spatial dimensions. A search for evidence of extra spatial dimensions in the diphoton channel has been performed using the full set of proton-proton collisions at \(\sqrt {s} = 7\) TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. This dataset corresponds to an integrated luminosity of \(4.9 fb^{-1}\). The diphoton invariant mass spectrum is observed to be in good agreement with the Standard Model expectation. In the context of the model proposed by Arkani-Hamed, Dimopoulos and Dvali, 95\% confidence level lower limits of between 2.52 and 3.92 TeV are set on the ultraviolet cutoff scale MS depending on the number of extra dimensions and the theoretical formalism used. In the context of the Randall-Sundrum model, a lower limit of 2.06 (1.00) TeV at 95\% confidence level is set on the mass of the lightest graviton for couplings of \(k/\overline {M}_{Pl} = 0.1 (0.01)\). Combining with the ATLAS dilepton searches based on the 2011 data, the 95\% confidence level lower limit on the Randall-Sundrum graviton mass is further tightened to 2.23 (1.03) TeV for \(k/\overline {M}_{Pl} = 0.1 (0.01)\).}, language = {en} } @article{WeibelBasseLuesebrinkHessetal.2013, author = {Weibel, Stephanie and Basse-Luesebrink, Thomas Christian and Hess, Michael and Hofmann, Elisabeth and Seubert, Carolin and Langbein-Laugwitz, Johanna and Gentschev, Ivaylo and Sturm, Volker J{\"o}rg Friedrich and Ye, Yuxiang and Kampf, Thomas and Jakob, Peter Michael and Szalay, Aladar A.}, title = {Imaging of Intratumoral Inflammation during Oncolytic Virotherapy of Tumors by \(^{19}\)F-Magnetic Resonance Imaging (MRI)}, series = {PLoS ONE}, volume = {8}, journal = {PLoS ONE}, number = {3}, doi = {10.1371/journal.pone.0056317}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-130311}, pages = {e56317}, year = {2013}, abstract = {Background Oncolytic virotherapy of tumors is an up-coming, promising therapeutic modality of cancer therapy. Unfortunately, non-invasive techniques to evaluate the inflammatory host response to treatment are rare. Here, we evaluate \(^{19}\)F magnetic resonance imaging (MRI) which enables the non-invasive visualization of inflammatory processes in pathological conditions by the use of perfluorocarbon nanoemulsions (PFC) for monitoring of oncolytic virotherapy. Methodology/Principal Findings The Vaccinia virus strain GLV-1h68 was used as an oncolytic agent for the treatment of different tumor models. Systemic application of PFC emulsions followed by \(^1H\)/\(^{19}\)F MRI of mock-infected and GLV-1h68-infected tumor-bearing mice revealed a significant accumulation of the \(^{19}\)F signal in the tumor rim of virus-treated mice. Histological examination of tumors confirmed a similar spatial distribution of the \(^{19}\)F signal hot spots and \(CD68^+\)-macrophages. Thereby, the \(CD68^+\)-macrophages encapsulate the GFP-positive viral infection foci. In multiple tumor models, we specifically visualized early inflammatory cell recruitment in Vaccinia virus colonized tumors. Furthermore, we documented that the \(^{19}\)F signal correlated with the extent of viral spreading within tumors. Conclusions/Significance These results suggest \(^{19}\)F MRI as a non-invasive methodology to document the tumor-associated host immune response as well as the extent of intratumoral viral replication. Thus, \(^{19}\)F MRI represents a new platform to non-invasively investigate the role of the host immune response for therapeutic outcome of oncolytic virotherapy and individual patient response.}, language = {en} } @article{OPUS4-13155, title = {Search for excited electrons and muons in \(\sqrt {s}=8\) TeV proton-proton collisions with the ATLAS detector}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {093011}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/9/093011}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131557}, year = {2013}, abstract = {The ATLAS detector at the Large Hadron Collider is used to search for excited electrons and excited muons in the channel \(pp →ℓℓ^\ast→ℓℓ\gamma \), assuming that excited leptons are produced via contact interactions. The analysis is based on \(13 fb^{-1}\) of pp collisions at a centre-of-mass energy of 8 TeV. No evidence for excited leptons is found, and a limit is set at the 95\% credibility level on the cross section times branching ratio as a function of the excitedlepton mass \(m_{ℓ^\ast}\) . For \(m_{ℓ^\ast}\) ≽0.8 TeV, the respective upper limits on \(\sigma B(ℓ→ ℓ\gamma)\) are 0.75 and 0.90 fb for the \(e^\ast\) and \(μ^\ast\) searches. Limits on \(\sigma B\) are converted into lower bounds on the compositeness scale \(\Lambda\). In the special case where \(\Lambda = m_{ℓ^\ast}\), excited-electron and excited-muon masses below 2.2 TeV are excluded.}, language = {en} } @article{OPUS4-13156, title = {Search for WH production with a light Higgs boson decaying to prompt electron-jets in proton-proton collisions at \(\sqrt {s}=7\)  TeV with the ATLAS detector}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {043009}, organization = {The ATLAS Collaboration}, doi = {10.1088/1367-2630/15/4/043009}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131565}, year = {2013}, abstract = {A search is performed for WH production with a light Higgs boson decaying to hidden-sector particles resulting in clusters of collimated electrons, known as electron-jets. The search is performed with \(2.04 fb^{-1}\) of data collected in 2011 with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at \(\sqrt {s}=7 TeV\) . One event satisfying the signal selection criteria is observed, which is consistent with the expected background rate. Limits on the product of the WH production cross section and the branching ratio of a Higgs boson decaying to prompt electron-jets are calculated as a function of a Higgs boson mass in the range from 100 to 140 GeV.}, language = {en} } @article{OostingaMaierSchueffelgenetal.2013, author = {Oostinga, Jeroen B. and Maier, Luis and Sch{\"u}ffelgen, Peter and Knott, Daniel and Ames, Christopher and Br{\"u}ne, Christoph and Tkachov, Grigory and Buhmann, Hartmut and Molenkamp, Laurens W.}, title = {Josephson Supercurrent through the Topological Surface States of Strained Bulk HgTe}, series = {Physical Review X}, volume = {3}, journal = {Physical Review X}, number = {021007}, doi = {10.1103/PhysRevX.3.021007}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129834}, year = {2013}, abstract = {Strained bulk HgTe is a three-dimensional topological insulator, whose surface electrons have a high mobility (~ 30 000 cm\(^2\)=Vs), while its bulk is effectively free of mobile charge carriers. These properties enable a study of transport through its unconventional surface states without being hindered by a parallel bulk conductance. Here, we show transport experiments on HgTe-based Josephson junctions to investigate the appearance of the predicted Majorana states at the interface between a topological insulator and a superconductor. Interestingly, we observe a dissipationless supercurrent flow through the topological surface states of HgTe. The current-voltage characteristics are hysteretic at temperatures below 1 K, with critical supercurrents of several microamperes. Moreover, we observe a magnetic-field-induced Fraunhofer pattern of the critical supercurrent, indicating a dominant \(2\pi\)-periodic Josephson effect in the unconventional surface states. Our results show that strained bulk HgTe is a promising material system to get a better understanding of the Josephson effect in topological surface states, and to search for the manifestation of zero-energy Majorana states in transport experiments.}, language = {en} } @article{NordbeckBoenhofHilleretal.2013, author = {Nordbeck, Peter and B{\"o}nhof, Leoni and Hiller, Karl-Heinz and Voll, Sabine and Arias-Loza, Paula and Seidlmaier, Lea and Williams, Tatjana and Ye, Yu-Xiang and Gensler, Daniel and Pelzer, Theo and Ertl, Georg and Jakob, Peter M. and Bauer, Wolfgang R. and Ritter, Oliver}, title = {Impact of Thoracic Surgery on Cardiac Morphology and Function in Small Animal Models of Heart Disease: A Cardiac MRI Study in Rats}, series = {PLoS ONE}, volume = {8}, journal = {PLoS ONE}, number = {8}, doi = {10.1371/journal.pone.0068275}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-130064}, pages = {e68275}, year = {2013}, abstract = {Background Surgical procedures in small animal models of heart disease might evoke alterations in cardiac morphology and function. The aim of this study was to reveal and quantify such potential artificial early or long term effects in vivo, which might account for a significant bias in basic cardiovascular research, and, therefore, could potentially question the meaning of respective studies. Methods Female Wistar rats (n = 6 per group) were matched for weight and assorted for sham left coronary artery ligation or control. Cardiac morphology and function was then investigated in vivo by cine magnetic resonance imaging at 7 Tesla 1 and 8 weeks after the surgical procedure. The time course of metabolic and inflammatory blood parameters was determined in addition. Results Compared to healthy controls, rats after sham surgery showed a lower body weight both 1 week (267.5±10.6 vs. 317.0±11.3 g, n<0.05) and 8 weeks (317.0±21.1 vs. 358.7±22.4 g, n<0.05) after the intervention. Left and right ventricular morphology and function were not different in absolute measures in both groups 1 week after surgery. However, there was a confined difference in several cardiac parameters normalized to the body weight (bw), such as myocardial mass (2.19±0.30/0.83±0.13 vs. 1.85±0.22/0.70±0.07 mg left/right per g bw, p<0.05), or enddiastolic ventricular volume (1.31±0.36/1.21±0.31 vs. 1.14±0.20/1.07±0.17 µl left/right per g bw, p<0.05). Vice versa, after 8 weeks, cardiac masses, volumes, and output showed a trend for lower values in sham operated rats compared to controls in absolute measures (782.2±57.2/260.2±33.2 vs. 805.9±84.8/310.4±48.5 mg, p<0.05 for left/right ventricular mass), but not normalized to body weight. Matching these findings, blood testing revealed only minor inflammatory but prolonged metabolic changes after surgery not related to cardiac disease. Conclusion Cardio-thoracic surgical procedures in experimental myocardial infarction cause distinct alterations upon the global integrity of the organism, which in the long term also induce circumscribed repercussions on cardiac morphology and function. This impact has to be considered when analyzing data from respective animal studies and transferring these findings to conditions in patients.}, language = {en} } @article{ThierschmannHenkeKnorretal.2013, author = {Thierschmann, H. and Henke, M. and Knorr, J. and Maier, L. and Heyn, C. and Hansen, W. and Buhmann, H. and Molenkamp, L. W.}, title = {Diffusion thermopower of a serial double quantum dot}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {123010}, doi = {10.1088/1367-2630/15/12/123010}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129714}, year = {2013}, abstract = {We have experimentally studied the diffusion thermopower of a serial double quantum dot, defined electrostatically in a GaAs/AlGaAs heterostructure. We present the thermopower stability diagram for a temperature difference 1T = (20±10)mK across the device and find a maximum thermovoltage signal of several μV in the vicinity of the triple points. Along a constant energy axis in this regime, the data show a characteristic pattern which is in agreement with Mott's relation and can be well understood within a model of sequential transport.}, language = {en} } @article{BentmannReinert2013, author = {Bentmann, Hendrik and Reinert, Friedrich}, title = {Enhancing and reducing the Rashba-splitting at surfaces by adsorbates: Na and Xe on Bi/Cu(111)}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {115011}, doi = {10.1088/1367-2630/15/11/115011}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129722}, year = {2013}, abstract = {The surface alloy Bi/Cu(111) shows a paradigmatic free-electron-like surface state with a very large Rashba-type spin-orbit splitting. Using angle-resolved photoemission we investigate how adsorbates of different chemical nature influence the size of the spin splitting in this system. We find that the adsorption of small amounts of monovalent Na atoms leads to an enhancement of the spin splitting while an overlayer of the closed-shell rare gas Xe causes a reduction. The latter result is in contrast to the Au(111) surface for which an increased splitting size after Xe-adsorption was observed. We discuss these experimental findings in terms of the characteristic differences of the surface state wave functions and their spatial deformation in the presence of different types of adsorbates. Our results provide insight into the complex interplay of atomic and interface potential gradients governing the Rashba effect.}, language = {en} } @article{ScholzSauerWiessneretal.2013, author = {Scholz, M. and Sauer, C. and Wiessner, M. and Nguyen, N. and Scholl, A. and Reinert, F.}, title = {Structure formation in organic thin films observed in real time by energy dispersive near-edge x-ray absorption fine-structure spectroscopy}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {083052}, doi = {10.1088/1367-2630/15/8/083052}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129749}, year = {2013}, abstract = {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.}, language = {en} } @article{AstakhovFuchsSoltamovetal.2013, author = {Astakhov, Georgy V. and Fuchs, F. and Soltamov, V. A. and V{\"a}th, S. and Baranov, P. G. and Mokhov, E. N. and Dyakonov, V.}, title = {Silicon carbide light-emitting diode as a prospective room temperature source for single photons}, series = {Scientific Reports}, journal = {Scientific Reports}, doi = {10.1038/srep01637}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-96308}, year = {2013}, abstract = {Generation of single photons has been demonstrated in several systems. However, none of them satisfies all the conditions, e.g. room temperature functionality, telecom wavelength operation, high efficiency, as required for practical applications. Here, we report the fabrication of light-emitting diodes (LEDs) based on intrinsic defects in silicon carbide (SiC). To fabricate our devices we used a standard semiconductor manufacturing technology in combination with high-energy electron irradiation. The room temperature electroluminescence (EL) of our LEDs reveals two strong emission bands in the visible and near infrared (NIR) spectral ranges, associated with two different intrinsic defects. As these defects can potentially be generated at a low or even single defect level, our approach can be used to realize electrically driven single photon source for quantum telecommunication and information processing.}, language = {en} } @article{WinterKampfHelluyetal.2013, author = {Winter, Patrick and Kampf, Thomas and Helluy, Xavier and Gutjahr, Fabian T. and Meyer, Cord B. and Rommel, Eberhard and Bauer, Wolfgang R. and Jakob, Peter M. and Herold, Volker}, title = {Fast retrospectively triggered local pulse-wave velocity measurements in mice with CMR-microscopy using a radial trajectory}, series = {Journal of Cardiovascular Magnetic Resonance}, journal = {Journal of Cardiovascular Magnetic Resonance}, doi = {10.1186/1532-429X-15-88}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-96602}, year = {2013}, abstract = {Background The aortic pulse-wave velocity (PWV) is an important indicator of cardiovascular risk. In recent studies MRI methods have been developed to measure this parameter noninvasively in mice. Present techniques require additional hardware for cardiac and respiratory gating. In this work a robust self-gated measurement of the local PWV in mice without the need of triggering probes is proposed. Methods The local PWV of 6-months-old wild-type C57BL/6J mice (n=6) was measured in the abdominal aorta with a retrospectively triggered radial Phase Contrast (PC) MR sequence using the flow-area (QA) method. A navigator signal was extracted from the CMR data of highly asymmetric radial projections with short repetition time (TR=3 ms) and post-processed with high-pass and low-pass filters for retrospective cardiac and respiratory gating. The self-gating signal was used for a reconstruction of high-resolution Cine frames of the aortic motion. To assess the local PWV the volume flow Q and the cross-sectional area A of the aorta were determined. The results were compared with the values measured with a triggered Cartesian and an undersampled triggered radial PC-Cine sequence. Results In all examined animals a self-gating signal could be extracted and used for retrospective breath-gating and PC-Cine reconstruction. With the non-triggered measurement PWV values of 2.3±0.2 m/s were determined. These values are in agreement with those measured with the triggered Cartesian (2.4±0.2 m/s) and the triggered radial (2.3±0.2 m/s) measurement. Due to the strong robustness of the radial trajectory against undersampling an acceleration of more than two relative to the prospectively triggered Cartesian sampling could be achieved with the retrospective method. Conclusion With the radial flow-encoding sequence the extraction of a self-gating signal is feasible. The retrospective method enables a robust and fast measurement of the local PWV without the need of additional trigger hardware.}, language = {en} } @phdthesis{Meyer2013, author = {Meyer, Sebastian}, title = {Model System for Correlation Phenomena in Reduced Dimensions - Gold-induced Atomic Chains on Germanium}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-77723}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Atomic chains, often called nanowires, form in a self-organized process after the adsorption of metal atoms. These wires are spatially well confined representing a close approach of a true one-dimensional structure. The low-dimensional architecture thereby often leads to anisotropic electronic states with vanishing interchain interaction. In the presence of weak coupling to the substrate a one-dimensional metal can experience a phase transition according to Peierls into an insulating ground state upon temperature, which is accompanied by a periodic lattice distortion. Without any coupling a strict onedimensional regime is reached, where the common Fermi liquid description breaks down with the quasi-particles being replaced by collective excitations of spin and charge. This state is referred to as a Tomonaga-Luttinger liquid (TLL), which has been observed so far only in anisotropic bulk materials. An experimental fingerprint for both phenomena can be obtained from the electronic states close to the chemical potential, i.e. the Fermi energy. Using a semiconducting substrate provides the best observation conditions since any bulk projection onto the interesting bands is avoided. In case of Au/Ge(001) the growth of gold-induced chains is guided by the dimerized bare Ge (2×1) reconstruction yielding two different domains of wires rotated by 90° going from one terrace to the next by a single height step. The superior wetting capabilities of gold on germanium enables a complete coverage of the Ge(001) surface with longrange ordered wires. Their length scale and defect density is limited by the underlying substrate, for which a cleaning procedure is introduced based on wet-chemical etching followed by thermal dry oxidation. The band structure of Au/Ge(001) is investigated by angle-resolved photoelectron spectroscopy as a function of temperature. Two states are observed: a two-dimensional metallic state with hole-like dispersion and a one-dimensional electron pocket, whose band-integrated spectral function does not show the typical Fermi distribution at the chemical potential. Instead, a decrease of spectral weight applies following a power-law. This behavior can be well explained within the Tomonaga-Luttinger liquid theory which replaces the Fermi-Landau formalism in strictly one-dimensional systems. To enable theoretical modeling, a structural analysis was performed on the basis of surface x-ray diffraction (SXRD). From the in-plane scattering data a Patterson-map could be extracted leading to in-plane distances between gold atoms in the unit cell. This provides the first step towards a complete structural model and therefore towards a band structure calculation. First successful attempts have been made to manipulate the system by controlled adsorption of potassium. Here, an n-type doping effect is observed for submonolayer coverage whereas slightly increased coverages in combination with thermal energy lead to a new surface reconstruction.}, subject = {Nanodraht}, language = {en} } @phdthesis{Wagenpfahl2013, author = {Wagenpfahl, Alexander Johannes}, title = {Numerical simulations on limitations and optimization strategies of organic solar cells}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-90119}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Continuously increasing energy prices have considerably influenced the cost of living over the last decades. At the same time increasingly extreme weather conditions, drought-filled summers as well as autumns and winters with heavier rainfall and worsening storms have been reported. These are possibly the harbingers of the expected approaching global climate change. Considering the depletability of fossil energy sources and a rising distrust in nuclear power, investigations into new and innovative renewable energy sources are necessary to prepare for the coming future. In addition to wind, hydro and biomass technologies, electricity generated by the direct conversion of incident sunlight is one of the most promising approaches. Since the syntheses and detailed studies of organic semiconducting polymers and fullerenes were intensified, a new kind of solar cell fabrication became conceivable. In addition to classical vacuum deposition techniques, organic cells were now also able to be processed from a solution, even on flexible substrates like plastic, fabric or paper. An organic solar cell represents a complex electrical device influenced for instance by light interference for charge carrier generation. Also charge carrier recombination and transport mechanisms are important to its performance. In accordance to Coulomb interaction, this results in a specific distribution of the charge carriers and the electric field, which finally yield the measured current-voltage characteristics. Changes of certain parameters result in a complex response in the investigated device due to interactions between the physical processes. Consequently, it is necessary to find a way to generally predict the response of such a device to temperature changes for example. In this work, a numerical, one-dimensional simulation has been developed based on the drift-diffusion equations for electrons, holes and excitons. The generation and recombination rates of the single species are defined according to a detailed balance approach. The Coulomb interaction between the single charge carriers is considered through the Poisson equation. An analytically non-solvable differential equation system is consequently set-up. With numerical approaches, valid solutions describing the macroscopic processes in organic solar cells can be found. An additional optical simulation is used to determine the spatially resolved charge carrier generation rates due to interference. Concepts regarding organic semiconductors and solar cells are introduced in the first part of this work. All chapters are based on previous ones and logically outline the basic physics, device architectures, models of charge carrier generation and recombination as well as the mathematic and numerical approaches to obtain valid simulation results. In the second part, the simulation is used to elaborate issues of current interest in organic solar cell research. This includes a basic understanding of how the open circuit voltage is generated and which processes limit its value. S-shaped current-voltage characteristics are explained assigning finite surface recombination velocities at metal electrodes piling-up local space charges. The power conversion efficiency is identified as a trade-off between charge carrier accumulation and charge extraction. This leads to an optimum of the power conversion efficiency at moderate to high charge carrier mobilities. Differences between recombination rates determined by different interpretations of identical experimental results are assigned to a spatially inhomogeneous recombination, relevant for almost all low mobility semiconductor devices.}, subject = {Organische Solarzelle}, language = {en} } @phdthesis{Sperlich2013, author = {Sperlich, Andreas}, title = {Electron Paramagnetic Resonance Spectroscopy of Conjugated Polymers and Fullerenes for Organic Photovoltaics}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-81244}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {In the presented thesis, the various excited states encountered in conjugated organic semiconductors are investigated with respect to their utilization in organic thin-film solar cells. Most of these states are spin-baring and can therefore be addressed by means of magnetic resonance spectroscopy. The primary singlet excitation (spin 0), as well as positive and negative polaronic charge carriers (spin 1/2) are discussed. Additionally, triplet excitons (spin 1) and charge transfer complexes are examined, focussing on their differing spin-spin interaction strength. For the investigation of these spin-baring states especially methods of electron paramagnetic resonance (EPR) are best suited. Therefore according experimental methods were implemented in the course of this work to study conjugated polymers, fullerenes and their blends with continuous wave as well as time-resolved EPR and optically detected magnetic resonance.}, subject = {Organische Solarzelle}, language = {en} } @phdthesis{Ruff2013, author = {Ruff, Andreas}, title = {On the importance of electronic correlations in potassium-doped organic semiconductors}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83635}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {The present thesis is concerned with the impact of alkali metal-doping on the electronic structure of semiconducting organic thin films. The organic molecular systems which have been studied are the polycyclic aromatic hydrocarbons picene, pentacene, and coronene. Motivated by reports about exceptional behavior like superconductivity and electronic correlations of their alkali metal-doped compounds, high quality films fabricated from the above named molecules have been studied. The electronic structure of the pristine materials and their doped compounds has been investigated using photoelectron spectroscopy. Core level and valence band studies of undoped films yield excellent photoemission spectra agreeing with or even outperforming previously reported data from the literature. Alkali metal-doping manifests itself in a uniform manner in the electronic structure for all probed samples: Opposed to reports from the literature about metallicity and even superconductivity in alkali metal-doped picene, pentacene, and coronene, all films exhibit insulating nature with an energy gap of the order of one electron-volt. Remarkably, this is independent of the doping concentration and the type of dopant, i.e., potassium, cesium, or sodium. Based on the interplay between narrow bandwidths in organic semiconductors and sufficiently high on-molecule Coulomb repulsion, the non-metallicity is attributed to the strong influence of electronic correlations leading to the formation of a Mott insulator. In the case of picene, this is consolidated by calculations using a combination of density functional theory and dynamical mean-field theory. Beyond the extensive considerations regarding electronic correlations, further intriguing aspects have been observed. The deposition of thin picene films leads to the formation of a non-equilibrium situation between substrate and film surface. Here, the establishment of a homogeneous chemical potential is hampered due to the only weak van der Waals-interactions between the molecular layers in the films. Consequently, spectral weight is measurable above the reference chemical potential in photoemission. Furthermore, it has been found that the acceptance of additional electrons in pentacene is limited. While picene and coronene are able to host up to three extra electrons, in pentacene the limit is already reached for one electron. Finally, further extrinsic effects, coming along with alkali metal-doping, have been scrutinized. The oxidation of potassium atoms induced by the reaction with molecular oxygen in the residual gas of the ultra-high vacuum system turned out to significantly influence the electronic structure of alkali metal-doped picene and coronene. Moreover, also the applied X-ray and UV irradiation caused a certain impact on the photoemission spectra. Surprisingly, both effects did not play a role in the studies of potassium-doped pentacene.}, subject = {Organischer Halbleiter}, language = {en} } @phdthesis{Pohl2013, author = {Pohl, Christoph}, title = {Silicon Based MBE of Manganese-Silicide and Silicon-Suboxide}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83757}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {The present thesis deals with the fabrication, optimization of growth process and characterization of silicon based materials with molecular beam epitaxy. Two material systems are investigated in the course of this work: silicon/silicon suboxide multilayer structures and mono manganese silicide thin films. Mono manganese silicide (MnSi) is grown on Si(111) substrates with an hydrogen passivated surface, that is prepared by wet chemical processes. The growth start is performed by deposition of an amorphous Mn wetting layer that is subsequently annealed to form a MnSi seed layer on which the MnSi molecular beam epitaxy (MBE) is achieved. An amorphous or a crystalline Si cap layer is deposited onto the MnSi film to finalize the growth process and protect the sample from oxidation. With Raman spectroscopy it is shown that the crystalline cap layer is in fact single crystalline silicon. Results of x-ray diffraction and Raman spectroscopy confirm the growth of mono manganese silicide in contrast to other existing manganese silicide phases. In addition, in-plane and out-of-plane residual strain, and twinning of the MnSi thin film is detected with x-ray diffraction of symmetric and asymmetric reflections. Orientation between the Si substrate and the MnSi film is determined with the parallel lattice planes MnSi(210) and Si(511). Transport measurements show a T^2 dependence of the resistivity below 30K and metallic behavior above, a magneto resistance of 0.9\% and an unusual memory like effect of the resistance for an in-plane magnetic field sweep measurement. Silicon/Silicon suboxide (SiOx) multilayer structures are grown on Si(100) by interrupting the Si growth and oxidizing the surface with molecular oxygen. During oxidation the RHEED pattern changes from the Si(2x1) reconstruction to an amorphous pattern. When silicon growth is resumed a spotty RHEED pattern emerges, indicating a rough, three dimensional surface. The rough surface can be smoothed out with Si growth at substrate temperatures between 600°C and 700°C. Measurements with transmission electron microscopy show that a silicon suboxide layer of about 1nm embedded in single crystalline silicon is formed with the procedure. Multilayer structures are achieved by repeating the oxidation procedure when the Si spacer layer has a smooth and flat surface. The oxygen content of the suboxide layers can be varied between 7.6\% and 26.8\%, as determined with secondary ion mass spectrometry and custom-built simulations models for the x-ray diffraction. Structural stability of the multilayer structures is investigated by x-ray diffraction before and after rapid thermal annealing. For temperatures up to 1000°C the multilayer structures show no modification of the SiOx layer in x-ray diffraction.}, subject = {Molekularstrahlepitaxie}, language = {en} } @phdthesis{Scholz2013, author = {Scholz, Markus}, title = {Energy-Dispersive NEXAFS: A Novel Tool for the Investigation of Intermolecular Interaction and Structural Phase Dynamics}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83839}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {In the context of this thesis, the novel method soft X-ray energy-dispersive NEXAFS spectroscopy was explored and utilized to investigate intermolecular coupling and post-growth processes with a temporal resolution of seconds. 1,4,5,8- naphthalene tetracarboxylic acid dianhydride (NTCDA)multilayer films were the chosen model system for these investigations. The core hole-electron correlation in coherently coupled molecules was studied by means of energy-dispersive near-edge X-ray absorption fine-structure spectroscopy. A transient phase was found which exists during the transition between a disordered condensed phase and the bulk structure. This phase is characterized by distinct changes in the spectral line shape and energetic position of the X-ray absorption signal at the C K-edge. The findings were explained with the help of theoretical models based on the coupling of transition dipole moments, which are well established for optically excited systems. In consequence, the experimental results provides evidence for a core hole-electron pair delocalized over several molecules. Furthermore, the structure formation of NTCDA multilayer films on Ag(111) surfaces was investigated. With time-resolved and energy-dispersive NEXAFS experiments the intensity evolution in s- and p-polarization showed a very characteristic behavior. By combining these findings with the results of time-dependent photoemission measurements, several sub-processes were identified in the post- growth behavior. Upon annealing, the amorphous but preferentially flat-lying molecules flip into an upright orientation. After that follows a phase characterized by strong intermolecular coupling. Finally, three-dimensional islands are established. Employing the Kolmogorov-Johnson-Mehl-Avrami model, the activation energies of the sub-processes were determined.}, subject = {Organisches Molek{\"u}l}, language = {en} } @phdthesis{Ebel2013, author = {Ebel, Lars Frederik}, title = {Molecular Beam Epitaxy and Characterization of Ferromagnetic Bulk and Thin (Ga,Mn)As Layers/Heterostructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83942}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Die vorgelegte Arbeit untersucht den ferromagnetischen Halbleiter (Ga,Mn)As mit seinen komplexen Eigenschaften im Hinblick auf die Optimierung der Materialeigenschaften sehr d{\"u}nner (4 nm) (Ga,Mn)As Schichten, welche mit der Technologie der Molekularstrahlepitaxie (MBE) hergestellt wurden. Zuerst werden die strukturellen, ferromagnetischen und elektrischen Eigenschaften von (Ga,Mn)As vorgestellt. Die Einfl{\"u}sse von Punktdefekten, Grenzfl{\"a}chen- und Oberfl{\"a}chen-Effekten auf dicke und d{\"u}nne (Ga,Mn)As Schichten werden mit Hilfe von vereinfachten, selbstkonsistenten Berechnungen der Bandkantenverl{\"a}ufe diskutiert. Der Experimental-Teil ist in drei Teile unterteilt: Der erste Teil untersucht den Einfluss der epitaktischen Wachstumsbedingungen auf die elektrischen und magnetischen Eigenschaften von dicken (70 nm) (Ga,Mn)As Schichten. Der zweite Teil f{\"u}hrt ein alternatives, parabolisches Mn-Dotierprofil mit effektiver Schichtdicke von 4 nm ein im Vergleich zu einer gleich d{\"u}nnen Schicht mit homogenem Mn-Dotierprofil. Es konnten einerseits verbesserte Eigenschaften dieser parabolischen Schicht erreicht werden, anderseits sind die magnetischen und elektrischen Eigenschaften vergleichbar zu dicken (Ga,Mn)As Schichten mit gleichem Mn-Gehalt von 4\%. MBE Wachstumsbedingungen f{\"u}r (Ga,Mn)As Schichten mit parabolischem Mn-Dotierprofil und verringertem nominellem Mn-Gehalt von 2.5\% wurden ebenfalls untersucht. Ein schmales Wachstumsfenster wurde hierbei ermittelt, in dem die Tieftemperatur-Eigenschaften verbessert sind. Der letzte Teil der Arbeit pr{\"a}sentiert eine Anwendung der magnetischen Anisotropiekontrolle einer dicken (Ga,Mn)As Schicht.}, subject = {Molekularstrahlepitaxie}, language = {en} } @phdthesis{Foertig2013, author = {F{\"o}rtig, Alexander}, title = {Recombination Dynamics in Organic Solar Cells}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83895}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Neben herk{\"o}mmlichen, konventionellen anorganischen Solarzellen — haupts{\"a}chlich auf Silizium basierend — ist die Organische Photovoltaik (OPV) auf dem besten Wege in naher Zukunft eine kosteng{\"u}nstige, umweltfreundliche, komplement{\"a}re Technolgie darzustellen. Die Produktionskosten, die Lebenszeit der Solarzellen sowie deren Wirkungsgrad m{\"u}ssen dabei weiter optimiert werden, um einen Markteintritt der OPV zu erm{\"o}glichen. Die vorliegende Arbeit befasst sich mit der Effizienz organischer Solarzellen und deren Limitierung durch die Rekombination von Ladungstr{\"a}gern. Um funktionsf{\"a}hige Zellen zu untersuchen, werden zeitaufgel{\"o}ste 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 fl{\"u}ssig prozessierte als auch aufgedampfte Proben, unterschiedliche Materialzusammensetzungen und verschiedene Probengeometrien. Das Standardmaterialsystem der OPV, P3HT:PC61BM, wird bei verschiedenen emperaturen und Beleuchtungsst{\"a}rken auf die Lebenszeit und Dichte der photogenerierten Ladungstr{\"a}ger {\"u}berpr{\"u}ft. F{\"u}r den Fall spannungsunabh{\"a}ngiger Generation von Ladungstr{\"a}gern zeigt sich die Anwendbarkeit der Shockley-Gleichung auf organische Solarzellen. Des Weiteren wird ein konsistentes Modell erl{\"a}utert, welches den Idealtit{\"a}tsfaktor direkt mit der Rekombination von freien mit gefangenen, exponentiell verteilten Ladungstr{\"a}gern verkn{\"u}pft. Ein Ansatz, bekannt unter der Bezeichung j=V Rekonstruktion, erm{\"o}glicht 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. W{\"a}hrend in der Zelle hergestellt aus reinem Chlorbenzol betr{\"a}chtliche geminale wie nichtgeminale Verluste zu beobachten sind, erleichtert die Zugabe eines L{\"o}sungsmittelzusatzes die Polaronenpaartrennung, was zu einer starken Reduktion geminaler Verluste f{\"u}hrt. In einer Kooperation mit dem IMEC Institut in Leuven, werden abschließend die beiden bedeutensten Probenarchitekturen organischer Solarzellen, die planare und die Misch{\"u}bergang Struktur, jeweils basierend auf CuPC und C60, bez{\"u}glich nichtgeminaler Rekombination und Ladungstr{\"a}gerverteilung miteinander verglichen. Neben den beiden experimentellen Techniken um TPV und CE werden makroskopische Simulationen herangezogen, um den Ursprung unterschiedlichen Voc vs. Lichtintensit{\"a}t-Verhaltens zu erkl{\"a}ren.}, subject = {Organische Solarzelle}, language = {en} } @phdthesis{Baerwald2013, author = {Baerwald, Philipp}, title = {Neutrinos from gamma-ray bursts, and the multi-messenger connection}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-85333}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {In this work, we take a look at the connection of gamma-ray bursts (GRBs) and ultra-high-energy cosmic rays (UHECR) as well as the possibilities how to verify this connection. The currently most promising approach is based on the detection of high-energy neutrinos, which are associated with the acceleration of cosmic rays. We detail how the prompt gamma-ray emission is connected to the prediction of a neutrino signal. We focus on the interactions of photons and protons in this regard. At the example of the current ANTARES GRB neutrino analysis, we show the differences between numerical predictions and older analytical methods. Moreover, we discuss the possibilities how cosmic ray particles can escape from GRBs, assuming that UHECR are entirely made up of protons. For this, we compare the commonly assumed neutron escape model with a new component of direct proton escape. Additionally, we will show that the different components, which contribute to the cosmic ray flux, strongly depend on the burst parameters, and test the applicability on some chosen GRBs. In a further step, we continue with the considerations regarding the connection of GRBs and UHECR by connecting the GRB source model with the cosmic ray observations using a simple cosmic ray propagation code. We test if it is possible to achieve the observed cosmic ray energy densities with our simple model and what the consequences are regarding the prompt GRB neutrino flux predictions as well as the cosmogenic neutrinos. Furthermore, we consider the question of neutrino lifetime and how it affects the prompt GRB neutrino flux predictions. In a final chapter, we show that it is possible to apply the basic source model with photohadronic interactions to other types of sources, using the example of the microquasar Cygnus X-3.}, subject = {Neutrino}, language = {en} } @phdthesis{Kern2013, author = {Kern, Julia}, title = {Field Dependence of Charge Carrier Generation in Organic Bulk Heterojunction Solar Cells}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-91963}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {In the field of organic photovoltaics, one of the most intensely researched topics to date is the charge carrier photogeneration in organic bulk heterojunction solar cells whose thorough understanding is crucial for achieving higher power conversion efficiencies. In particular, the mechanism of singlet exciton dissociation at the polymer-fullerene interface is still controversially debated. This work addresses the dissociation pathway via relaxed charge transfer states (CTS) by investigating its field dependence for reference material systems consisting of MDMO-PPV and one of the fullerene derivatives PC61BM, bisPCBM and PC71BM. Field dependent photoluminescence (PL(F)) and transient absorption (TA(F)) measurements give insight into the recombination of charge transfer excitons (CTE) and the generation of polarons, respectively. Optically detected magnetic resonance and atomic force microscopy are used to characterize the morphology of the samples. The comparison of the experimental field dependent exciton recombination recorded by PL(F) and the theoretical exciton dissociation probability given by the Onsager-Braun model yields the exciton binding energy as one of the key parameters determining the dissociation efficiency. The binding energies of both the singlet exciton in neat MDMO-PPV and the CTE in MDMO-PPV:PC61BM 1:1 are extracted, the latter turning out to be significantly reduced with respect to the one of the singlet exciton. Based on these results, the field dependence of CTE dissociation is evaluated for MDMO-PPV:PC61BM blends with varying fullerene loads by PL(F) and TA(F). For higher PC61BM contents, the CTE binding energies decrease notably. This behavior is ascribed to a larger effective dielectric constant for well-intermixed blends and to an interplay between dielectric constant and CTE delocalization length for phase separated morphologies, emphasizing the importance of high dielectric constants for the charge carrier photogeneration process. Finally, the CTE binding energies are determined for MDMO-PPV blends with different fullerene derivatives, focusing on the influence of the acceptor LUMO energy. Here, the experimental results suggest the latter having no or at least no significant impact on the binding energy of the CTE. Variations of this binding energy are rather related to different trap levels in the acceptors which seem to be involved in CTS formation.}, subject = {Organische Solarzelle}, language = {en} } @phdthesis{Bollmann2013, author = {Bollmann, Stefan}, title = {Structural Dynamics of Oligopeptides determined by Fluorescence Quenching of Organic Dyes}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-92191}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {For determination of structures and structural dynamics of proteins organic fluorophores are a standard instrument. Intra- and intermolecular contact of biomolecular structures are determined in time-resolved and stationary fluorescence microscopy experiments by quenching of organic fluorophores due to Photoinduced Electron Transfer (PET) and dimerization interactions. Using PET we show in this work that end-to-end contact dynamics of serine-glycine peptides are slowed down by glycosylation. This slow down is due to a change in reaction enthalpy for end-to-end contact and is partly compensated by entropic effects. In a second step we test how dimerization of MR121 fluorophore pairs reports on end-to-end contact dynamics. We show that in aqueous solutions containing strong denaturants MR121 dimerization reports advantageously on contact dynamics for glycine-serine oligopeptides compared to the previously used MR121/tryptophane PET reporters. Then we analyze dimer interactions and quenching properties of different commercially available fluorophores being standards in F{\"o}rster Resonance Energy Transfer (FRET) measurements. Distances in biomolecules are determinable using FRET, but for very flexible biomolecules the analysis of masurement data can be distorted if contact of the two FRET fluorophores is likely. We quantify how strong the quenching of fluorophore pairs with two different or two identical fluorophores is. Dimer spectra and association constants are quantified to estimate if fluophores are applicable in various applications, e.g. in FRET measurements with unstructured peptides and proteins.}, subject = {Fluorophore}, language = {en} } @phdthesis{Constantino2013, author = {Constantino, Jennifer Anne}, title = {Characterization of Novel Magnetic Materials: Ultra-Thin (Ga,Mn)As and Epitaxial-Growth MnSi Thin Films}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-90578}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {The study of magnetic phases in spintronic materials is crucial to both our fundamental understanding of magnetic interactions and for finding new effects for future applications. In this thesis, we study the basic electrical and magnetic transport properties of both epitaxially-grown MnSi thin films, a helimagnetic metal only starting to be developed within our group, and parabolic-doped ultra-thin (Ga,Mn)As layers for future studies and applications.}, subject = {Galliumarsenid}, language = {en} } @phdthesis{Michalska2013, author = {Michalska, Marta}, title = {Molecular Imaging of atherosclerosis}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-73243}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Atherosklerose ist eine aktive und progressive Erkrankung, bei der vaskul{\"a}re Adh{\"a}sionsmolek{\"u}le wie VCAM-1 eine entscheidende Rolle durch Steuerung der Rekrutierung von Immunzellen in den fr{\"u}hen und fortgeschrittenen Plaques spielen. Ein zielgerichteter Einsatz von VCAM-1-Molek{\"u}len mit spezifischen Kontrastmitteln ist daher eine M{\"o}glichkeit, die VCAM-1-Expression zu kontrollieren, Plaquewachstum ab einem fr{\"u}hen Zeitpunkt zu visualisieren und eine fr{\"u}he Pr{\"a}vention von Atherosklerose vor Beginn der Thrombusbildung zu etablieren. Des Weiteren bietet die nichtinvasive Magnetresonanz (MR)-Bildgebung den Vorteil der Kombination molekularer und morphologischer Daten. Sie erm{\"o}glicht, mithilfe von entwickelten VCAM-1-markierten Eisenoxidpartikeln, den spezifischen Nachweis entz{\"u}ndlicher Prozesse w{\"a}hrend der Atherosklerose. Diese Arbeit belegt, dass mit dem VCAM-1-Konzept eine vielversprechende Herangehensweise gefunden wurde und dass das, mit spezifischen superparamagnetischen Eisenoxid (USPIO) konjugierte VCAM-1-Peptid, gegen{\"u}ber unspezifischer USPIOs ein erh{\"o}htes Potenzial bei der Untersuchung der Atherosklerose in sich tr{\"a}gt. Im ersten Teil der Arbeit konnte im Mausmodell gezeigt werden, dass gerade das VCAM-1-Molek{\"u}l ein sinnvoller Ansatzpunkt zur Darstellung und Bildgebung von Atherosklerose ist, da in der fr{\"u}hen Phase der Entz{\"u}ndung die vaskul{\"a}ren Zelladh{\"a}sionsmolek{\"u}le {\"u}berexprimiert und auch kontinuierlich, w{\"a}hrend der fortschreitenden Plaquebildung, hochreguliert werden. Weiterhin beschreibt diese Arbeit die Funktionst{\"u}chtigkeit und das Verm{\"o}gen des neu gestalteten USPIO Kontrastmittels mit dem zyklischen Peptid, in seiner Spezialisierung auf die VCAM-1 Erkennung. Experimentelle Studien mit ultra-Hochfeld-MRT erm{\"o}glichten weitere ex vivo und in vivo Nachweise der eingesetzten USPIO-VCAM-1-Partikel innerhalb der Region um die Aortenwurzel in fr{\"u}hen und fortgeschrittenen atherosklerotischen Plaques von 12 und 30 Wochen alten Apolipoprotein E-defizienten (ApoE-/-) M{\"a}usen. Mit ihrer Kombination aus Histologie und Elektronenmikroskopie zeigt diese Studie zum ersten Mal die Verteilung von VCAM-1-markierten USPIO Partikeln nicht nur in luminalem Bereich der Plaques, sondern auch in tieferen Bereichen der medialen Muskelzellen. Dieser spezifische und sensitive Nachweis der fr{\"u}hen und fortgeschrittenen Stadien der Plaquebildung bringt auf molekularer Ebene neue M{\"o}glichkeiten zur Fr{\"u}herkennung von atherosklerotischen Plaques vor dem Entstehen von 8 Rupturen. Im Gegensatz zum USPIO-VCAM-1-Kontrastmittel scheiterten unspezifische USPIO Partikel an der Identifikation fr{\"u}her Plaqueformen und begrenzten die Visualisierung von Atherosklerose auf fortgeschrittene Stadien in ApoE-/- M{\"a}usen.}, subject = {VCAM}, language = {en} }