@article{KimZhangWangetal.2016, author = {Kim, Seonghoon and Zhang, Bo and Wang, Zhaorong and Fischer, Julian and Brodbeck, Sebastian and Kamp, Martin and Schneider, Christian and H{\"o}fling, Sven and Deng, Hui}, title = {Coherent Polariton Laser}, series = {Physical Review X}, volume = {6}, journal = {Physical Review X}, number = {011026}, doi = {10.1103/PhysRevX.6.011026}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166597}, year = {2016}, abstract = {The semiconductor polariton laser promises a new source of coherent light, which, compared to conventional semiconductor photon lasers, has input-energy threshold orders of magnitude lower. However, intensity stability, a defining feature of a coherent state, has remained poor. Intensity noise many times the shot noise of a coherent state has persisted, attributed to multiple mechanisms that are difficult to separate in conventional polariton systems. The large intensity noise, in turn, limits the phase coherence. Thus, the capability of the polariton laser as a source of coherence light is limited. Here, we demonstrate a polariton laser with shot-noise-limited intensity stability, as expected from a fully coherent state. This stability is achieved by using an optical cavity with high mode selectivity to enforce single-mode lasing, suppress condensate depletion, and establish gain saturation. Moreover, the absence of spurious intensity fluctuations enables the measurement of a transition from exponential to Gaussian decay of the phase coherence of the polariton laser. It suggests large self-interaction energies in the polariton condensate, exceeding the laser bandwidth. Such strong interactions are unique to matter-wave lasers and important for nonlinear polariton devices. The results will guide future development of polariton lasers and nonlinear polariton devices.}, language = {en} } @article{ChenariSeibelHauschildetal.2016, author = {Chenari, Hossein Mahmoudi and Seibel, Christoph and Hauschild, Dirk and Reinert, Friedrich and Abdollahian, Hossein}, title = {Titanium Dioxide Nanoparticles: Synthesis, X-Ray Line Analysis and Chemical Composition Study}, series = {Materials Research}, volume = {19}, journal = {Materials Research}, number = {6}, doi = {10.1590/1980-5373-MR-2016-0288}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-165807}, pages = {1319-1323}, year = {2016}, abstract = {TiO2 nanoparticleshave been synthesized by the sol-gel method using titanium alkoxide and isopropanolas a precursor. The structural properties and chemical composition of the TiO2 nanoparticles were studied usingX-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy.The X-ray powder diffraction pattern confirms that the particles are mainly composed of the anatase phase with the preferential orientation along [101] direction. The physical parameters such as strain, stress and energy density were investigated from the Williamson- Hall (W-H) plot assuming a uniform deformation model (UDM), and uniform deformation energy density model (UDEDM). The W-H analysis shows an anisotropic nature of the strain in nanopowders. The scanning electron microscopy image shows clear TiO2 nanoparticles with particle sizes varying from 60 to 80nm. The results of mean particle size of TiO2 nanoparticles show an inter correlation with the W-H analysis and SEM results. Our X-ray photoelectron spectroscopy spectra show that nearly a complete amount of titanium has reacted to TiO2}, language = {en} } @article{HargartRoyChoudhuryJohnetal.2016, author = {Hargart, F and Roy-Choudhury, K and John, T and Portalupi, S L and Schneider, C and H{\"o}fling, S and Kamp, M and Hughes, S and Michler, P}, title = {Probing different regimes of strong field light-matter interaction with semiconductor quantum dots and few cavity photons}, series = {New Journal of Physics}, volume = {18}, journal = {New Journal of Physics}, doi = {10.1088/1367-2630/aa5198}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166278}, year = {2016}, abstract = {In this work we present an extensive experimental and theoretical investigation of different regimes of strong field light-matter interaction for cavity-driven quantum dot (QD) cavity systems. The electric field enhancement inside a high-Q micropillar cavity facilitates exceptionally strong interaction with few cavity photons, enabling the simultaneous investigation for a wide range of QD-laser detuning. In case of a resonant drive, the formation of dressed states and a Mollow triplet sideband splitting of up to 45 μeV is measured for amean cavity photon number \(\leq\) 1. In the asymptotic limit of the linear ACStark effect we systematically investigate the power and detuning dependence of more than 400 QDs. Some QD-cavity systems exhibit an unexpected anomalous Stark shift, which can be explained by an extended dressed 4-levelQDmodel.Weprovide a detailed analysis of the QD-cavity systems properties enabling this novel effect. The experimental results are successfully reproduced using a polaron master equation approach for the QD-cavity system, which includes the driving laser field, exciton-cavity and exciton-phonon interactions}, language = {en} } @article{RedlichLingnauHolzingeretal.2016, author = {Redlich, Christoph and Lingnau, Benjamin and Holzinger, Steffen and Schlottmann, Elisabeth and Kreinberg, S{\"o}ren and Schneider, Christian and Kamp, Martin and H{\"o}fling, Sven and Wolters, Janik and Reitzenstein, Stephan and L{\"u}dge, Kathy}, title = {Mode-switching induced super-thermal bunching in quantum-dot microlasers}, series = {New Journal of Physics}, volume = {18}, journal = {New Journal of Physics}, number = {063011}, doi = {10.1088/1367-2630/18/6/063011}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-166286}, year = {2016}, abstract = {The super-thermal photon bunching in quantum-dot (QD) micropillar lasers is investigated both experimentally and theoretically via simulations driven by dynamic considerations. Using stochastic multi-mode rate equations we obtain very good agreement between experiment and theory in terms of intensity profiles and intensity-correlation properties of the examined QD micro-laser's emission. Further investigations of the time-dependent emission show that super-thermal photon bunching occurs due to irregular mode-switching events in the bimodal lasers. Our bifurcation analysis reveals that these switchings find their origin in an underlying bistability, such that spontaneous emission noise is able to effectively perturb the two competing modes in a small parameter region. We thus ascribe the observed high photon correlation to dynamical multistabilities rather than quantum mechanical correlations.}, language = {en} } @article{WiedenmannBocquillonDeaconetal.2016, author = {Wiedenmann, J. and Bocquillon, E. and Deacon, R.S. and Hartinger, S. and Herrmann, O. and Klapwijk, T.M. and Maier, L. and Ames, C. and Br{\"u}ne, C. and Gould, C. and Oiwa, A. and Ishibashi, K. and Tarucha, S. and Buhmann, H. and Molenkamp, L.W.}, title = {4π-periodic Josephson supercurrent in HgTe-based topological Josephson junctions}, series = {Nature Communications}, volume = {7}, journal = {Nature Communications}, doi = {10.1038/ncomms10303}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-175353}, year = {2016}, abstract = {The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4π-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction made of a HgTe weak link. The response is understood as due to a 4π-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.}, language = {en} } @article{SiminSoltamovPoshakinskiyetal.2016, author = {Simin, D. and Soltamov, V. A. and Poshakinskiy, A. V. and Anisimov, A. N. and Babunts, R. A. and Tolmachev, D. O. and Mokhov, E. N. and Trupke, M. and Tarasenko, S. A. and Sperlich, A. and Baranov, P. G. and Dyakonov, V. and Astakhov, G. V.}, title = {All-Optical dc Nanotesla Magnetometry Using Silicon Vacancy Fine Structure in Isotopically Purified Silicon Carbide}, series = {Physical Review X}, volume = {6}, journal = {Physical Review X}, doi = {10.1103/PhysRevX.6.031014}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-147682}, pages = {031014}, year = {2016}, abstract = {We uncover the fine structure of a silicon vacancy in isotopically purified silicon carbide (4H-\(^{28}\)SiC) and reveal not yet considered terms in the spin Hamiltonian, originated from the trigonal pyramidal symmetry of this spin-3/2 color center. These terms give rise to additional spin transitions, which would be otherwise forbidden, and lead to a level anticrossing in an external magnetic field. We observe a sharp variation of the photoluminescence intensity in the vicinity of this level anticrossing, which can be used for a purely all-optical sensing of the magnetic field. We achieve dc magnetic field sensitivity better than 100  nT/√Hz within a volume of 3×10\(^{-7}\)mm\(^3\) at room temperature and demonstrate that this contactless method is robust at high temperatures up to at least 500 K. As our approach does not require application of radio-frequency fields, it is scalable to much larger volumes. For an optimized light-trapping waveguide of 3  mm\(^3\), the projection noise limit is below 100  fT/√Hz.}, language = {en} } @article{KrausHeiberVaethetal.2016, author = {Kraus, Hannes and Heiber, Michael C. and V{\"a}th, Stefan and Kern, Julia and Deibel, Carsten and Sperlich, Andreas and Dyakonov, Vladimir}, title = {Analysis of Triplet Exciton Loss Pathways in PTB7:PC\(_{71}\)BM Bulk Heterojunction Solar Cells}, series = {Scientific Reports}, volume = {6}, journal = {Scientific Reports}, number = {29158}, doi = {10.1038/srep29158}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-147413}, year = {2016}, abstract = {A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC\(_{71}\)BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC\(_{71}\)BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC\(_{71}\)BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway.}, language = {en} } @phdthesis{Pfeuffer2016, author = {Pfeuffer, Rebekka Christina}, title = {Growth and characterization of II-VI semiconductor nanowires grown by Au catalyst assisted molecular beam epitaxy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-141385}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {In the present PhD thesis the control of the morphology, such as the diameter, the length, the orientation, the density, and the crystalline quality of 1D ZnSe NWs grown by MBE for optical and transport applications has been achieved.}, subject = {Zinkselenid}, language = {en} } @phdthesis{Vaeth2016, author = {V{\"a}th, Stefan Kilian}, title = {On the Role of Spin States in Organic Semiconductor Devices}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-141894}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {The present work addressed the influence of spins on fundamental processes in organic semiconductors. In most cases, the role of spins in the conversion of sun light into electricity was of particular interest. However, also the reversed process, an electric current creating luminescence, was investigated by means of spin sensitive measurements. In this work, many material systems were probed with a variety of innovative detection techniques based on electron paramagnetic resonance spectroscopy. More precisely, the observable could be customized which resulted in the experimental techniques photoluminescence detected magnetic resonance (PLDMR), electrically detected magnetic resonance (EDMR), and electroluminescence detected magnetic resonance (ELDMR). Besides the commonly used continuous wave EPR spectroscopy, this selection of measurement methods yielded an access to almost all intermediate steps occurring in organic semiconductors during the conversion of light into electricity and vice versa. Special attention was paid to the fact that all results were applicable to realistic working conditions of the investigated devices, i.e. room temperature application and realistic illumination conditions.}, subject = {Organischer Halbleiter}, language = {en} } @phdthesis{Schreyeck2016, author = {Schreyeck, Steffen}, title = {Molecular Beam Epitaxy and Characterization of Bi-Based V\(_2\)VI\(_3\) Topological Insulators}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-145812}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {The present thesis is addressed to the growth and characterization of Bi-based V2VI3 topological insulators (TIs). The TIs were grown by molecular beam epitaxy (MBE) on differently passivated Si(111) substrates, as well as InP(111) substrates. This allows the study of the influence of the substrate on the structural and electrical properties of the TIs. The Bi2Se3 layers show a change of mosaicity-tilt and -twist for growth on the differently prepared Si(111) substrates, as well as a significant increase of crystalline quality for growth on the lateral nearly lattice matched InP(111). The rocking curve FWHMs observed for thick layers grown on InP are comparable to these of common zincblende layers, which are close to the resolution limit of standard high resolution X-ray diffraction (HRXRD) setups. The unexpected high structural crystalline quality achieved in this material system is remarkable due to the presence of weak van der Waals bonds between every block of five atomic layers, i.e. a quintuple layer (QL), in growth direction. In addition to the mosaicity also twin domains, present in films of the V2VI3 material system, are studied. The twin defects are observed in Bi2Se3 layers grown on Si(111) and lattice matched InP(111) suggesting that the two dimensional surface lattice of the substrates can not determine the stacking order ABCABC... or ACBACB... in locally separated growth seeds. Therefore the growth on misoriented and rough InP(111) is analyzed. The rough InP(111) with its facets within a hollow exceeding the height of a QL is able to provide its stacking information to the five atomic layers within a QL. By varying the roughness of the InP substrate surface, due to thermal annealing, the influence on the twinning within the layer is confirmed resulting in a complete suppression of twin domains on rough InP(111). Focusing on the electrical properties of the Bi2Se3 films, the increased structural quality for films grown on lattice matched flat InP(111)B results in a marginal reduction of carrier density by about 10\% compared to the layers grown on H-passivated Si(111), whereas the suppression of twin domains for growth on rough InP(111)B resulted in a reduction of carrier density by an order of magnitude. This implies, that the twin domains are a main crystal defect responsible for the high carrier density in the presented Bi2Se3 thin films. Besides the binary Bi2Se3 also alloys with Sb and Te are fabricated to examine the influence of the compound specific point defects on the carrier density. Therefore growth series of the ternary materials Bi2Te(3-y)Se(y), Bi(2-x)Sb(x)Se3, and Bi(2-x)Sb(x)Te3, as well as the quaternary Bi(2-x)Sb(x)Te(3-y)Se(y) are studied. To further reduce the carrier density of twin free Bi2Se3 layers grown on InP(111)B:Fe a series of Bi(2-x)Sb(x)Se3 alloys were grown under comparable growth conditions. This results in a reduction of the carrier density with a minimum in the composition range of about x=0.9-1.0. The Bi(2-x)Sb(x)Te3 alloys exhibit a pn-transition, due to the dominating n-type and p-type point defects in its binary compounds, which is determined to reduce the bulk carrier density enabling the study the TI surface states. This pn-transition plays a significant role in realizing predicted applications and exotic effects, such as the quantum anomalous Hall effect. The magnetic doping of topological insulators with transition metals is studied by incorporating Cr and V in the alloy Bi(2-x)Sb(x)Te3 by codeposition. The preferential incorporation of Cr on group-V sites is confirmed by EDX and XRD, whereas the incorporation of Cr reduces the crystalline quality of the layer. Magnetotransport measurements of the Cr-doped TIs display an anomalous Hall effect confirming the realization of a magnetic TI thin film. The quantum anomalous Hall effect is observed in V-doped Bi(2-x)Sb(x)Te3, where the V-doping results in higher Curie temperatures, as well as higher coercive fields compared to the Cr-doping of the TIs. Moreover the present thesis contributes to the understanding of the role of the substrate concerning the crystalline quality of van der Waals bonded layers, such as the V2VI3 TIs, MoS2 and WoTe2. Furthermore, the fabrication of the thin film TIs Bi(2-x)Sb(x)Te(3-y)Se(y) in high crystalline quality serves as basis to explore the physics of topological insulators.}, subject = {Bismutverbindungen}, language = {en} } @phdthesis{Kessel2016, author = {Kessel, Maximilian}, title = {HgTe shells on CdTe nanowires: A low-dimensional topological insulator from crystal growth to quantum transport}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149069}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {A novel growth method has been developed, allowing for the growth of strained HgTe shells on CdTe nanowires (NWs). The growth of CdTe-HgTe core-shell NWs required high attention in controlling basic parameters like substrate temperature and the intensity of supplied material fluxes. The difficulties in finding optimized growth conditions have been successfully overcome in this work. We found the lateral redistribution of liquid growth seeds with a ZnTe growth start to be crucial to trigger vertical CdTe NW growth. Single crystalline zinc blende CdTe NWs grew, oriented along [111]B. The substrate temperature was the most critical parameter to achieve straight and long wires. In order to adjust it, the growth was monitored by reflection high-energy electron diffraction, which was used for fine tuning of the temperature over time in each growth run individually. For optimized growth conditions, a periodic diffraction pattern allowed for the detailed analysis of atomic arrangement on the surfaces and in the bulk. The ability to do so reflected the high crystal quality and ensemble uniformity of our CdTe NWs. The NW sides were formed by twelve stable, low-index crystalline facets. We observed two types stepped and polar sides, separated by in total six flat and non-polar facets. The high crystalline quality of the cores allowed to grow epitaxial HgTe shells around. We reported on two different heterostructure geometries. In the first one, the CdTe NWs exhibit a closed HgTe shell, while for the second one, the CdTe NWs are overgrown mainly on one side. Scanning electron microscopy and scanning transmission electron microscopy confirmed, that many of the core-shell NWs are single crystalline zinc blende and have a high uniformity. The symmetry of the zinc blende unit cell was reduced by residual lattice strain. We used high-resolution X-ray diffraction to reveal the strain level caused by the small lattice mismatch in the heterostructures. Shear strain has been induced by the stepped hetero-interface, thereby stretching the lattice of the HgTe shell by 0.06 \% along a direction oriented with an angle of 35 ° to the interface. The different heterostructures obtained, were the base for further investigation of quasi-one-dimensional crystallites of HgTe. We therefore developed methods to reliably manipulate, align, localize and contact individual NWs, in order to characterize the charge transport in our samples. Bare CdTe cores were insulating, while the HgTe shells were conducting. At low temperature we found the mean free path of charge carriers to be smaller, but the phase coherence length to be larger than the sample size of several hundred nanometers. We observed universal conductance fluctuations and therefore drew the conclusion, that the trajectories of charge carriers are defined by elastic backscattering at randomly distributed scattering sites. When contacted with superconducting leads, we saw induced superconductivity, multiple Andreev reflections and the associated excess current. Thus, we achieved HgTe/superconductor interfaces with high interfacial transparency. In addition, we reported on the appearance of peaks in differential resistance at Delta/e for HgTe-NW/superconductor and 2*Delta/e for superconductor/HgTe-NW/superconductor junctions, which is possibly related to unconventional pairing at the HgTe/superconductor interface. We noticed that the great advantage of our self-organized growth is the possibility to employ the metallic droplet, formerly seeding the NW growth, as a superconducting contact. The insulating wire cores with a metallic droplet at the tip have been overgrown with HgTe in a fully in-situ process. A very high interface quality was achieved in this case.}, subject = {Quecksilbertellurid}, language = {en} } @article{NitscheKimRoumposetal.2016, author = {Nitsche, Wolfgang H. and Kim, Na Young and Roumpos, Georgios and Schneider, Christian and H{\"o}fling, Sven and Forchel, Alfred and Yamamoto, Yoshihisa}, title = {Spatial correlation of two-dimensional bosonic multimode condensates}, series = {Physical Review A}, volume = {93}, journal = {Physical Review A}, number = {5}, doi = {10.1103/PhysRevA.93.053622}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-188897}, pages = {53622}, year = {2016}, abstract = {The Berezinskii-Kosterlitz-Thouless (BKT) theorem predicts that two-dimensional bosonic condensates exhibit quasi-long-range order which is characterized by a slow decay of the spatial coherence. However previous measurements on exciton-polariton condensates revealed that their spatial coherence can decay faster than allowed under the BKT theory, and different theoretical explanations have already been proposed. Through theoretical and experimental study of exciton-polariton condensates, we show that the fast decay of the coherence can be explained through the simultaneous presence of multiple modes in the condensate.}, language = {en} } @article{LaihoPresslSchlageretal.2016, author = {Laiho, K. and Pressl, B. and Schlager, A. and Suchomel, H. and Kamp, M. and H{\"o}fling, S. and Schneider, C. and Weihs, G.}, title = {Uncovering dispersion properties in semiconductor waveguides to study photon-pair generation}, series = {Nanotechnology}, volume = {27}, journal = {Nanotechnology}, number = {43}, doi = {10.1088/0957-4484/27/43/434003}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-187025}, pages = {434003}, year = {2016}, abstract = {We investigate the dispersion properties of ridge Bragg-reflection waveguides to deduce their phasematching characteristics. These are crucial for exploiting them as sources of parametric down-conversion (PDC). In order to estimate the phasematching bandwidth we first determine the group refractive indices of the interacting modes via Fabry-Perot experiments in two distant wavelength regions. Second, by measuring the spectra of the emitted PDC photons, we gain access to their group index dispersion. Our results offer a simple approach for determining the PDC process parameters in the spectral domain, and provide important feedback for designing such sources, especially in the broadband case.}, language = {en} } @article{LundtKlembtCherotchenkoetal.2016, author = {Lundt, Nils and Klembt, Sebastian and Cherotchenko, Evgeniia and Betzold, Simon and Iff, Oliver and Nalitov, Anton V. and Klaas, Martin and Dietrich, Christof P. and Kavokin, Alexey V. and H{\"o}fling, Sven and Schneider, Christian}, title = {Room-temperature Tamm-plasmon exciton-polaritons with a WSe\(_{2}\) monolayer}, series = {Nature Communications}, volume = {7}, journal = {Nature Communications}, doi = {10.1038/ncomms13328}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-169470}, year = {2016}, abstract = {Solid-state cavity quantum electrodynamics is a rapidly advancing field, which explores the frontiers of light-matter coupling. Metal-based approaches are of particular interest in this field, as they carry the potential to squeeze optical modes to spaces significantly below the diffraction limit. Transition metal dichalcogenides are ideally suited as the active material in cavity quantum electrodynamics, as they interact strongly with light at the ultimate monolayer limit. Here, we implement a Tamm-plasmon-polariton structure and study the coupling to a monolayer of WSe\(_{2}\), hosting highly stable excitons. Exciton-polariton formation at room temperature is manifested in the characteristic energy-momentum dispersion relation studied in photoluminescence, featuring an anti-crossing between the exciton and photon modes with a Rabi-splitting of 23.5 meV. Creating polaritonic quasiparticles in monolithic, compact architectures with atomic monolayers under ambient conditions is a crucial step towards the exploration of nonlinearities, macroscopic coherence and advanced spinor physics with novel, low-mass bosons.}, language = {en} } @phdthesis{Fella2016, author = {Fella, Christian}, title = {High-Resolution X-ray Imaging based on a Liquid-Metal-Jet-Source with and without X-ray Optics}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-145938}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {With increasing miniaturization in industry and medical technology, non-destructive testing techniques are an area of everincreasing importance. In this framework, X-ray microscopy offers an efficient tool for the analysis, understanding and quality assurance of microscopic species, in particular as it allows reconstructing three-dimensional data sets of the whole sample's volumevia computed tomography (CT). The following thesis describes the conceptualization, design, construction and characterization of a compact laboratory-based X-ray microscope in the hard X-ray regime around 9 keV, corresponding to a wavelength of 0.134 nm. Hereby, the main focus is on the optimization of resolution and contrast at relatively short exposure times. For this, a novel liquid-metal-jet anode source is the basis. Such only recently commercially available X-ray source reaches a higher brightness than other conventional laboratory sources, i.e. the number of emitted photons (X-ray quanta) per area and solid angle is exceptionally high. This is important in order to reach low exposure times. The reason for such high brightness is the usage of the rapidly renewing anode out of liquid metal which enables an effective dissipation of heat, normally limiting the creation of high intensities on a small area. In order to cover a broad range of different samples, the microscope can be operated in two modes. In the "micro-CT mode", small pixels are realized with a crystal-scintillator and an optical microscope via shadow projection geometry. Therefore, the resolution is limited by the emitted wavelength of the scintillator, as well as the blurring of the screen. However, samples in the millimeter range can be scanned routinely with low exposure times. Additionally, this mode is optimized with respect to in-line phase contrast, where edges of an object are enhanced and thus better visible. In the second "nano-CT mode", a higher resolution can be reached via X-ray lenses. However, their production process is due to the physical properties of the hard X-ray range - namely high absorption and low diffraction - extremely difficult, leading typically to low performances. In combination with a low brightness, this leads to long exposure times and high requirements in terms of stability, which is one of the key problems of laboratory-based X-ray microscopy. With the here-developed setup and the high brightness of its source, structures down to 150 nm are resolved at moderate exposure times (several minutes per image) and nano-CTs can be obtained.}, subject = {computed tomography}, language = {en} } @article{AnisimovSiminSoltamovetal.2016, author = {Anisimov, A. N. and Simin, D. and Soltamov, V. A. and Lebedev, S. P. and Baranov, P. G. and Astakhov, G. V. and Dyakonov, V.}, title = {Optical thermometry based on level anticrossing in silicon carbide}, series = {Scientific Reports}, volume = {6}, journal = {Scientific Reports}, number = {33301}, doi = {10.1038/srep33301}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-147809}, year = {2016}, abstract = {We report a giant thermal shift of 2.1 MHz/K related to the excited-state zero-field splitting in the silicon vacancy centers in 4H silicon carbide. It is obtained from the indirect observation of the optically detected magnetic resonance in the excited state using the ground state as an ancilla. Alternatively, relative variations of the zero-field splitting for small temperature differences can be detected without application of radiofrequency fields, by simply monitoring the photoluminescence intensity in the vicinity of the level anticrossing. This effect results in an all-optical thermometry technique with temperature sensitivity of 100 mK/Hz\(^{1/2}\) for a detection volume of approximately 10\(^{-6}\) mm\(^3\). In contrast, the zero-field splitting in the ground state does not reveal detectable temperature shift. Using these properties, an integrated magnetic field and temperature sensor can be implemented on the same center.}, language = {en} } @article{KiermaschRiederTvingstedtetal.2016, author = {Kiermasch, David and Rieder, Philipp and Tvingstedt, Kristofer and Baumann, Andreas and Dyakonov, Vladimir}, title = {Improved charge carrier lifetime in planar perovskite solar cells by bromine doping}, series = {Scientific Reports}, volume = {6}, journal = {Scientific Reports}, doi = {10.1038/srep39333}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-147976}, pages = {39333}, year = {2016}, abstract = {The charge carrier lifetime is an important parameter in solar cells as it defines, together with the mobility, the diffusion length of the charge carriers, thus directly determining the optimal active layer thickness of a device. Herein, we report on charge carrier lifetime values in bromine doped planar methylammonium lead iodide (MAPbI\(_3\)) solar cells determined by transient photovoltage. The corresponding charge carrier density has been derived from charge carrier extraction. We found increased lifetime values in solar cells incorporating bromine compared to pure MAPbI\(_3\) by a factor of ~2.75 at an illumination intensity corresponding to 1 sun. In the bromine containing solar cells we additionally observe an anomalously high value of extracted charge, which we deduce to originate from mobile ions.}, language = {en} } @article{HsuKuegelKemmeretal.2016, author = {Hsu, Pin-Jui and K{\"u}gel, Jens and Kemmer, Jeannette and Toldin, Francesco Parisen and Mauerer, Tobias and Vogt, Matthias and Assaad, Fakher and Bode, Matthias}, title = {Coexistence of charge and ferromagnetic order in fcc Fe}, series = {Nature Communications}, volume = {7}, journal = {Nature Communications}, doi = {10.1038/ncomms10949}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173969}, year = {2016}, abstract = {Phase coexistence phenomena have been intensively studied in strongly correlated materials where several ordered states simultaneously occur or compete. Material properties critically depend on external parameters and boundary conditions, where tiny changes result in qualitatively different ground states. However, up to date, phase coexistence phenomena have exclusively been reported for complex compounds composed of multiple elements. Here we show that charge- and magnetically ordered states coexist in double-layer Fe/Rh(001). Scanning tunnelling microscopy and spectroscopy measurements reveal periodic charge-order stripes below a temperature of 130 K. Close to liquid helium temperature, they are superimposed by ferromagnetic domains as observed by spin-polarized scanning tunnelling microscopy. Temperature-dependent measurements reveal a pronounced cross-talk between charge and spin order at the ferromagnetic ordering temperature about 70 K, which is successfully modelled within an effective Ginzburg-Landau ansatz including sixth-order terms. Our results show that subtle balance between structural modifications can lead to competing ordering phenomena.}, language = {en} } @article{SessiBiswasBathonetal.2016, author = {Sessi, Paolo and Biswas, Rudro R. and Bathon, Thomas and Storz, Oliver and Wilfert, Stefan and Barla, Alessandro and Kokh, Konstantin A. and Tereshchenko, Oleg E. and Fauth, Kai and Bode, Matthias and Balatsky, Alexander V.}, title = {Dual nature of magnetic dopants and competing trends in topological insulators}, series = {Nature Communications}, volume = {7}, journal = {Nature Communications}, doi = {10.1038/ncomms12027}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172704}, year = {2016}, abstract = {Topological insulators interacting with magnetic impurities have been reported to host several unconventional effects. These phenomena are described within the framework of gapping Dirac quasiparticles due to broken time-reversal symmetry. However, the overwhelming majority of studies demonstrate the presence of a finite density of states near the Dirac point even once topological insulators become magnetic. Here, we map the response of topological states to magnetic impurities at the atomic scale. We demonstrate that magnetic order and gapless states can coexist. We show how this is the result of the delicate balance between two opposite trends, that is, gap opening and emergence of a Dirac node impurity band, both induced by the magnetic dopants. Our results evidence a more intricate and rich scenario with respect to the once generally assumed, showing how different electronic and magnetic states may be generated and controlled in this fascinating class of materials.}, language = {en} } @phdthesis{Herrmann2016, author = {Herrmann, Oliver}, title = {Graphene-based single-electron and hybrid devices, their lithography, and their transport properties}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-146924}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {This work explores three different aspects of graphene, a single-layer of carbon atoms arranged in a hexagonal lattice, with regards to its usage in future electronic devices; for instance in the context of quantum information processing. For a long time graphene was believed to be thermodynamically unstable. The discovery of this strictly two-dimensional material completed the family of carbon based structures, which had already been subject of intensive research with focus on zero-dimensional fullerenes and one-dimensional carbon nanotubes. Within only a few years of its discovery, the field of graphene related research has grown into one of today's most diverse and prolific areas in condensed matter physics, highlighted by the award of the 2010 Nobel Prize in Physics to A.K. Geim and K. Noveselov for "their groundbreaking experiments regarding the two-dimensional material graphene". From the point of view of an experimental physicist interested in the electronic properties of a material system, the most intriguing characteristic of graphene is found in the Dirac-like nature of its charge carriers, a peculiar fact that distinguishes graphene from all other known standard semiconductors. The dynamics of charge carriers close to zero energy are described by a linear energy dispersion relation, as opposed to a parabolic one, which can be understood as a result of the underlying lattice symmetry causing them to behave like massless relativistic particles. This fundamentally different behavior can be expected to lead to the observation of completely new phenomena or the occurrence of deviations in well-known effects. Following a brief introduction of the material system in chapter 2, we present our work studying the effect of induced superconductivity in mesoscopic graphene Josephson junctions by proximity to superconducting contacts in chapter 3. We explore the use of Nb as the superconducting material driven by the lack of high critical temperature and high critical magnetic field superconductor technology in graphene devices at that time. Characterization of sputter-deposited Nb films yield a critical transition temperature of \(T_{C}\sim 8{\rm \,mK}\). A prerequisite for successful device operation is a high interface quality between graphene and the superconductor. In this context we identify the use of an Ti as interfacial layer and incorporate its use by default in our lithography process. Overall we are able to increase the interface transparency to values as high as \(85\\%\). With the prospect of interesting effects in the ballistic regime we try to enhance the electronic quality of our Josephson junction devices by substrate engineering, yet with limited success. We achieve moderate charge carrier mobilities of up to \(7000{\rm \,cm^2/Vs}\) on a graphene/Boron-nitride heterostructure (fabrication details are covered in chapter 5) putting the junction in the diffusive regime (\(L_{device}