@phdthesis{Scheffler2023,
  author    = {Scheffler, Lukas},
  title     = {Molecular beam epitaxy of the half-Heusler antiferromagnet CuMnSb},
  doi       = {10.25972/OPUS-32283},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322839},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {This work presents a newly developed method for the epitaxial growth of the half-Heusler antiferromagnet CuMnSb. All necessary process steps, from buffer growth to the deposition of a protective layer, are presented in detail. Using structural, electrical, and magnetic characterization, the material parameters of the epitaxial CuMnSb layers are investigated. The successful growth of CuMnSb by molecular beam epitaxy is demonstrated on InAs (001), GaSb (001), and InP (001) substrates. While CuMnSb can be grown pseudomorphically on InAs and GaSb, the significant lattice mismatch for growth on InP leads to relaxation already at low film thicknesses. Due to the lower conductivity of GaSb compared to InAs, GaSb substrates are particularly suitable for the fabrication of CuMnSb layers for lateral electrical transport experiments. However, by growing a high-resistive ZnTe interlayer below the CuMnSb layer, lateral transport experiments on CuMnSb layers grown on InAs can also be realized. Protective layers of Ru and Al2O3 have proven to be suitable for protecting the CuMnSb layers from the environment. Structural characterization by high resolution X-ray diffraction (full width at half maximum of 7.7 ′′ of the rocking curve) and atomic force microscopy (root mean square surface roughness of 0.14 nm) reveals an outstanding crystal quality of the epitaxial CuMnSb layers. The half-Heusler crystal structure is confirmed by scanning transmission electron microscopy and the stoichiometric material composition by Rutherford backscattering spectrometry. In line with the high crystal quality, a new minimum value of the residual resistance of CuMnSb (𝜌0 = 35 μΩ ⋅ cm) could be measured utilizing basic electrical transport experiments. An elaborate study of epitaxial CuMnSb grown on GaSb reveals a dependence of the vertical lattice parameter on the Mn/Sb flux ratio. This characteristic enables the growth of tensile, unstrained, and compressive strained CuMnSb layers on a single substrate material. Additionally, it is shown that the N{\´e}el temperature has a maximum of 62 K at stoichiometric material composition and thus can be utilized as a selection tool for stoichiometric CuMnSb samples. Mn-related defects are believed to be the driving force for these observations. The magnetic characterization of the epitaxial CuMnSb films is performed by superconducting quantum interference device magnetometry. Magnetic behavior comparable to the bulk material is found, however, an additional complex magnetic phase appears in thin CuMnSb films and/or at low magnetic fields, which has not been previously reported for CuMnSb. This magnetic phase is believed to be localized at the CuMnSb surface and exhibits both superparamagnetic and spin-glass-like behavior. The exchange bias effect of CuMnSb is investigated in combination with different in- and out-of-plane ferromagnets. It is shown that the exchange bias effect can only be observed in combination with in-plane ferromagnets. Finally, the first attempts at the growth of fully epitaxial CuMnSb/NiMnSb heterostructures are presented. Both magnetic and structural studies by secondary-ion mass spectrometry indicate the interdiffusion of Cu and Ni atoms between the two half-Heusler layers, however, an exchange bias effect can be observed for the CuMnSb/NiMnSb heterostructures. Whether this exchange bias effect originates from exchange interaction between the CuMnSb and NiMnSb layers, or from ferromagnetic inclusions in the antiferromagnetic layer can not be conclusively identified.},
  subject      = {Molekularstrahlepitaxie},
  language  = {en}
}
@phdthesis{Fischer2023,
  author    = {Fischer, Mathias},
  title     = {Transient Phenomena and Ionic Kinetics in Hybrid Metal Halide Perovskite Solar Cells},
  doi       = {10.25972/OPUS-32220},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322204},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {The fact that photovoltaics is a key technology for climate-neutral energy production can be taken as a given. The question to what extent perovskite will be used for photovoltaic technologies has not yet been fully answered. From a photophysical point of view, however, it has the potential to make a useful contribution to the energy sector. However, it remains to be seen whether perovskite-based modules will be able to compete with established technologies in terms of durability and cost efficiency. The additional aspect of ionic migration poses an additional challenge. In the present work, primarily the interaction between ionic redistribution, capacitive properties and recombination dynamics was investigated. This was done using impedance spectroscopy, OCVD and IV characteristics as well as extensive numerical drift-diffusion simulations. The combination of experimental and numerical methods proved to be very fruitful. A suitable model for the description of solar cells with respect to mobile ions was introduced in chapter 4.4. The formal mathematical description of the model was transferred by a non-dimensionalization and suitable numerically solvable form. The implementation took place in the Julia language. By intelligent use of structural properties of the sparse systems of equations, automatic differentiation and the use of efficient integration methods, the simulation tool is not only remarkably fast in finding the solution, but also scales quasi-linearly with the grid resolution. The software package was released under an open source license. In conventional semiconductor diodes, capacitance measurements are often used to determine the space charge density. In the first experimental chapter 5, it is shown that although this is also possible for the ionic migration present in perovskites, it cannot be directly understood as doping related, since the space charge distribution strongly depends on the preconditions and can be manipulated by an externally applied voltage. The exact form of this behavior depends on the perovskite composition. This shows, among other things, that experimental results can only be interpreted within the framework of conventional semiconductors to a very limited extent. Nevertheless, the built-in 99 potential of the solar cell can be determined if the experiments are carried out properly. A statement concerning the type and charge of the mobile ions is not possible without further effort, while their number can be determined. The simulations were applied to experimental data in chapter 6. Thus, it could be shown that mobile ions make a significant contribution to the OCVD of perovskite solar cells. j-V characteristics and OCVD transients measured as a function of temperature and illumination intensities could be quantitatively modeled simultaneously using a single global set of parameters. By the simulations it was further possible to derive a simple experimental procedure to determine the concentration and the diffusivity of the mobile ions. The possibility of describing different experiments in a uniform temperaturedependent manner strongly supports the model of mobile ions in perovskites. In summary, this work has made an important contribution to the elucidation of ionic contributions to the (photo)electrical properties of perovskite solar cells. Established experimental techniques for conventional semiconductors have been reinterpreted with respect to ionic mass transport and new methods have been proposed to draw conclusions on the properties for ionic transport. As a result, the published simulation tools can be used for a number of further studies.},
  subject      = {Simulation},
  language  = {en}
}
@phdthesis{Gram2023,
  author    = {Gram, Maximilian},
  title     = {Neue Methoden der Spin-Lock-basierten Magnetresonanztomographie: Myokardiale T\(_{1ρ}\)-Quantifizierung und Detektion magnetischer Oszillationen im nT-Bereich},
  doi       = {10.25972/OPUS-32255},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322552},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {Das Ziel der vorliegenden Arbeit war die Entwicklung neuer, robuster Methoden der Spin-Lock-basierten MRT. Im Fokus stand hierbei vorerst die T1ρ-Quantifizierung des Myokards im Kleintiermodell. Neben der T1ρ-Bildgebung bietet Spin-Locking jedoch zus{\"a}tzlich die M{\"o}glichkeit der Detektion ultra-schwacher, magnetischer Feldoszillationen. Die Projekte und Ergebnisse, die im Rahmen dieses Promotionsvorhabens umgesetzt und erzielt wurden, decken daher ein breites Spektrum der Spin-lock basierten Bildgebung ab und k{\"o}nnen grob in drei Bereiche unterteilt werden. Im ersten Schritt wurde die grundlegende Pulssequenz des Spin-Lock-Experimentes durch die Einf{\"u}hrung des balancierten Spin-Locks optimiert. Der zweite Schritt war die Entwicklung einer kardialen MRT-Sequenz f{\"u}r die robuste Quantifizierung der myokardialen T1ρ-Relaxationszeit an einem pr{\"a}klinischen Hochfeld-MRT. Im letzten Schritt wurden Konzepte der robusten T1ρ-Bildgebung auf die Methodik der Felddetektion mittels Spin-Locking {\"u}bertragen. Hierbei wurden erste, erfolgreiche Messungen magnetischer Oszillationen im nT-Bereich, welche lokal im untersuchten Gewebe auftreten, an einem klinischen MRT-System im menschlichen Gehirn realisiert.},
  subject      = {Kernspintomografie},
  language  = {de}
}
@phdthesis{Genheimer2023,
  author    = {Genheimer, Ulrich},
  title     = {The Photophysics of Small Organic Molecules for Novel Light Emitting Devices},
  doi       = {10.25972/OPUS-32031},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-320313},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {This PhD thesis addresses the photophysics of selected small organic molecules with the purpose of using them for efficient and even novel light sources. In particular, the studies presented focused on revealing the underlying exciton dynamics and determining the transition rates between different molecular states. It was shown how the specific properties and mechanisms of light emission in fluorescent molecules, molecules with phosphorescence or thermally activated delayed fluorescence (TADF), biradicals, and multichromophores can be utilized to build novel light-emitting devices. The main tool employed here was the analysis of the emitters' photon statistics, i.e. the analysis of the temporal distribution of emitted photons, during electrical or optical excitation. In the introduction of this work, the working principle of an organic light-emitting diode (OLED) was introduced, while Chapter 2 provided the physical background of the relevant properties of organic molecules and their interaction with light. In particular, the occurrence of discrete energy levels in organic semiconductors and the process of spontaneous light emission were discussed. Furthermore, in this chapter a mathematical formalism was elaborated with the goal to find out what kind of information about the studied molecule can be obtained by analyzing its photon statistics. It was deduced that the intensity correlation function g (2)(t) contains information about the first two factorial moments of the photon statistics and that higher order factorial moments do not contain any additional information about the system under study if the system is always in the same state after the emission of a photon. To conclude the introductory part, Chapter 3 introduced the utilized characterization methods including confocal microscopy of single molecules, time correlated single photon counting and temperature dependent photoluminescence measurements. To provide the background necessary for an understanding of for the following result chapters, in Section 4.1 a closer look was taken at the phenomenon of blinking and photobleaching of individual molecules. For a squaraine-based fluorescent emitter rapid switching between a bright and dark state was observed during photoexcitation. Using literature transition rates between the molecular states, a consistent model was developed that is able to explain the distribution of the residence times of the molecule in the bright and dark states. In particular, an exponential and a power-law probability distribution was measured for the time the molecule resides in tis bright and dark state, respectively. This behavior as well as the change in photoluminescence intensity between the two states was conclusively explained by diffusion of residual oxygen within the sample, which had been prepared in a nitrogen-filled glovebox. For subsequent samples of this work, thin strips of atomic aluminum were deposited on the matrices to serve as oxygen getter material. This not only suppressed the efficiency of photobleaching, but also noticeably prolonged the time prior to photobleaching, which made many of the following investigations possible in the first place. For emitters used in displays, emission properties such as narrow-band luminescence and short fluorescence lifetimes are desired. These properties can be influenced not only by the emitter molecule itself, but also by the interaction with the chosen environment. Therefore, before focusing on the photophysics of individual small organic molecules, Section 4.2 highlighted the interaction of a perylene bisimide-based molecular species with its local environment in a disordered polymethyl methacrylate matrix. In a statistical approach, individual photophysical properties were measured for 32 single molecules and correlations in the variation of the properties were analyzed. This revealed how the local polarity of the molecules' environment influences their photophysics. In particular, it was shown how an increase in local polarity leads to a red-shifted emission, narrower emission lines, broader vibronic splitting between different emission lines in combination with a smaller Huang-Rhys parameter, and a longer fluorescence lifetime. In the future, these results may help to embed individual chromophores into larger macromolecules to provide the chromophore with the optimal local environment to exhibit the desired emission properties. The next two sections focused on a novel and promising class of chromophores, namely linear coordinated copper complexes, synthesized in the group of Dr. Andreas Steffen at the Institute of Inorganic Chemistry at the University of W{\"u}rzburg. In copper atoms, the d-orbitals are fully occupied, which prevents undesirable metal-centered d-d⋆ states, which tend to lie low in energy and recombine non-radiatively. Simultaneously, the copper atom provides a flexible coordination geometry, while complexes in their linear form are expected to exhibit the least amount of excited state distortions. Depending on the chosen ligands, these copper complexes can exhibit phosphorescence as well as temperature activated delayed fluorescence. In Section 4.3, a phosphorescent copper complex with a chlorine atom and a 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethyl-2-pyrrolidine-ylidene- ligand was tested for its suitability as an optically active material in an OLED. For this purpose, an OLED with a polyspirobifluorene-based copolymer matrix and the dopant at a concentration of 20 wt\% was electrically excited. Deconvolution of the emission spectrum in contributions from the matrix and the dopant revealed that 60 \% of the OLEDs emission was due to the copper complex. It was also shown that the shape of the emission spectrum of the copper complex remains unchanged upon incorporation into the OLED, but is red-shifted by about 233 meV. In Section 4.4, a second copper complex exhibiting thermally activated delayed fluorescence was analyzed. This complex comprised a carbazolate as well as a 2-(2,6- diisopropyl)-phenyl-1,1-diphenyl-isoindol-2-ium-3-ide ligand and was examined in the solid state and at the single-molecule level, where single photon emission was recorded up to an intensity of 78'000 counts per second. The evaluation of the second-order autocorrelation function of the emitted light proved an efficient transition between singlet and triplet excited states on the picosecond time scale. In the solid state, the temperature- dependent fluorescence decay of the complex was analyzed after pulsed photoexcitation in the temperature range between 300 K and 5 K. From these measurements, a small singlet-triplet energy gap of only 65 meV and a triplet sublevel splitting of 3.0 meV were derived. The transition rates between molecular states could also be determined. Here, the fast singlet decay time of τS1 = 9.8ns proved the efficient thermally activated delayed fluorescence process, which was demonstrated for the first time for this new class of copper(I) complexes thus. While the use of thermally activated delayed fluorescence is a potential way to harness otherwise long-living dark triplet states, radicals completely avoid dark triplet states. However, this usually comes with the huge drawback of the molecules being chemically unstable. Therefore, two chemically stable biradical species were synthesized in the framework of the DFG research training school GRK 2112 on Molecular biradicals: structure, properties and reactivity, by Yohei Hattori in the group of Prof. Dr. Christoph Lambert and Rodger Rausch in the group of Prof. Dr. Frank W{\"u}rthner at the Institute of Organic Chemistry at the University of W{\"u}rzburg, respectively. In Section 4.5, it was investigated how these molecules can be used in OLEDs. In the first isoindigo based biradical (6,6'-bis(3,5-di-tert-butyl-4-phenoxyl)-1,1'-bis(2- ethylhexyl)-[3,3'-biindolinyl-idene]-2,2'-dione) two tert-butyl moieties kinetically block chemical reactions at the place of the lone electrons and an electron-withdrawing core shifts the electron density into the center of the chromophore. With these properties, it was possible to realize a poly(p-phenylene vinylene) copolymer based OLED doped with the biradical and to observe luminescence during optical as well as electrical excitation. Analyzing shapes of the photo- and electroluminescence spectra at different doping concentrations, F{\"o}rster resonance energy transfer was determined to be the dominant transition mechanism for excitons from the matrix to the biradical dopants. Likewise, OLEDs could be realized with the second diphenylmethylpyridine based birad- ical (4-(5-(bis(2,4,6-trichlorophenyl)methyl)-4,6-dichloropyridin-2-yl)-N-(4-(5-(bis(2,4,6- -trichlorophenyl)methyl)-4,6-dichloropyridin-2-yl)phenyl)-N-(4-methoxyphenyl)aniline) as dopant. In this biradical, chlorinated diphenylmethyl groups protect the two unpaired electrons. Photo- and electroluminescence spectra showed an emission in the near in- frared spectral range between 750 nm and 1000 nm. Also, F{\"o}rster resonance energy trans- fer was the dominant energy transfer mechanism with an transfer efficiency close to 100 \% even at doping concentrations of only 5 wt\%. In addition to demonstrating the working OLEDs based in biradicals, the detection of luminescence of the two biradical species in devices also constitutes an important step toward making use of experimental techniques such as optically detected electron spin resonance, which could provide information about the electronic states of the emitter and their spin manifold during OLED operation. Another class of emitters studied are molecules in which several chromophores are co- valently linked to form a macrocyclic system. The properties of these multichromophores were highlighted in Section 4.6. Here, it was analyzed how the photophysical behavior of the molecules is affected by the covalent linking, which determines the interaction be- tween the chromophores. The first multichromophore, 2,2'-ditetracene, was synthesized by Lena Ross in the group of Prof. Dr. Anke Kr{\"u}ger at the Institute of Organic Chemistry at the University of W{\"u}rzburg and was analyzed in this work both at the single-molecule level and in its aggregated crystalline form. While the single crystals were purified and grown in a vertical sublimation oven, the samples for the single molecule studies were prepared in matrices of amorphous polymethyl methacrylate and crystalline anthracene. Tetracene was analyzed concurrently to evaluate the effects of covalent linking. In samples where the distance between two molecules is sufficiently large, tetracene and 2,2'-ditracene show matching emission profiles with the only difference in the Franck-Condon factors and a de- creased photoluminescence decay time constant from 14 ns for tetracene to 5 ns for 2,2'- ditracene, which can be attributed to the increased density of the vibrational modes in 2,2'-ditracene. Evaluation of the photon statistics of individual 2,2'-ditracene molecules however showed that the system does not behave as two individual chromophores but as a collective state, preserving the spectral properties of the two tetracene chromophores. Complementary calculations performed by Marian Deutsch in the group of Prof. Dr. Bernd Engels at the Institute of Physical and Theoretical Chemistry at the University of W{\"u}rzburg helped to understand the processes in the materials and could show that the electronic and vibronic modes of 2,2'-ditracene are superpositions of the modes occurring in tetracene. In contrast, single-crystalline 2,2'-ditetracene behaves significantly different than tetracene, namely exhibiting a red shift in photoluminescence of 150 meV, caused by an altered crys- talline packing that lowers the S1-state energy level. Temperature-dependent photolu- minescence measurements revealed a rich emission pattern from 2,2'-ditetracene single crystals. The mechanisms behind this were unraveled using photoluminescence lifetime density analysis in different spectral regions of the emission spectrum and at different tem- peratures. An excimer state was identified that is located about 5 meV below the S1-state, separated by a 1 meV barrier, and which can decay to the ground state with a time constant of 9 ns. Also, as the S1-state energy level is lowered below the E(S1) ≥ 2 ×E(T1) threshold, singlet fission is suppressed in 2,2'-ditetracene in contrast to tetracene. Therefore, at low temperatures, photoluminescence is enhanced by a factor of 46, which could make 2,2'- ditetracene a useful material for future applications in devices such as OLEDs or lasers. The second multichromophore species, para-xylylene bridged perylene bisimide macrocycles, were synthesized by Peter Spenst in the group of Prof. Dr. Frank W{\"u}rthner at the Institute of Organic Chemistry at the University of W{\"u}rzburg, by linking three and four perylene bisimides, respectively. To reveal the exciton dynamics in these macrocycles, highly diluted monomers as well as trimers and tetramers were doped into matrices of polymethyl methacrylate to create thin films in which individual macrocycles could be analyzed. The emission spectra of the macrocycles remained identical to those of the monomers, indicating weak coupling between the chromophores. Single photon emission could be verified for monomers as well as macrocycles, as exciton-exciton annihilation processes suppress the simultaneous emission of two photons from one macrocycle. Nevertheless, the proof of the occurrence of a doubly excited state was obtained by excitation power dependent photon statistics measurements. The formalism developed in the theory part of this thesis for calculating the photon statistics of multichromophore systems was used here to find a theoretical model that matches the experimental results. The main features of this model are a doubly excited state, fast singlet-singlet annihilation, and an efficient transition from the doubly excited state to a dark triplet state. The occurrence of triplet-triplet annihilation was demonstrated in a subsequent experiment in which the macrocycles were excited at a laser intensity well above the saturation intensity of the monomer species. In contrast to the monomers, the trimers and tetramers exhibited neither a complete dark state nor saturation of photoluminescence. Both processes, efficient singlet-singlet and triplet-triplet annihilation make perylene bisimide macrocycles exceptionally bright single photon emitters. These advantages were utilized to realize a room temperature electrically driven fluorescent single photon source. For this purpose, OLEDs were fabricated using polyvinylcarbazole and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol blends as a host material for perylene bisimide trimers. Photon antibunching could be observed in both optically and electrically driven devices, representing the first demonstration of electrically driven single photon sources using fluorescent emitters at room temperature. As expected from the previous optical experiments, the electroluminescence of the molecules was exceptionally bright, emitting about 105 photons per second, which could be seen even by eye under the microscope. Finally, in the last section 4.7 of this thesis, two additional measurement schemes were proposed as an alternative to the measurement of the second-order correlation function g (2)(t) of single molecules, which only provides information about the first two factorial moments of the molecules' photon statistics. In the first scheme, the g (3)(t) function was measured with three photodiodes, which is a consequential extension of the Hanbury Brown and Twiss measurement with two photodiodes. It was demonstrated how measuring the g (3)(t) function is able to identify interfering emitters with non-Poisson statistics in the experiment. The second setup was designed with an electro-optic modulator that repeatedly gen- erates photoexcitation in the form of a step function. The recording of luminescence transients for different excitation intensities yields the same results as the correspond- ing g (2)-functions measured on single emitters, both in their shape and in their depen- dence on excitation power. To demonstrate this concept, the TADF emitter TXO-TPA (2- [4-(diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one) was doped at a concen- tration of 10-4 wt\% in a mCP (1,3-Bis(N-carbazolyl)benzene) matrix. This concentration was low enough that TXO-TPA molecules did not interact with each other, but an ensem- ble of molecules was still present in the detection volume. The intramolecular transition rates between singlet and triplet states of TXO-TPA could be derived with an error of at most 5 \%. Other experimental techniques designed to obtain this information require ei- ther lengthy measurements on single molecules, where sample preparation is also often a challenge, or temperature-dependent fluorescence lifetime measurements, which require a cryostat, which in turn places constraints on the sample design used. In future, this ap- proach could establish a powerful method to study external factors influencing molecular transition rates. Overall, this thesis has introduced new molecular materials, revealed their photophys- ical properties, and demonstrated how they can be used to fabricate efficient and even novel light sources.},
  subject      = {Fotophysik},
  language  = {en}
}
@phdthesis{Friedrich2023,
  author    = {Friedrich, Felix},
  title     = {Magnetic Excitations in Single and Coupled Atoms on Surfaces: From the Kondo Effect to Yu-Shiba-Rusinov States},
  doi       = {10.25972/OPUS-32069},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-320699},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {Magnetic systems underlie the physics of quantum mechanics when reaching the limit of few or even single atoms. This behavior limits the minimum size of magnetic bits in data storage devices as spontaneous switching of the magnetization leads to the loss of information. On the other hand, exactly these quantum mechanic properties allow to use such systems in quantum computers. Proposals to realize qubits involve the spin states of single atoms as well as topologically protected Majorana zero modes, that emerge in coupled systems of magnetic atoms in proximity to a superconductor. In order to implement and control the proposed applications, a detailed understanding of atomic spins and their interaction with the environment is required. In this thesis, two different systems of magnetic adatoms coupled to metallic and superconducting surfaces are studied by means of scanning tunneling microscopy (STM) and spectroscopy: Co atoms on the clean Cu(111) were among the first systems exhibiting signatures of the Kondo effect in an individual atom. Yet, a recent theoretical work proposed an alternative interpretation of these early experimental results, involving a newly described many-body state. Spin-averaged and -polarized experiments in high magnetic fields presented in this thesis confirm effects beyond the Kondo effect that determine the physics in these Co atoms and suggest a potentially even richer phenomenology than proposed by theory. The second studied system are single and coupled Fe atoms on the superconducting Nb(110) surface. Magnetic impurities on superconducting surfaces locally induce Yu-Shiba-Rusinov (YSR) states inside the superconducting gap due to their pair breaking potential. Coupled systems of such impurities exhibit YSR bands and, if the bands cross the Fermi level such that the band structure is inverted, host Majorana zero modes. Using the example of Fe atoms on Nb(110), the YSR states' dependence on the adatom-substrate interaction as well as the interatomic YSR state coupling is investigated. In the presence of oxygen on the Nb surface, the adatom-substrate interaction is shown to be heavily modified and the YSR states are found to undergo a quantum phase transition, which can be directly linked to a modified Kondo screening. STM tips functionalized with CO molecules allow to resolve self-assembled one-dimensional chains of Fe atoms on the clean Nb(110) surface to study the YSR states' coupling. Mapping out the states' wave functions reveals their symmetry, which is shown to alter as a function of the states' energy and number of atoms in the chain. These experimental results are reproduced in a simple tight-binding model, demonstrating a straightforward possibility to describe also more complex YSR systems toward engineered, potentially topologically non-trivial states.},
  subject      = {Rastertunnelmikroskopie},
  language  = {en}
}
@phdthesis{Lutter2023,
  author    = {Lutter, Fabian},
  title     = {Elementsensitive Bildgebung - Einsatz chromatischer Pixelarrays in R{\"o}ntgen nano-CT},
  doi       = {10.25972/OPUS-31995},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-319955},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {Diese Arbeit befasst sich mit der Weiterentwicklung und Charakterisierung des XRM-II nanoCT Systems, sowie dessen M{\"o}glichkeiten zur Materialtrennung und Elementbestimmung in der nano-Computertomographie. Beim XRM-II nanoCT System handelt es sich um ein R{\"o}ntgenmikroskop, welches in ein Rasterelektronenmikroskop integriert ist, und auf dem Prinzip der geometrischen Vergr{\"o}ßerung basiert. Neben zweidimensionalen Durchstrahlungsbildern ist dieses Mikroskop auch zur dreidimensionalen Bildgebung mittels Computertomographie f{\"a}hig. Der Ausgangspunkt f{\"u}r die Weiterentwicklung ist das XRM-II, mit welchem bereits Computertomographien im Nanometerbereich m{\"o}glich waren. Deren Aufnahmedauer liegt zwischen 14 und 21 Tagen, was das System trotz seiner hohen Aufl{\"o}sung wenig praktikabel macht. Durch eine Anpassung der Blendeneinstellungen am Rasterelektronenmikroskop konnte der Strahlstrom um den Faktor 40 erh{\"o}ht und damit die Aufnahmedauer auf 24 Stunden reduziert werden, wobei weiterhin eine zweidimensionale Aufl{\"o}sung von \(167 \pm 9\) nm erreicht wird. Durch die Trennung von Objekt- und Targetmanipulator lassen sich beide unabh{\"a}ngig und genauer bewegen, wodurch es m{\"o}glich ist selbst 50 nm große Strukturen abzubilden. Die Charakterisierung erfolgt sowohl f{\"u}r das komplette System als auch getrennt in die entscheidenden Komponenten wie Target und Detektor. F{\"u}r das R{\"o}ntgentarget werden Monte-Carlo Simulationen zur Brennfleckgr{\"o}ße, welche entscheidend f{\"u}r die erreichbare Aufl{\"o}sung ist, durchgef{\"u}hrt und mit Aufl{\"o}sungstests verglichen. Der R{\"o}ntgendetektor wird hinsichtlich seiner spektralen Aufl{\"o}sung {\"u}berpr{\"u}ft, welche haupts{\"a}chlich vom Charge Sharing Effekt beeinflusst wird. Die Charakterisierung des Gesamtsystems erfolgt durch den Vergleich mit einer h{\"o}her aufl{\"o}senden Bildgebungsmethode, der FIB Tomographie. Hierbei wird die gleiche Probe, ein Bruchst{\"u}ck einer CPU, mit beiden Methoden unter der Voraussetzung einer {\"a}hnlichen Aufnahmezeit (24 h) untersucht. In der nano-CT kann ein 12 mal gr{\"o}ßeres Volumen analysiert werden, was jedoch eine geringere r{\"a}umliche Aufl{\"o}sung als die FIB Tomographie mit sich bringt. Da die spektrale Aufl{\"o}sung des Detektors aufgrund des Charge Sharing begrenzt ist, lassen sich nur Materialien mit einem großen Unterschied in der Ordnungszahl mittels der Energieschwellen des Detektors trennen. Jedoch kann in Verbindung mit der geeigneten Wahl des Targetmaterials der Absorptionskontrast f{\"u}r leichte Materialien, wie beispielsweise \(SiO_2\) verbessert werden. Dar{\"u}ber hinaus ist es am XRM-II nanoCT m{\"o}glich, durch das integrierte EDX-System, Elemente in der Computertomographie zu identifizieren. Dies wird anhand eines Drei-Wegekatalysators und eines NCA-Partikel gezeigt.},
  subject      = {Computertomographie},
  language  = {de}
}
@phdthesis{Stuehler2023,
  author    = {St{\"u}hler, Rudolf Raul Albert},
  title     = {Growth and Spectroscopy of the Two-dimensional Topological Insulator Bismuthene on SiC(0001)},
  doi       = {10.25972/OPUS-32008},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-320084},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {A plethora of novel material concepts are currently being investigated in the condensed matter research community. Some of them hold promise to shape our everyday world in a way that silicon-based semiconductor materials and the related development of semiconductor devices have done in the past. In this regard, the last decades have witnessed an explosion of studies concerned with so called ''quantum materials'' with emerging novel functionalities. These could eventually lead to new generations of electronic and/or spintronic devices. One particular material class, the so called topological materials, play a central role. As far as their technological applicability is concerned, however, they are still facing outstanding challenges to date. Predicted for the first time in 2005 and experimentally verified in 2007, two-dimensional topological insulators (2D TIs) (a.k.a. quantum spin Hall insulators) exhibit the outstanding property of hosting spin-polarized metallic states along the boundaries of the insulating 2D bulk material, which are protected from elastic single-particle backscattering and give rise to the quantum spin Hall effect (QSHE). Owing to these peculiar properties the QSHE holds promise for dissipationless charge and/or spin transport. However, also in today's best 2D TIs the observation of the QSHE is still limited to cryogenic temperatures of maximum 100 K. Here, the discovery of bismuthene on SiC(0001) has marked a milestone towards a possible realization of the QSHE at or beyond room-temperature owing to the massively increased electronic bulk energy gap on the order of 1 eV. This thesis is devoted to and motivated by the goal of advancing its synthesis and to build a deeper understanding of its one-particle and two-particle electronic properties that goes beyond prior work. Regarding the aspect of material synthesis, an improved growth procedure for bismuthene is elaborated that increases the domain size of the material considerably (by a factor of ≈ 3.2 - 6.5 compared to prior work). The improved film quality is an important step towards any future device application of bismuthene, but also facilitates all further basic studies of this material. Moreover, the deposition of magnetic transition metals (Mn and Co) on bismuthene is investigated. Thereby, the formation of ordered magnetic Bi-Mn/Co alloys is realized, their structure is resolved with scanning tunneling microscopy (STM), and their pristine electronic properties are resolved with scanning tunneling spectroscopy (STS) and photoemission spectroscopy (PES). It is proposed that these ordered magnetic Bi-Mn/Co-alloys offer the potential to study the interplay between magnetism and topology in bismuthene in the future. In this thesis, a wide variety of spectroscopic techniques are employed that aim to build an understanding of the single-particle, as well as two-particle level of description of bismuthene's electronic structure. The techniques involve STS and angle-resolved PES (ARPES) on the one hand, but also optical spectroscopy and time-resolved ARPES (trARPES), on the other hand. Moreover, these experiments are accompanied by advanced numerical modelling in form of GW and Bethe-Salpeter equation calculations provided by our theoretical colleagues. Notably, by merging many experimental and theoretical techniques, this work sets a benchmark for electronic structure investigations of 2D materials in general. Based on the STS studies, electronic quasi-particle interferences in quasi-1D line defects in bismuthene that are reminiscent of Fabry-P{\´e}rot states are discovered. It is shown that they point to a hybridization of two pairs of helical boundary modes across the line defect, which is accompanied by a (partial) lifting of their topological protection against elastic single-particle backscattering. Optical spectroscopy is used to reveal bismuthene's two-particle elecronic structure. Despite its monolayer thickness, a strong optical (two-particle) response due to enhanced electron-hole Coulomb interactions is observed. The presented combined experimental and theoretical approach (including GW and Bethe-Salpeter equation calculations) allows to conclude that two prominent optical transitions can be associated with excitonic transitions derived from the Rashba-split valence bands of bismuthene. On a broader scope this discovery might promote further experiments to elucidate links of excitonic and topological physics. Finally, the excited conduction band states of bismuthene are mapped in energy and momentum space employing trARPES on bismuthene for the first time. The direct and indirect band gaps are succesfully extracted and the effect of excited charge carrier induced gap-renormalization is observed. In addition, an exceptionally fast excited charge carrier relaxation is identified which is explained by the presence of a quasi-metallic density of states from coupled topological boundary states of domain boundaries.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@phdthesis{CrespoVidal2023,
  author    = {Crespo Vidal, Can Raphael},
  title     = {Spectroscopic investigation of the three-dimensional topological insulators (MnBi\(_2\)Te\(_4\))(Bi\(_2\)Te\(_3\))\(_n\) and HgTe: band structure, orbital symmetries, and influence of the cation \(d\)-states},
  doi       = {10.25972/OPUS-31293},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-312931},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {This thesis examines the electronic properties of two materials that promise the realization and observation of novel exotic quantum phenomena. For this purpose, angle-resolved photoemission forms the experimental basis for the investigation of the electronic properties. Furthermore, the magnetic order is investigated utilizing X-ray dichroism measurements. First, the bulk and surface electronic structure of epitaxially grown HgTe in its three-dimensional topological insulator phase is investigated. In this study, synchrotron radiation is used to address the three-dimensional band structure and orbital composition of the bulk states by employing photon-energy-dependent and polarization-dependent measurements, respectively. In addition, the topological surface state is examined on in situ grown samples using a laboratory photon source. The resulting data provide a means to experimentally localize the bulk band inversion in momentum space and to evidence the momentum-dependent change in the orbital character of the inverted bulk states. Furthermore, a rather new series of van der Waals compounds, (MnBi\(_2\)Te\(_4\))(Bi\(_2\)Te\(_3\))\(_n\), is investigated. First, the magnetic properties of the first two members of the series, MnBi\(_2\)Te\(_4\) and MnBi\(_4\)Te\(_7\), are studied via X-ray absorption-based techniques. The topological surface state on the two terminations of MnBi\(_4\)Te\(_7\) is analyzed using circular dichroic, photon-energy-dependent, and spin-resolved photoemission. The topological state on the (MnBi\(_2\)Te\(_4\))-layer termination shows a free-standing Dirac cone with its Dirac point located in the bulk band gap. In contrast, on the (Bi\(_2\)Te\(_3\))-layer termination the surface state hybridizes with the bulk valences states, forming a spectral weight gap, and exhibits a Dirac point that is buried within the bulk continuum. Lastly, the lack of unambiguous evidence in the literature showing a temperature-dependent mass gap opening in these magnetic topological insulators is discussed through MnBi\(_2\)Te\(_4\).},
  subject      = {ARPES},
  language  = {en}
}
@phdthesis{Bauernfeind2023,
  author    = {Bauernfeind, Maximilian Josef Xaver},
  title     = {Epitaxy and Spectroscopy of Two-Dimensional Adatom Systems: the Elemental Topological Insulator Indenene on SiC},
  doi       = {10.25972/OPUS-31166},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-311662},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {Two-dimensional (2D) topological insulators are a new class of materials with properties that are promising for potential future applications in quantum computers. For example, stanene represents a possible candidate for a topological insulator made of Sn atoms arranged in a hexagonal lattice. However, it has a relatively fragile low-energy spectrum and sensitive topology. Therefore, to experimentally realize stanene in the topologically non-trivial phase, a suitable substrate that accommodates stanene without compromising these topological properties must be found. A heterostructure consisting of a SiC substrate with a buffer layer of adsorbed group-III elements constitutes a possible solution for this problem. In this work, 2D adatom systems of Al and In were grown epitaxially on SiC(0001) and then investigated structurally and spectroscopically by scanning tunneling microscopy (STM) and photoelectron spectroscopy. Al films in the high coverage regime \( (\Theta_{ML}\approx2\) ML\( ) \) exhibit unusually large, triangular- and rectangular-shaped surface unit cells. Here, the low-energy electron diffraction (LEED) pattern is brought into accordance with the surface topography derived from STM. Another Al reconstruction, the quasi-one-dimensional (1D) Al phase, exhibits a striped surface corrugation, which could be the result of the strain imprinted by the overlayer-substrate lattice mismatch. It is suggested that Al atoms in different surface areas can occupy hexagonal close-packed and face-centered cubic lattice sites, respectively, which in turn lead to close-packed transition regions forming the stripe-like corrugations. On the basis of the well-known herringbone reconstruction from Au(111), a first structural model is proposed, which fits well to the structural data from STM. Ultimately, however, thermal treatments of the sample could not generate lower coverage phases, i.e. in particular, a buffer layer structure. Strong metallic signatures are found for In high coverage films \( (\Theta_{ML}\approx3\) to \(2\) ML\() \) by scanning tunneling spectroscopy (STS) and angle-resolved photoelectron spectroscopy (ARPES), which form a \( (7\times7) \), \( (6\times4\sqrt{3}) \), and \( (4\sqrt{3}\times4\sqrt{3}) \) surface reconstruction. In all these In phases electrons follow the nearly-free electron model. Similar to the Al films, thermal treatments could not obtain the buffer layer system. Surprisingly, in the course of this investigation a triangular In lattice featuring a \( (1\times1) \) periodicity is observed to host massive Dirac-like bands at \( K/K^{\prime} \) in ARPES. Based on this strong electronic similarity with graphene at the Brillouin zone boundary, this new structure is referred to as \textit{indenene}. An extensive theoretical analysis uncovers the emergence of an electronic honeycomb network based on triangularly arranged In \textit{p} orbitals. Due to strong atomic spin-orbit coupling and a comparably small substrate-induced in-plane inversion symmetry breaking this material system is rendered topologically non-trivial. In indenene, the topology is intimately linked to a bulk observable, i.e., the energy-dependent charge accumulation sequence within the surface unit cell, which is experimentally exploited in STS to confirm the non-trivial topological character. The band gap at \( K/K^{\prime} \), a signature of massive Dirac fermions, is estimated by ARPES to approximately 125 meV. Further investigations by X-ray standing wave, STM, and LEED confirm the structural properties of indenene. Thus, this thesis presents the growth and characterization of the novel quantum spin Hall insulator material indenene.},
  subject      = {Dreiecksgitter},
  language  = {en}
}
@phdthesis{Tcakaev2023,
  author    = {Tcakaev, Abdul-Vakhab},
  title     = {Soft X-ray Spectroscopic Study of Electronic and Magnetic Properties of Magnetic Topological Insulators},
  doi       = {10.25972/OPUS-30378},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-303786},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {After the discovery of three-dimensional topological insulators (TIs), such as tetradymite chalcogenides Bi\$_2\$Se\$_3\$, Bi\$_2\$Te\$_3\$ and Sb\$_2\$Te\$_3\$ - a new class of quantum materials characterized by their unique surface electronic properties - the solid state community got focused on topological states that are driven by strong electronic correlations and magnetism. An important material class is the magnetic TI (MTI) exhibiting the quantum anomalous Hall (QAH) effect, i.e. a dissipationless quantized edge-state transport in the absence of external magnetic field, originating from the interplay between ferromagnetism and a topologically non-trivial band structure. The unprecedented opportunities offered by these new exotic materials open a new avenue for the development of low-dissipation electronics, spintronics, and quantum computation. However, the major concern with QAH effect is its extremely low onset temperature, limiting its practical application. To resolve this problem, a comprehensive understanding of the microscopic origin of the underlying ferromagnetism is necessary. V- and Cr-doped (Bi,Sb)\$_2\$Te\$_3\$ are the two prototypical systems that have been widely studied as realizations of the QAH state. Finding microscopic differences between the strongly correlated V and Cr impurities would help finding a relevant model of ferromagnetic coupling and eventually provide better control of the QAH effect in these systems. Therefore, this thesis first focuses on the V- and Cr-doped (Bi,Sb)\$_2\$Te\$_3\$ systems, to better understand these differences. Exploiting the unique capabilities of x-ray absorption spectroscopy and magnetic circular dichroism (XAS/XMCD), combined with advanced modeling based on multiplet ligand-field theory (MLFT), we provide a detailed microscopic insight into the local electronic and magnetic properties of these systems and determine microscopic parameters crucial for the comparison with theoretical models, which include the \$d\$-shell filling, spin and orbital magnetic moments. We find a strongly covalent ground state, dominated by the superposition of one and two Te-ligand-hole configurations, with a negligible contribution from a purely ionic 3+ configuration. Our findings indicate the importance of the Te \$5p\$ states for the ferromagnetism in (Bi, Sb)\$_2\$Te\$_3\$ and favor magnetic coupling mechanisms involving \$pd\$-exchange. Using state-of-the-art density functional theory (DFT) calculations in combination with XMCD and resonant photoelectron spectroscopy (resPES), we reveal the important role of the \$3d\$ impurity states in mediating magnetic exchange coupling. Our calculations illustrate that the kind and strength of the exchange coupling varies with the impurity \$3d\$-shell occupation. We find a weakening of ferromagnetic properties upon the increase of doping concentration, as well as with the substitution of Bi at the Sb site. Finally, we qualitatively describe the origin of the induced magnetic moments at the Te and Sb sites in the host lattice and discuss their role in mediating a robust ferromagnetism based on a \$pd\$-exchange interaction scenario. Our findings reveal important clues to designing higher \$T_{\text{C}}\$ MTIs. Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of TIs, which is favorable for the realization of the high-temperature QAH effect. Therefore, we have further focused on Eu-doped Bi\$_2\$Te\$_3\$ and scrutinized whether the conditions for formation of a substantial gap in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the \$M_{4,5}\$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu\$^{2+}\$ valence, unlike most other rare earth elements, and confirms a large magnetic moment. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic ordering. This is in good agreement with DFT results, which predict AFM interactions between the Eu impurities due to the direct overlap of the impurity wave functions. Our results support the notion of antiferromagnetism coexisting with topological surface states in rare-earth doped Bi\$_2\$Te\$_3\$ and corroborate the potential of such doping to result in an antiferromagnetic TI with exotic quantum properties. The doping with impurities introduces disorder detrimental for the QAH effect, which may be avoided in stoichiometric, well-ordered magnetic compounds. In the last part of the thesis we have investigated the recently discovered intrinsic magnetic TI (IMTI) MnBi\$_6\$Te\$_{10}\$, where we have uncovered robust ferromagnetism with \$T_{\text{C}} \approx 12\$ K and connected its origin to the Mn/Bi intermixing. Our measurements reveal a magnetically intact surface with a large moment, and with FM properties similar to the bulk, which makes MnBi\$_6\$Te\$_{10}\$ a promising candidate for the QAH effect at elevated temperatures. Moreover, using an advanced ab initio MLFT approach we have determined the ground-state properties of Mn and revealed a predominant contribution of the \$d^5\$ configuration to the ground state, resulting in a \$d\$-shell electron occupation \$n_d = 5.31\$ and a large magnetic moment, in excellent agreement with our DFT calculations and the bulk magnetometry data. Our results together with first principle calculations based on the DFT-GGA\$+U\$, performed by our collaborators, suggest that carefully engineered intermixing plays a crucial role in achieving a robust long-range FM order and therefore could be the key for achieving enhanced QAH effect properties. We expect our findings to aid better understanding of MTIs, which is essential to help increasing the temperature of the QAH effect, thus facilitating the realization of low-power electronics in the future.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@phdthesis{Rueckert2023,
  author    = {R{\"u}ckert, Martin Andreas},
  title     = {Rotationsdriftspektroskopie},
  doi       = {10.25972/OPUS-26863},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-268631},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {Die wachsende Verf{\"u}gbarkeit von magnetischen Nanopartikeln (MNPs) mit funktionalisierten Partikeloberfl{\"a}chen er{\"o}ffnet weitreichende M{\"o}glichkeiten f{\"u}r chemische, biologische und klinische Analysemethoden. Durch Funktionalisierung kann eine gezielte Interaktion mit Molek{\"u}len bewirkt werden, die im Allgemeinen auch die Beweglichkeit der MNPs ver{\"a}ndern. Methoden zur Charakterisierung von MNPs wie bspw. AC-Suszeptometrie, Magnetorelaxometrie (MRX) oder Magnetic Particle Spectroscopy (MPS) k{\"o}nnen diese {\"A}nderung der Beweglichkeit bei MNPs messen, wenn es sich um MNPs handelt, deren magnetisches Moment im Partikel fixiert ist. Damit ist mit funktionalisierten MNPs indirekt auch die spezifische Messung von Molek{\"u}lkonzentrationen m{\"o}glich. MNPs k{\"o}nnen zudem in biokompatibler Form hergestellt werden und sind dadurch auch als in-vivo Marker einsetzbar. Das 2005 das erste Mal ver{\"o}ffentlichte Magnetic Particle Imaging (MPI) kann als ein mittels Gradientenfeldern um die r{\"a}umliche Kodierung erweitertes MPS betrachtet werden. Dank biokompatibler MNPs handelt es sich dabei um eine in-vivo-taugliche, nicht-invasive Bildgebungsmethode. Mit funktionalisierten MNPs als Marker ist damit im Prinzip auch molekulare Bildgebung m{\"o}glich, die durch Detektion der beteiligten Molek{\"u}le (Biomarker) Stoffwechselprozesse r{\"a}umlich abbilden kann. Im Vergleich zur Bildgebung von Gewebe- und Knochenstrukturen lassen sich die diagnostischen M{\"o}glichkeiten durch molekulare Bildgebung erheblich erweitern. Rotationsdriftspektroskopie (Rotational Drift Spectroscopy, RDS) ist eine in dieser Arbeit entwickelte Methode f{\"u}r die induktive Messung der Beweglichkeit von MNPs in fl{\"u}ssiger Suspension. Es verwendet die Rotationsdrift von MNPs in rotierenden magnetischen Feldern als Grundlage und bietet das Potential die {\"A}nderungen der Beweglichkeit von MNPs mit einer Empfindlichkeit messen zu k{\"o}nnen, welche potentiell um mehrere Gr{\"o}ßenordnungen h{\"o}her sein kann als mit den oben erw{\"a}hnten Verfahren. Die vorliegende Arbeit konzentriert sich auf die Verwendbarkeit dieses Effekts als Spektroskopiemethode. Die Eigenschaften des RDS-Signals sind jedoch auch als Grundlage f{\"u}r r{\"a}umliche Kodierung vielversprechend. In weiterf{\"u}hrenden Projekten soll daher auch die Entwicklung von Rotationsdriftbildgebung (Rotating Drift Imaging, RDI) als ein nicht-invasives Verfahren f{\"u}r molekulare Bildgebung angestrebt werden. Der Grundgedanke von RDS entlehnt sich aus einem in 2006 ver{\"o}ffentlichten Sensordesign basierend auf magnetische Mikropartikel in einem schwachen rotierenden Magnetfeld. Das rotierende Magnetfeld ist dabei so schwach gew{\"a}hlt, dass sich das Partikel aufgrund der viskosen Reibung nicht mehr synchron mit dem externen Feld drehen kann. Die Frequenz der resultierenden asynchronen Rotationsdrift liegt unterhalb der Frequenz des externen Rotationsfelds und ist Abh{\"a}ngig von der viskosen Reibung. Aufgrund dieser Abh{\"a}ngigkeit k{\"o}nnen {\"A}nderungen im Reibungskoeffizienten des Partikels {\"u}ber {\"A}nderungen in der Rotationsdriftfrequenz gemessen werden. RDS zielt darauf ab, diese Rotationsdrift bei suspendierten MNPs {\"u}ber deren makroskopische Magnetisierung messen zu k{\"o}nnen. Damit wird u.a. auch die nicht-invasive Messung von MNPs innerhalb opaker biologischer Proben m{\"o}glich. MNP-Suspensionen sind großzahlige Nanopartikel-ensembles und k{\"o}nnen nicht wie ein einzelnes Mikropartikel gemessen werden. F{\"u}r die induktive Messung ist vor dem Start eine Ausrichtung aller magnetischen Momente n{\"o}tig, da sich deren makroskopische Magnetisierung andernfalls zu Null addiert. Aufgrund von Rotationsdiffusion bleibt diese Ausrichtung nur eine begrenzte Zeit bestehen, so dass auch die eigentliche Messung des RDS-Signals nur eine begrenzte Zeit m{\"o}glich ist. Diese Ausrichtung wurde in den ersten Experimenten durch einen kurzen Magnetfeldpuls erzeugt. In der Empfangsspule ist die Induktion durch das Rotationsfeld typischer Weise um mehrere Gr{\"o}ßenordnungen h{\"o}her als das zu erwartende Signal und muss durch einen Tiefpass unterdr{\"u}ckt werden. In diesem Tiefpassfilter ruft jedoch die Einkopplung des Anfangspulses eine Pulsantwort hervor, die ebenso mehrere Gr{\"o}ßenordnungen des zu erwartenden Signals betragen kann und {\"a}hnlich langsam wie typische Signale abklingt. Die Unterdr{\"u}ckung dieser Pulsantwort stellte in den ersten Experimenten die gr{\"o}ßte H{\"u}rde da. Der erste Aufbau hatte eine Relaisschaltung zur Pulsunterdr{\"u}ckung und resultierte in einer Totzeit von 3 ms zwischen Anfangspuls und Start der Messung. Aufgrund dieser Totzeit waren die ersten Messungen auf gr{\"o}ßere Agglomerate und Sedimente von MNPs beschr{\"a}nkt, da nur in diesem Fall eine hinreichend lange Zerfallsdauer der Probenmagnetisierung vorlag. Das Verhalten derartiger Partikelsysteme ist jedoch aufgrund von mechanischer und magnetischer Interpartikelwechselwirkung vergleichsweise komplex und theoretisch schwer modellierbar. Das prim{\"a}re Zielsystem f{\"u}r RDS hingegen, Eindom{\"a}nenpartikel mit im Partikel fixierter Magnetisierung und Punktsymmetrie bzgl. des Reibungstensors, erlaubt die Aufstellung einer parametrisierten Funktion f{\"u}r den Signalverlauf. Es erm{\"o}glicht somit aufgrund der besseren Berechenbarkeit eine solidere Auswertung des RDS-Signals. Um Eindom{\"a}nenpartikel in w{\"a}ssriger Suspension mit typischen Partikeldurchmessern um 100 nm messen zu k{\"o}nnen ist eine Verk{\"u}rzung der Totzeit auf mindestens 1/10 erforderlich. Prinzipiell kann diese Problematik durch die Verwendung schneller Halbleiterschalter in Verbindung mit einer pr{\"a}zise abstimmbaren induktiven Entkopplung des Spulensystems gemindert werden. Simulationen des RDS-Signals f{\"u}r verschiedene RDS-Sequenzen zeigen jedoch noch zwei weitere M{\"o}glichkeiten auf, die ohne aufw{\"a}ndigen Eingriffe in der Hardware auskommen. Zum einen kann durch orthogonales Frequenzmischen mit geeignetem Frequenz- und Phasenverh{\"a}ltnis eine Ausrichtung der magnetischen Momente bewirkt werden. Da die ben{\"o}tigten Frequenzen vollst{\"a}ndig im Sperrband des Tiefpassfilters liegen k{\"o}nnen, l{\"a}sst sich damit die Pulsantwort bei hinreichend „weichem" Umschalten zwischen der Polarisierungssequenz und der RDS-Sequenz vollst{\"a}ndig vermeiden. Dar{\"u}ber hinaus zeigt sich, dass es bei Anwesenheit eines schwachen Offsetfelds (< 10 \% der Rotationsfeldamplitude) zu einer Ausrichtung der magnetischen Momente kommt, wenn das magnetische Rotationsfeld seine Richtung {\"a}ndert und diese {\"A}nderung nicht abrupt erfolgt, sondern das Rotationsfeld {\"u}bergangsweise in ein linear oszillierendes Feld {\"u}bergeht. Hingegen wird die Wirkung des Offsetfelds durch das Rotationsfeld vor und nach dem Wechsel nahezu vollst{\"a}ndig neutralisiert, so dass damit das St{\"o}rsignale generierende Schalten eines Offsetfelds ersetzt werden kann. Es ist auf diese Weise nicht m{\"o}glich, Echosequenzen zu erzeugen, da hier bei der f{\"u}r Echosequenzen ben{\"o}tigten Richtungsumkehr des Rotationsfelds die zuvor aufgepr{\"a}gte Phasenverteilung durch das Offsetfeld zerst{\"o}rt wird und somit anstelle einer Signalechogenerierung eine neue RDS-Messung gestartet wird. Obwohl es Echosequenzen mit Anfangspuls erlauben, mehr MNP Parameter zu messen, bietet dieser Ansatz dennoch entscheidende Vorteile. So ergibt sich eine massive Vereinfachung der Hardware und es sind bei gleicher Rotationsfrequenz deutlich h{\"o}here Wiederholraten m{\"o}glich. Die Vermeidung von Schaltvorg{\"a}ngen durch die Verwendung von Offsetfeldern erm{\"o}glicht es, mit dem urspr{\"u}nglichem Aufbau auch Partikelsysteme zu untersuchen, deren Relaxationszeit weit unter 3 ms liegt. Hier zeigt sich, dass sich f{\"u}r unterschiedliche Partikelsysteme teils sehr charakteristische Signalmuster ergeben. Diese lassen sich grob in drei Kategorien einteilen. Die erste Kategorie sind suspendierte Eindom{\"a}nenpartikel mit einer nicht vernachl{\"a}ssigbaren Relaxationszeit. Hier handelt es sich um das bevorzugte Zielsystem f{\"u}r RDS, das durch die Langevin-Gleichung beschrieben werden kann. Die zweite Kategorie sind Partikelsysteme, bei denen die Relaxationsdauer vernachl{\"a}ssigbar ist. In diesem Fall kann der Signalverlauf mit der Langevinfunktion beschrieben werden. Die dritte Kategorie umfasst alle {\"u}brigen Partikelsysteme, insbesondere Suspensionen von MNP-Clustern, die u.a. aufgrund von Interpartikelwechselwirkung komplexe Signalverl{\"a}ufe ergeben, die sich praktisch nicht berechnen lassen. Spektroskopische Untersuchungen sind damit dennoch durch das Anlegen entsprechender Referenzdatenbanken m{\"o}glich (Fingerprinting). Multiparametrisches RDS, d.h. die Wiederholung der Messung f{\"u}r z.B. unterschiedliche Amplituden oder unterschiedliche Viskosit{\"a}ten des Suspensionsmediums, erzeugt aufgrund mehrerer nichtlinearer Abh{\"a}ngigkeiten massive Unterschiede im resultierenden multidimensionalen Datensatz. Das verspricht die Erreichbarkeit hoher spektroskopischer Trennsch{\"a}rfen bei geeigneter Partikel- und Sequenzoptimierung. Die Simulationen und experimentellen Ergebnisse dieser Arbeit zeigen grunds{\"a}tzliche H{\"u}rden und M{\"o}glichkeiten f{\"u}r das ebenfalls in dieser Arbeit eingef{\"u}hrte RDS auf. Es zeigt damit grundlegende Aspekte auf, die f{\"u}r die Entwicklung von RDS-Hardware und die Optimierung von MNP-Suspensionen n{\"o}tig sind. Mit RDS wird in weiterf{\"u}hrenden Arbeiten die Entwicklung von hochempfindlichen Bioassays und die Erweiterung um die r{\"a}umliche Kodierung angestrebt (RDI), da der zugrunde liegende Effekt zugleich sehr vielversprechend als Grundlage f{\"u}r molekulare Bildgebung ist.},
  subject      = {Magnetteilchen},
  language  = {de}
}
@phdthesis{Stahlhut2023,
  author    = {Stahlhut, Philipp},
  title     = {Konzeption und Aufbau einer Nanofokus Labor CT Anlage in Reflexionsgeometrie auf Basis eines Rasterelektronenmikroskops},
  doi       = {10.25972/OPUS-30264},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-302648},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {In der vorliegenden Arbeit werden die Konzeption und Realisierung eines Computertomographen zur Materialanalyse auf Basis eines Rasterelektronenmikroskops mit einem r{\"a}umlichen Aufl{\"o}sungsverm{\"o}gen im Nanometerbereich diskutiert. Durch einen fokussierten Elektronenstrahl, der mit einer Beschleunigungsspannung von 30 kV auf eine mikrostrukturierte Wolframnadel mit einem Spitzenradius von bis zu 50 nm gezielt wird, entsteht ein kleiner R{\"o}ntgenbrennfleck {\"u}ber den mit geometrischer Vergr{\"o}ßerung hochaufl{\"o}sende Projektionen eines zu untersuchenden Objekts erzeugt werden. Durch Rotation des Testobjekts werden Projektionen aus verschiedenen Blickwinkeln aufgenommen und {\"u}ber einen speziellen Rekonstruktionsalgorithmus zu einem 3-dimensionalen Bild zusammengef{\"u}gt. Bei der Beurteilung der Einzelkomponenten des Ger{\"a}ts wird insbesondere auf Struktur, Form und den elektrochemischen Herstellungsprozess der R{\"o}ntgenquelle eingegangen. Eine ausreichend genaue Positionierung von Messobjekt und R{\"o}ntgenbrennfleck wird {\"u}ber Piezoachsen realisiert, w{\"a}hrend die Stabilit{\"a}t des R{\"o}ntgenbrennflecks {\"u}ber die Elektronenoptik des Rasterelektronenmikroskops und die Form der Quellnadel optimiert wird. Das r{\"a}umliche Aufl{\"o}sungsverm{\"o}gen wird {\"u}ber die Linienspreizfunktion an Materialkanten abgesch{\"a}tzt. F{\"u}r eine Wolfram-Block-Quelle ergibt sich dabei ein Aufl{\"o}sungsverm{\"o}gen von 325 nm - 400 nm in 3D, w{\"a}hrend der Quellfleck einer Wolframnadel das Aufl{\"o}sungsverm{\"o}gen der Anlage auf 65 nm - 90 nm in 2D und 170 nm - 300 nm in 3D bei Messungen an einem AlCu29-Testobjekt anhebt. Außerdem werden die Auswirkungen der Phasenkontrastcharakteristik der R{\"o}ntgenquelle auf die rekonstruierten Bilder nach Anwendung eines Paganin-Filters diskutiert. Dabei zeigt sich, dass durch Anwendung des Filters ein verbessertes Signal-zu-Rausch-Verh{\"a}ltnis auf Kosten der r{\"a}umlichen Bildaufl{\"o}sung erzielt werden kann. Eine Vergleichsmessung mit einem kommerziell verf{\"u}gbaren R{\"o}ntgenmikroskop zeigt die St{\"a}rken des vorgestellten Systems bei Untersuchung von stark absorbierenden Messobjekten. Das kompakte Design erlaubt eine Weiterentwicklung in Richtung eines nanoCT-Moduls als Upgrade Option f{\"u}r Rasterelektronenmikroskope im Gegensatz zu den weitaus teureren bisher verbreiteten nanoCT-Ger{\"a}ten.},
  subject      = {Computertomographie},
  language  = {de}
}
@phdthesis{Jung2023,
  author    = {Jung, Johannes},
  title     = {Wechselwirkungen zwischen Kantenzust{\"a}nden auf dem topologisch kristallinen Isolator Pb\(_{1-x}\)Sn\(_x\)Se},
  doi       = {10.25972/OPUS-29861},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-298616},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {Einerseits besteht die einfachste M{\"o}glichkeit zum Ladungs- und Informationstransport zwischen zwei Punkten in deren direkter Verbindung durch eindimensionale Kan{\"a}le. Andererseits besitzen topologische Materialien exotische und {\"a}ußerst vorteilhafte Eigenschaften, weshalb es nahe liegt, dass schon bald neue Anwendungen aus ihnen realisiert werden. Wenn diese beiden Entwicklungen zusammenkommen, dann ist ein grundlegendes Verst{\"a}ndnis von Quanteninterferenz oder Hybridisierungseffekten in eindimensionalen, topologischen Kan{\"a}len von fundamentaler Wichtigkeit. Deshalb werden in der vorliegenden Arbeit Wechselwirkungen von eindimensionalen, topologisch gesch{\"u}tzten Kantenzust{\"a}nden, die an ungeradzahligen Stufenkanten auf der (001)-Oberfl{\"a}che von Pb1-xSnxSe auftreten, untersucht. Aufgrund der lateralen Lokalisierung auf wenige Nanometer um eine Stufenkante herum und der Notwendigkeit zwischen gerad- und ungeradzahligen Stufenkantenh{\"o}hen zu unterscheiden, bieten sich die Rastertunnelmikroskopie und -spektroskopie als Methoden an. Die neu entdeckten Kopplungs- bzw. Wechselwirkungseffekte zwischen benachbarten Kantenzust{\"a}nden treten auf, sobald der Stufe zu Stufe Abstand einen kritischen Wert von dkri ≈ 25nm unterschreitet. Dieses Kriterium kann durch verschiedene r{\"a}umliche Anordnungen von Stufenkanten erf{\"u}llt werden. Infolgedessen werden sich kreuzende, parallel verlaufende und zusammenlaufende Stufenkanten genauer untersucht. Bei letzteren ver{\"a}ndert sich entlang der Struktur kontinuierlich der Abstand und damit die Kopplungsst{\"a}rke zwischen den beiden Randkan{\"a}len. Infolgedessen wurden drei Koppelungsregime identifiziert. (I) Ausgehend von einer schwachen Wechselwirkung zeigt der f{\"u}r die Kantenzust{\"a}nde charakteristische Peak im Spektrum zun{\"a}chst eine Verbreiterung und Verminderung der Intensit{\"a}t. (II) Mit weiter zunehmender Wechselwirkung beginnt sich der Zustand in zwei Peaks aufzuspalten, sodass ab dkri ≈ 15nm an beiden Stufenkanten durchgehen eine Doppelpeak zu beobachten ist . Mit weiter abnehmendem Abstand erreicht die Aufspaltung Werte von einigen 10 meV, w{\"a}hrend sich die Intensit{\"a}t weiter reduziert. (III) Sobald zwei Stufenkanten weniger als etwa 5nm voneinander getrennt sind, konvergieren aufgrund der schwindenden Intensit{\"a}t und des sinkenden energetischen Abstands der beiden Peaks zu den van Hove Singularit{\"a}ten die Spektren an den Stufenkanten gegen das Spektrum {\"u}ber einer Terrasse. i Die Aufspaltung verl{\"a}uft in den Bereichen I und II asymmetrisch, d. h. ein Peak verbleibt ungef{\"a}hr bei der Ausgangsenergie, w{\"a}hrend der andere mit zunehmender Kopplung immer weiter weg schiebt. Bez{\"u}glich der Asymmetrie kann kein Unterschied festgestellt werden, ob die zusammenlaufenden Stufenkanten eine Insel oder Fehlstelleninsel bilden oder ob die Stufenkanten sogar g{\"a}nzlich parallel verlaufen. Es zeigt sich keine Pr{\"a}ferenz, ob zun{\"a}chst der niederenergetische oder der hochenergetische Peak schiebt. Erst im Regime starker Kopplung (III) kann beobachtet werden, dass beide Peaks die Ausgangsenergie deutlich verlassen. Im Gegensatz dazu kann bei sich kreuzenden Stufen ein erheblicher Einfluss der Geometrie, in Form des eingeschlossenen Winkels, auf das Spektrum beobachtet werden. Unabh{\"a}ngig vom Winkel existiert am Kreuzungspunkt selbst kein Kantenzustand mehr. Die Zust{\"a}nde an den vier Stufen beginnen, abh{\"a}ngig vom Winkel, etwa 10-15nm vor dem Kreuzungspunkt abzuklingen. {\"U}berraschenderweise zeigt sich dabei, dass im Fall rechtwinkliger Stufen gar keine Aufspaltung zu beobachten ist, w{\"a}hrend bei allen anderen Winkeln ein Doppelpeak festgestellt werden kann. Diese Entdeckung deutet auf Orthogonalit{\"a}t bez{\"u}glich einer Quantenzahl bei den beteiligten Kantenzust{\"a}nde hin. Neben einer nur theoretisch vorhergesagten Spinpolarisation kann dieser Effekt auch von dem orbitalem Charakter der beteiligten Dirac-Kegel verursacht sein. Da der topologische Schutz in Pb1-xSnxSe durch Kristallsymmetrien garantiert ist, wird als letzter intrinsischer Effekt der Einfluss von eindimensionalen Defekten auf die Kantenzust{\"a}nde untersucht. Ber{\"u}cksichtigt werden dabei ein nicht n{\"a}her klassifizierbarer, oberfl{\"a}chennaher Defekt und Schraubversetzungen. In beiden F{\"a}llen kann ebenfalls eine Aufspaltung des Kantenzustands in einen Doppelpeak gezeigt werden. Im zweiten Teil dieser Arbeit werden die Grundlagen f{\"u}r eine Wiederverwendung von (Pb,Sn)Se-Oberfl{\"a}chen bei zuk{\"u}nftige Experimenten mit (magnetischen) Adatomen geschaffen. Durch Kombination von Inoenzerst{\"a}ubung und Tempern wird dabei nicht nur eine gereinigte Oberfl{\"a}che erzeugt, sondern es kann auch das Ferminiveau gezielt erh{\"o}ht oder gesenkt werden. Dieser Effekt beruht auf eine Modifikation der Sn- Konzentration und der von ihr kontrollierten Anzahl an Defektelektronen. Als letztes sind erste Messungen an Cu- und Fe-dotierte Proben gezeigt. Durch die Adatome tritt eine n-Dotierung auf, welche den Dirac-Punkt des Systems in Richtung des Ferminiveaus verschiebt. Sobald er dieses erreicht hat kommt es zu Wechselwirkungsph{\"a}nomenen an freistehenden Stufenkanten. Dies f{\"u}hrt zu einer Doppelpeakstruktur mit einer feinen Aufspaltung von wenigen meV. Das Ph{\"a}nomen ist auf ein schmales Energiefenster beschr{\"a}nkt, bei dem die Lage des Dirac-Punkts nur etwa 5 meV (in beide Richtungen) von der des Ferminiveaus abweichen darf.},
  subject      = {Topologischer Isolator},
  language  = {de}
}
@phdthesis{Niehoerster2022,
  author    = {Nieh{\"o}rster, Thomas},
  title     = {Spektral aufgel{\"o}ste Fluoreszenzlebensdauer-Mikroskopie mit vielen Farben},
  doi       = {10.25972/OPUS-29657},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-296573},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Die Fluoreszenzmikroskopie ist eine vielseitig einsetzbare Untersuchungsmethode f{\"u}r biologische Proben, bei der Biomolek{\"u}le selektiv mit Fluoreszenzfarbstoffen markiert werden, um sie dann mit sehr gutem Kontrast abzubilden. Dies ist auch mit mehreren verschiedenartigen Zielmolek{\"u}len gleichzeitig m{\"o}glich, wobei {\"u}blicherweise verschiedene Farbstoffe eingesetzt werden, die {\"u}ber ihre Spektren unterschieden werden k{\"o}nnen. Um die Anzahl gleichzeitig verwendbarer F{\"a}rbungen zu maximieren, wird in dieser Arbeit zus{\"a}tzlich zur spektralen Information auch das zeitliche Abklingverhalten der Fluoreszenzfarbstoffe mittels spektral aufgel{\"o}ster Fluoreszenzlebensdauer-Mikroskopie (spectrally resolved fluorescence lifetime imaging microscopy, sFLIM) vermessen. Dazu wird die Probe in einem Konfokalmikroskop von drei abwechselnd gepulsten Lasern mit Wellenl{\"a}ngen von 485 nm, 532nm und 640nm angeregt. Die Detektion des Fluoreszenzlichtes erfolgt mit einer hohen spektralen Aufl{\"o}sung von 32 Kan{\"a}len und gleichzeitig mit sehr hoher zeitlicher Aufl{\"o}sung von einigen Picosekunden. Damit wird zu jedem detektierten Fluoreszenzphoton der Anregungslaser, der spektrale Kanal und die Ankunftszeit registriert. Diese detaillierte multidimensionale Information wird von einem Pattern-Matching-Algorithmus ausgewertet, der das Fluoreszenzsignal mit zuvor erstellten Referenzpattern der einzelnen Farbstoffe vergleicht. Der Algorithmus bestimmt so f{\"u}r jedes Pixel die Beitr{\"a}ge der einzelnen Farbstoffe. Mit dieser Technik konnten pro Anregungslaser f{\"u}nf verschiedene F{\"a}rbungen gleichzeitig dargestellt werden, also theoretisch insgesamt 15 F{\"a}rbungen. In der Praxis konnten mit allen drei Lasern zusammen insgesamt neun F{\"a}rbungen abgebildet werden, wobei die Anzahl der Farben vor allem durch die anspruchsvolle Probenvorbereitung limitiert war. In anderen Versuchen konnte die sehr hohe Sensitivit{\"a}t des sFLIM-Systems genutzt werden, um verschiedene Zielmolek{\"u}le voneinander zu unterscheiden, obwohl sie alle mit demselben Farbstoff markiert waren. Dies war m{\"o}glich, weil sich die Fluoreszenzeigenschaften eines Farbstoffmolek{\"u}ls geringf{\"u}gig in Abh{\"a}ngigkeit von seiner Umgebung {\"a}ndern. Weiterhin konnte die sFLIM-Technik mit der hochaufl{\"o}senden STED-Mikroskopie (STED: stimulated emission depletion) kombiniert werden, um so hochaufgel{\"o}ste zweifarbige Bilder zu erzeugen, wobei nur ein einziger gemeinsamer STED-Laser ben{\"o}tigt wurde. Die gleichzeitige Erfassung von mehreren photophysikalischen Messgr{\"o}ßen sowie deren Auswertung durch den Pattern-Matching-Algorithmus erm{\"o}glichten somit die Entwicklung von neuen Methoden der Fluoreszenzmikroskopie f{\"u}r Mehrfachf{\"a}rbungen.},
  subject      = {Fluoreszenzmikroskopie},
  language  = {de}
}
@phdthesis{Gruene2022,
  author    = {Gr{\"u}ne, Jeannine},
  title     = {Spin States and Efficiency-Limiting Pathways in Optoelectronic Materials and Devices},
  doi       = {10.25972/OPUS-29340},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-293405},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {This thesis addresses the identification and characterization of spin states in optoelectronic materials and devices using multiple spin-sensitive techniques. For this purpose, a systematic study focussing on triplet states as well as associated loss pathways and excited state kinetics was carried out. The research was based on comparing a range of donor:acceptor systems, reaching from organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) to organic photovoltaics (OPV) employing fullerene and multiple non-fullerene acceptors (NFAs). By developing new strategies, e.g., appropriate modeling, new magnetic resonance techniques and experimental frameworks, the influence of spin states in the fundamental processes of organic semiconductors has been investigated. Thereby, the combination of techniques based on the principle of electron paramagnetic resonance (EPR), in particular transient EPR (trEPR) and optically detected magnetic resonance (ODMR), with all-optical methods, such as transient electroluminescence (trEL) and transient absorption (TA), has been employed. As a result, excited spin states, especially molecular and charge transfer (CT) states, were investigated in terms of kinetic behavior and associated pathways, which revealed a significant impact of triplet states on efficiency-limiting processes in both optoelectronic applications.},
  subject      = {Elektronenspinresonanz},
  language  = {en}
}
@phdthesis{Hajer2022,
  author    = {Hajer, Jan},
  title     = {Mercury Telluride Nanowires for Topological Quantum Transport},
  doi       = {10.25972/OPUS-29322},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-293222},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Novel appraches to the molecular beam epitaxy of core-shell nanowires in the group II telluride material system were explored in this work. Significant advances in growth spurred the development of a flexible and reliable platform for a charge transport characterization of the topological insulator HgTe in a tubular nanowire geometry. The transport results presented provide an important basis for the design of future studies that strive for the experimental realization of topological charge transport in the quantum wire limit.},
  subject      = {Quecksilbertellurid},
  language  = {en}
}
@phdthesis{Schmitt2022,
  author    = {Schmitt, Fabian Bernhard},
  title     = {Transport properties of the three-dimensional topological insulator mercury telluride},
  doi       = {10.25972/OPUS-29173},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-291731},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {The subject of this thesis is the investigation of the transport properties of topological and massive surface states in the three-dimensional topological insulator Hg(Mn)Te. These surface states give rise to a variety of extraordinary transport phenomena, making this material system of great interest for research and technological applications. In this connection, many physical properties of the topological insulator Hg(Mn)Te still require in-depth exploration. The overall aim of this thesis is to analyze the quantum transport of HgTe-based devices ranging from hundreds of micrometers (macroscopic) down to a few micrometers in size (microscopic) in order to extend the overall understanding of surface states and the possibilities of their manipulation. In order to exploit the full potential of our high-quality heterostructures, it was necessary to revise and improve the existing lithographic fabrication process of macroscopic three-dimensional Hg(Mn)Te samples. A novel lithographic standard recipe for the fabrication of the HgTe-based macrostructures was developed. This recipe includes the use of an optimized Hall bar design and wet etching instead of etching with high-energy \(\mathrm{{Ar^{+}}}\)-ions, which can damage the samples. Further, a hafnium oxide insulator is applied replacing the SiO\(_{2}\)/Si\(_{3}\)N\(_{4}\) dielectric in order to reduce thermal load. Moreover, the devices are metallized under an alternating angle to avoid discontinuities of the metal layers over the mesa edges. It was revealed that the application of gate-dielectric and top-gate metals results in n-type doping of the devices. This phenomenon could be attributed to quasi-free electrons tunneling from the trap states, which form at the interface cap layer/insulator, through the cap into the active layer. This finding led to the development of a new procedure to characterize wafer materials. It was found that the optimized lithographic processing steps do not unintentionally react chemically with our heterostructures, thus avoiding a degradation of the quality of the Hg(Mn)Te layer. The implementation of new contact structures Ti/Au, In/Ti/Au, and Al/Ti/Au did not result in any improvement compared to the standard structure AuGe/Au. However, a novel sample recipe could be developed, resulting in an intermixing of the contact metals (AuGe and Au) and fingering of metal into the mesa. The extent of the quality of the ohmic contacts obtained through this process has yet to be fully established. This thesis further deals with the lithographic realization of three-dimensional HgTe-based microstructures measuring only a few micrometer in size. Thus, these structures are in the order of the mean free path and the spin relaxation length of topological surface state electrons. A lithographic process was developed enabling the fabrication of nearly any desired microscopic device structure. In this context, two techniques suitable for etching microscopic samples were realized, namely wet etching and the newly established inductively coupled plasma etching. While wet etching was found to preserve the crystal quality of the active layer best, inductively coupled plasma etching is characterized by high reproducibility and excellent structural fidelity. Hence, the etching technique employed depends on the envisaged type of experiment. Magneto-transport measurements were carried out on the macroscopic HgTe-based devices fabricated by means of improved lithographic processing with respect to the transport properties of topological and massive surface states. It was revealed that due to the low charge carrier density present in the leads to the ohmic contacts, these regions can exhibit an insulating behavior at high magnetic fields and extremely low temperatures. As soon as the filling factor of the lowest Landau levels dropped below a critical value (\(\nu_{\mathrm{{c}}}\approx0.8\)), the conductance of the leads decreased significantly. It was demonstrated that the carrier density in the leads can be increased by the growth of modulation doping layers, a back-gate-electrode, light-emitting diode illumination, and by the application of an overlapping top-gate layout. This overlapping top-gate and a back-gate made it possible to manipulate the carrier density of the surface states on both sides of the Hg(Mn)Te layer independently. With this setup, it was identified that topological and massive surface states contribute to transport simultaneously in 3D Hg(Mn)Te. A model could be developed allowing the charge carrier systems populated in the sample to be determined unambiguously. Based on this model, the process of the re-entrant quantum Hall effect observed for the first time in three-dimensional topological insulators could be explained by an interplay of n-type topological and p-type massive surface states. A well-pronounced \(\nu=-1\rightarrow\nu=-2\rightarrow\nu=-1\) sequence of quantum Hall plateaus was found in manganese-doped HgTe-based samples. It is postulated that this is the condensed-matter realization of the parity anomaly in three-dimensional topological insulators. The actual nature of this phenomenon can be the subject of further research. In addition, the measurements have shown that inter-scattering occurs between counter-propagating quantum Hall edge states. The good quantization of the Hall conductance despite this inter-scattering indicates that only the unpaired edge states determine the transport properties of the system as a whole. The underlying inter-scattering mechanism is the topic of a publication in preparation. Furthermore, three-dimensional HgTe-based microstructures shaped like the capital letter "H" were investigated regarding spin transport phenomena. The non-local voltage signals occurring in the measurements could be attributed to a current-induced spin polarization of the topological surface states due to electrons obeying spin-momentum locking. It was shown that the strength of this non-local signal is directly connected to the magnitude of the spin polarization and can be manipulated by the applied top-gate voltage. It was found that in these microstructures, the massive surface and bulk states, unlike the topological surface states, cannot contribute to this spin-associated phenomenon. On the contrary, it was demonstrated that the population of massive states results in a reduction of the spin polarization, either due to the possible inter-scattering of massive and topological surface states or due to the addition of an unpolarized electron background. The evidence of spin transport controllable by a top-gate-electrode makes the three-dimensional material system mercury telluride a promising candidate for further research in the field of spintronics.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@phdthesis{Fijalkowski2022,
  author    = {Fijalkowski, Kajetan Maciej},
  title     = {Electronic Transport in a Magnetic Topological Insulator (V,Bi,Sb)\(_2\)Te\(_3\)},
  doi       = {10.25972/OPUS-28230},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-282303},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {This thesis focuses on investigating magneto-transport properties of a ferromagnetic topological insulator (V,Bi,Sb)2Te3. This material is most famously known for exhibiting the quantum anomalous Hall effect, a novel quantum state of matter that has opened up possibilities for potential applications in quantum metrology as a quantum standard of resistance, as well as for academic investigations into unusual magnetic properties and axion electrodynamics. All of those aspects are investigated in the thesis.},
  subject      = {Topologischer Isolator},
  language  = {en}
}
@phdthesis{Uenzelmann2022,
  author    = {{\"U}nzelmann, Maximilian},
  title     = {Interplay of Inversion Symmetry Breaking and Spin-Orbit Coupling - From the Rashba Effect to Weyl Semimetals},
  doi       = {10.25972/OPUS-28310},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-283104},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Breaking inversion symmetry in crystalline solids enables the formation of spin-polarized electronic states by spin-orbit coupling without the need for magnetism. A variety of interesting physical phenomena related to this effect have been intensively investigated in recent years, including the Rashba effect, topological insulators and Weyl semimetals. In this work, the interplay of inversion symmetry breaking and spin-orbit coupling and, in particular their general influence on the character of electronic states, i.e., on the spin and orbital degrees of freedom, is investigated experimentally. Two different types of suitable model systems are studied: two-dimensional surface states for which the Rashba effect arises from the inherently broken inversion symmetry at the surface, and a Weyl semimetal, for which inversion symmetry is broken in the three-dimensional crystal structure. Angle-resolved photoelectron spectroscopy provides momentum-resolved access to the spin polarization and the orbital composition of electronic states by means of photoelectron spin detection and dichroism with polarized light. The experimental results shown in this work are also complemented and supported by ab-initio density functional theory calculations and simple model considerations. Altogether, it is shown that the breaking of inversion symmetry has a decisive influence on the Bloch wave function, namely, the formation of an orbital angular momentum. This mechanism is, in turn, of fundamental importance both for the physics of the surface Rashba effect and the topology of the Weyl semimetal TaAs.},
  subject      = {Rashba-Effekt},
  language  = {en}
}
@phdthesis{GraetzgebDittmann2022,
  author    = {Graetz [geb. Dittmann], Jonas},
  title     = {X-Ray Dark-Field Tensor Tomography : a Hitchhiker's Guide to Tomographic Reconstruction and Talbot Imaging},
  doi       = {10.25972/OPUS-28143},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-281437},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {X-ray dark-field imaging allows to resolve the conflict between the demand for centimeter scaled fields of view and the spatial resolution required for the characterization of fibrous materials structured on the micrometer scale. It draws on the ability of X-ray Talbot interferometers to provide full field images of a sample's ultra small angle scattering properties, bridging a gap of multiple orders of magnitude between the imaging resolution and the contrasted structure scale. The correspondence between shape anisotropy and oriented scattering thereby allows to infer orientations within a sample's microstructure below the imaging resolution. First demonstrations have shown the general feasibility of doing so in a tomographic fashion, based on various heuristic signal models and reconstruction approaches. Here, both a verified model of the signal anisotropy and a reconstruction technique practicable for general imaging geometries and large tensor valued volumes is developed based on in-depth reviews of dark-field imaging and tomographic reconstruction techniques. To this end, a wide interdisciplinary field of imaging and reconstruction methodologies is revisited. To begin with, a novel introduction to the mathematical description of perspective projections provides essential insights into the relations between the tangible real space properties of cone beam imaging geometries and their technically relevant description in terms of homogeneous coordinates and projection matrices. Based on these fundamentals, a novel auto-calibration approach is developed, facilitating the practical determination of perspective imaging geometries with minimal experimental constraints. A corresponding generalized formulation of the widely employed Feldkamp algorithm is given, allowing fast and flexible volume reconstructions from arbitrary tomographic imaging geometries. Iterative reconstruction techniques are likewise introduced for general projection geometries, with a particular focus on the efficient evaluation of the forward problem associated with tomographic imaging. A highly performant 3D generalization of Joseph's classic linearly interpolating ray casting algorithm is developed to this end and compared to typical alternatives. With regard to the anisotropic imaging modality required for tensor tomography, X-ray dark-field contrast is extensively reviewed. Previous literature is brought into a joint context and nomenclature and supplemented by original work completing a consistent picture of the theory of dark-field origination. Key results are explicitly validated by experimental data with a special focus on tomography as well as the properties of anisotropic fibrous scatterers. In order to address the pronounced susceptibility of interferometric images to subtle mechanical imprecisions, an efficient optimization based evaluation strategy for the raw data provided by Talbot interferometers is developed. Finally, the fitness of linear tensor models with respect to the derived anisotropy properties of dark-field contrast is evaluated, and an iterative scheme for the reconstruction of tensor valued volumes from projection images is proposed. The derived methods are efficiently implemented and applied to fiber reinforced plastic samples, imaged at the ID19 imaging beamline of the European Synchrotron Radiation Facility. The results represent unprecedented demonstrations of X-ray dark-field tensor tomography at a field of view of 3-4cm, revealing local fiber orientations of both complex shaped and low-contrast samples at a spatial resolution of 0.1mm in 3D. The results are confirmed by an independent micro CT based fiber analysis.},
  subject      = {Dreidimensionale Rekonstruktion},
  language  = {en}
}