@phdthesis{Andelovic2024,
  author    = {Andelovic, Kristina},
  title     = {Characterization of arterial hemodynamics using mouse models of atherosclerosis and tissue-engineered artery models},
  doi       = {10.25972/OPUS-30360},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-303601},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {Within this thesis, three main approaches for the assessment and investigation of altered hemodynamics like wall shear stress, oscillatory shear index and the arterial pulse wave velocity in atherosclerosis development and progression were conducted: 1. The establishment of a fast method for the simultaneous assessment of 3D WSS and PWV in the complete murine aortic arch via high-resolution 4D-flow MRI 2. The utilization of serial in vivo measurements in atherosclerotic mouse models using high-resolution 4D-flow MRI, which were divided into studies describing altered hemodynamics in late and early atherosclerosis 3. The development of tissue-engineered artery models for the controllable application and variation of hemodynamic and biologic parameters, divided in native artery models and biofabricated artery models, aiming for the investigation of the relationship between atherogenesis and hemodynamics Chapter 2 describes the establishment of a method for the simultaneous measurement of 3D WSS and PWV in the murine aortic arch at, using ultra high-field MRI at 17.6T [16], based on the previously published method for fast, self-navigated wall shear stress measurements in the murine aortic arch using radial 4D-phase contrast MRI at 17.6 T [4]. This work is based on the collective work of Dr. Patrick Winter, who developed the method and the author of this thesis, Kristina Andelovic, who performed the experiments and statistical analyses. As the method described in this chapter is basis for the following in vivo studies and undividable into the sub-parts of the contributors without losing important information, this chapter was not split into the single parts to provide fundamental information about the measurement and analysis methods and therefore better understandability for the following studies. The main challenge in this chapter was to overcome the issue of the need for a high spatial resolution to determine the velocity gradients at the vascular wall for the WSS quantification and a high temporal resolution for the assessment of the PWV without prolonging the acquisition time due to the need for two separate measurements. Moreover, for a full coverage of the hemodynamics in the murine aortic arch, a 3D measurement is needed, which was achieved by utilization of retrospective navigation and radial trajectories, enabling a highly flexible reconstruction framework to either reconstruct images at lower spatial resolution and higher frame rates for the acquisition of the PWV or higher spatial resolution and lower frame rates for the acquisition of the 3D WSS in a reasonable measurement time of only 35 minutes. This enabled the in vivo assessment of all relevant hemodynamic parameters related to atherosclerosis development and progression in one experimental session. This method was validated in healthy wild type and atherosclerotic Apoe-/- mice, indicating no differences in robustness between pathological and healthy mice. The heterogeneous distribution of plaque development and arterial stiffening in atherosclerosis [10, 12], however, points out the importance of local PWV measurements. Therefore, future studies should focus on the 3D acquisition of the local PWV in the murine aortic arch based on the presented method, in order to enable spatially resolved correlations of local arterial stiffness with other hemodynamic parameters and plaque composition. In Chapter 3, the previously established methods were used for the investigation of changing aortic hemodynamics during ageing and atherosclerosis in healthy wild type and atherosclerotic Apoe-/- mice using the previously established methods [4, 16] based on high-resolution 4D-flow MRI. In this work, serial measurements of healthy and atherosclerotic mice were conducted to track all changes in hemodynamics in the complete aortic arch over time. Moreover, spatially resolved 2D projection maps of WSS and OSI of the complete aortic arch were generated. This important feature allowed for the pixel-wise statistical analysis of inter- and intragroup hemodynamic changes over time and most importantly - at a glance. The study revealed converse differences of local hemodynamic profiles in healthy WT and atherosclerotic Apoe-/- mice, with decreasing longWSS and increasing OSI, while showing constant PWV in healthy mice and increasing longWSS and decreasing OSI, while showing increased PWV in diseased mice. Moreover, spatially resolved correlations between WSS, PWV, plaque and vessel wall characteristics were enabled, giving detailed insights into coherences between hemodynamics and plaque composition. Here, the circWSS was identified as a potential marker of plaque size and composition in advanced atherosclerosis. Moreover, correlations with PWV values identified the maximum radStrain could serve as a potential marker for vascular elasticity. This study demonstrated the feasibility and utility of high-resolution 4D flow MRI to spatially resolve, visualize and analyze statistical differences in all relevant hemodynamic parameters over time and between healthy and diseased mice, which could significantly improve our understanding of plaque progression towards vulnerability. In future studies the relation of vascular elasticity and radial strain should be further investigated and validated with local PWV measurements and CFD. Moreover, the 2D histological datasets were not reflecting the 3D properties and regional characteristics of the atherosclerotic plaques. Therefore, future studies will include 3D plaque volume and composition analysis like morphological measurements with MRI or light-sheet microscopy to further improve the analysis of the relationship between hemodynamics and atherosclerosis. Chapter 4 aimed at the description and investigation of hemodynamics in early stages of atherosclerosis. Moreover, this study included measurements of hemodynamics at baseline levels in healthy WT and atherosclerotic mouse models. Due to the lack of hemodynamic-related studies in Ldlr-/- mice, which are the most used mouse models in atherosclerosis research together with the Apoe-/- mouse model, this model was included in this study to describe changing hemodynamics in the aortic arch at baseline levels and during early atherosclerosis development and progression for the first time. In this study, distinct differences in aortic geometries of these mouse models at baseline levels were described for the first time, which result in significantly different flow- and WSS profiles in the Ldlr-/- mouse model. Further basal characterization of different parameters revealed only characteristic differences in lipid profiles, proving that the geometry is highly influencing the local WSS in these models. Most interestingly, calculation of the atherogenic index of plasma revealed a significantly higher risk in Ldlr-/- mice with ongoing atherosclerosis development, but significantly greater plaque areas in the aortic arch of Apoe-/- mice. Due to the given basal WSS and OSI profile in these two mouse models - two parameters highly influencing plaque development and progression - there is evidence that the regional plaque development differs between these mouse models during very early atherogenesis. Therefore, future studies should focus on the spatiotemporal evaluation of plaque development and composition in the three defined aortic regions using morphological measurements with MRI or 3D histological analyses like LSFM. Moreover, this study offers an excellent basis for future studies incorporating CFD simulations, analyzing the different measured parameter combinations (e.g., aortic geometry of the Ldlr-/- mouse with the lipid profile of the Apoe-/- mouse), simulating the resulting plaque development and composition. This could help to understand the complex interplay between altered hemodynamics, serum lipids and atherosclerosis and significantly improve our basic understanding of key factors initiating atherosclerosis development. Chapter 5 describes the establishment of a tissue-engineered artery model, which is based on native, decellularized porcine carotid artery scaffolds, cultured in a MRI-suitable bioreactor-system [23] for the investigation of hemodynamic-related atherosclerosis development in a controllable manner, using the previously established methods for WSS and PWV assessment [4, 16]. This in vitro artery model aimed for the reduction of animal experiments, while simultaneously offering a simplified, but completely controllable physical and biological environment. For this, a very fast and gentle decellularization protocol was established in a first step, which resulted in porcine carotid artery scaffolds showing complete acellularity while maintaining the extracellular matrix composition, overall ultrastructure and mechanical strength of native arteries. Moreover, a good cellular adhesion and proliferation was achieved, which was evaluated with isolated human blood outgrowth endothelial cells. Most importantly, an MRI-suitable artery chamber was designed for the simultaneous cultivation and assessment of high-resolution 4D hemodynamics in the described artery models. Using high-resolution 4D-flow MRI, the bioreactor system was proven to be suitable to quantify the volume flow, the two components of the WSS and the radStrain as well as the PWV in artery models, with obtained values being comparable to values found in literature for in vivo measurements. Moreover, the identification of first atherosclerotic processes like intimal thickening is achievable by three-dimensional assessment of the vessel wall morphology in the in vitro models. However, one limitation is the lack of a medial smooth muscle cell layer due to the dense ECM. Here, the utilization of the laser-cutting technology for the generation of holes and / or pits on a microscale, eventually enabling seeding of the media with SMCs showed promising results in a first try and should be further investigated in future studies. Therefore, the proposed artery model possesses all relevant components for the extension to an atherosclerosis model which may pave the way towards a significant improvement of our understanding of the key mechanisms in atherogenesis. Chapter 6 describes the development of an easy-to-prepare, low cost and fully customizable artery model based on biomaterials. Here, thermoresponsive sacrificial scaffolds, processed with the technique of MEW were used for the creation of variable, biomimetic shapes to mimic the geometric properties of the aortic arch, consisting of both, bifurcations and curvatures. After embedding the sacrificial scaffold into a gelatin-hydrogel containing SMCs, it was crosslinked with bacterial transglutaminase before dissolution and flushing of the sacrificial scaffold. The hereby generated channel was subsequently seeded with ECs, resulting in an easy-to-prepare, fast and low-cost artery model. In contrast to the native artery model, this model is therefore more variable in size and shape and offers the possibility to include smooth muscle cells from the beginning. Moreover, a custom-built and highly adaptable perfusion chamber was designed specifically for the scaffold structure, which enabled a one-step creation and simultaneously offering the possibility for dynamic cultivation of the artery models, making it an excellent basis for the development of in vitro disease test systems for e.g., flow-related atherosclerosis research. Due to time constraints, the extension to an atherosclerosis model could not be achieved within the scope of this thesis. Therefore, future studies will focus on the development and validation of an in vitro atherosclerosis model based on the proposed bi- and three-layered artery models. In conclusion, this thesis paved the way for a fast acquisition and detailed analyses of changing hemodynamics during atherosclerosis development and progression, including spatially resolved analyses of all relevant hemodynamic parameters over time and in between different groups. Moreover, to reduce animal experiments, while gaining control over various parameters influencing atherosclerosis development, promising artery models were established, which have the potential to serve as a new platform for basic atherosclerosis research.},
  subject      = {H{\"a}modynamik},
  language  = {en}
}
@phdthesis{Pres2024,
  author    = {Pres, Sebastian},
  title     = {Detection of a plasmon-polariton quantum wave packet by coherent 2D nanoscopy},
  doi       = {10.25972/OPUS-34824},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-348242},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {Plasmonic nanostructures are considered promising candidates for essential components of integrated quantum technologies because of their ability to efficiently localize broad-band electromagnetic fields on the nanoscale. The resulting local near field can be understood as a spatial superposition of spectrally different plasmon-polariton modes due to the spectrally broad optical excitation, and thus can be described as a classical wave packet. Since plasmon polaritons, in turn, can transmit and receive non-classical light states, the exciting question arises to what extent they have to be described as quantum mechanical wave packets, i.e. as a superposition of different quantum states. But how to probe, characterize and eventually manipulate the quantum state of such plasmon polaritons? Up to now, probing at room temperatures relied completely on analyzing quantum optical properties of the corresponding in-going and out-going far-field photon modes. However, these methods so far only allow a rather indirect investigation of the plasmon-polariton quantum state by means of transfer into photons. Moreover, these indirect methods lack spatial resolution and therefore do not provide on-site access to the plasmon-polariton quantum state. However, since the spectroscopic method of coherent two-dimensional (2D) nanoscopy offers the capability to follow the plasmon- polariton quantum state both in Hilbert space and in space and time domain a complete characterization of the plasmon polariton is possible. In this thesis a versatile coherent 2D nanoscopy setup is presented combining spectral tunability and femtosecond time resolution with spatial resolution on the nanometer scale due to the detection of optically excited nonlinear emitted electrons via photoemission electron microscopy (PEEM). Optical excitation by amplitude- and phase-shaped, systematically-modified and interferometric-stable multipulse sequences is realized, and characterized via Fourier-transform spectral interferometry (FTSI). This linear technique enables efficient data acquisition in parallel to a simultaneously performed experiment. The full electric-field reconstruction of every generated multipulse sequence is used to analyze the effect of non-ideal pulse sequences on the two-dimensional spectral data of population-based multidimensional spectroscopy methods like, e.g., the coherent 2D nanoscopy applied in this thesis. Investigation of the spatially-resolved nonlinear electron emission yield from plasmonic gold nanoresonators by coherent 2D nanoscopy requires a quasi-particle treatment of the addressed plasmon-polariton mode and development of a quantum model to adequately describe the plasmon-assisted multi-quantum electron emission from nanostructures. Good agreement between simulated and experimental data enables to connect certain spectral features to superpositions of non-adjacent plasmon-polariton quantum states, i.e, non-adjacent occupation-number states of the underlying quantized, harmonic oscillator, thus direct probing of the plasmon-polariton quantum wave packet at the location of the nanostructure. This is a necessary step to locally control and manipulate the plasmon-polariton quantum state and thus of general interest for the realization of nanoscale quantum optical devices.},
  subject      = {Coherent Multidimensional Spectroscopy},
  language  = {en}
}
@phdthesis{Bayer2024,
  author    = {Bayer, Florian},
  title     = {Investigating electromagnetic properties of topological surface states in mercury telluride},
  doi       = {10.25972/OPUS-35212},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-352127},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {This doctoral thesis investigates magneto-optical properties of mercury telluride layers grown tensile strained on cadmium telluride substrates. Here, layer thicknesses start above the usual quantum well thickness of about 20 nm and have a upper boundary around 100 nm due to lattice relaxation effects. This kind of layer system has been attributed to the material class of three-dimensional topological insulators in numerous publications. This class stands out due to intrinsic boundary states which cross the energetic band gap of the layer's bulk. In order to investigate the band structure properties in a narrow region around the Fermi edge, including possible boundary states, the method of highly precise time-domain Terahertz polarimetry is used. In the beginning, the state of the art of Teraherz technology at the start of this project is discussed, moving on to a detailed description and characterization of the self-built measurement setup. Typical standard deviation of a polarization rotation or ellipticity measurement are on the order of 10 to 100 millidegrees, according to the transmission strength through investigated samples. A range of polarization spectra, depending on external magnetic fields up to 10 Tesla, can be extracted from the time-domain signal via Fourier transformation. The identification of the actual band structure is done by modeling possible band structures by means of the envelope function approximation within the framework of the k·p method. First the bands are calculated based on well-established model parameters and from them the possible optical transitions and expected ellipticity spectra, all depending on external magnetic fields and the layer's charge carrier concentration. By comparing expected with measured spectra, the validity of k·p models with varying depths of detail is analyzed throughout this thesis. The rich information encoded in the ellipitcity spectra delivers key information for the attribution of single optical transitions, which are not part of pure absorption spectroscopy. For example, the sign of the ellipticity signals is linked to the mix of Landau levels which contribute to an optical transition, which shows direct evidence for bulk inversion asymmetry effects in the measured spectra. Throughout the thesis, the results are compared repeatedly with existing publications on the topic. It is shown that the models used there are often insufficient or, in worst case, plainly incorrect. Wherever meaningful and possible without greater detours, the differences to the conclusions that can be drawn from the k·p model are discussed. The analysis ends with a detailed look on remaining differences between model and measurement. It contains the quality of model parameters as well as different approaches to integrate electrostatic potentials that exist in the structures into the model. An outlook on possible future developments of the mercury cadmium telluride layer systems, as well as the application of the methods shown here onto further research questions concludes the thesis.},
  subject      = {Quecksilbertellurid},
  language  = {en}
}
@phdthesis{Miller2024,
  author    = {Miller, Kirill},
  title     = {Untersuchung von Nanostrukturen basierend auf LaAlO\(_3\)/SrTiO\(_3\) f{\"u}r Anwendungen in nicht von-Neumann-Rechnerarchitekturen},
  doi       = {10.25972/OPUS-35472},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-354724},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {Die Dissertation besch{\"a}ftigt sich mit der Analyse von oxidischen Nanostrukturen. Die Grundlage der Bauelemente stellt dabei die LaAlO3/SrTiO3-Heterostruktur dar. Hierbei entsteht an der Grenzfl{\"a}che beider {\"U}bergangsmetalloxide ein quasi zweidimensionales Elektronengas, welches wiederum eine F{\"u}lle von beachtlichen Eigenschaften und Charakteristika zeigt. Mithilfe lithographischer Verfahren wurden zwei unterschiedliche Bauelemente verwirklicht. Dabei handelt es sich einerseits um einen planaren Nanodraht mit lateralen Gates, welcher auf der Probenoberfl{\"a}che prozessiert wurde und eine bemerkenswerte Trialit{\"a}t aufweist. Dieses Bauelement kann unter anderem als ein herk{\"o}mmlicher Feldeffekttransistor agieren, wobei der Ladungstransport durch die lateral angelegte Spannung manipuliert wird. Zus{\"a}tzlich konnten auch Speichereigenschaften beobachtet werden, sodass das gesamte Bauelement als ein sogenannter Memristor fungieren kann. In diesem Fall h{\"a}ngt der Ladungstransport von der Elektronenakkumulation auf den lateralen potentialfreien Gates ab. Die Memristanz des Nanodrahts l{\"a}sst sich unter anderem durch Lichtleistungen im Nanowattbereich und mithilfe von kurzen Spannungspulsen ver{\"a}ndern. Dar{\"u}ber hinaus kann die Elektronenakkumulation auch in Form einer memkapazitiven Charakteristik beobachtet werden. Neben dem Nanodraht wurde auch eine Kreuzstruktur, die eine erg{\"a}nzende ferromagnetischen Elektrode beinhaltet, realisiert. Mit diesem neuartigen Bauteil wird die Umwandlung zwischen Spin- und Ladungsstr{\"o}men innerhalb der nanoskaligen Struktur untersucht. Hierbei wird die starke Spin-Bahn-Kopplung im quasi zweidimensionalen Elektronengas ausgenutzt.},
  subject      = {Memristor},
  language  = {de}
}
@phdthesis{Kagerer2024,
  author    = {Kagerer, Philipp Thomas},
  title     = {Two-Dimensional Ferromagnetism and Topology at the Surface of MnBi\(_2\)Te\(_4\) - Bi\(_2\)Te\(_3\) Heterostructures - MBE Growth, Magnetism and Electronic Properties},
  doi       = {10.25972/OPUS-36012},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-360121},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {In this thesis, a model system of a magnetic topological heterostructure is studied, namely a heterosystem consisting of a single ferromagnetic septuple-layer (SL) of \(MnBi_2Te_4\) on the surface of the three-dimensional topological insulator \(Bi_2Te_3\). Using MBE and developing a specialized experimental setup, the first part of this thesis deals with the growth of \(Bi_2Te_3\) and thin films of \(MnBi_2Te_4\) on \(BaF_2\)-substrates by the co-evaporation of its binary constituents. The structural analysis is conducted along several suitable probes such as X-ray diffraction (XRD, XRR), AFM and scanning tunnelling electron microscopy (STEM). It is furthermore found that the growth of a single septuple-layer of \(MnBi_2Te_4\) on the surface of \(Bi_2Te_3\) can be facilitated. By using X-ray absorption and circular magnetic dichroism (XAS, XMCD), the magnetic properties of \(MnBi_2Te_4\) are explored down to the monolayer limit. The layered nature of the vdW crystal and a strong uniaxial magnetocrystalline anisotropy establish stable out-of plane magnetic order at the surface of \(MnBi_2Te_4\), which is stable even down to the 2D limit. Pushing the material system to there, i.e. a single SL \(MnBi_2Te_4\) further allows to study the phase transition of this 2D ferromagnet and extract its critical behaviour with \(T_c \, = \, 14.89~k\) and \(\beta \, = \, 0.484\). Utilizing bulk crystals of the ferromagnetic \(Fe_3GeTe_2\) as substrate allows to influence, enhance and bias the magnetism in the single SL of \(MnBi_2Te_4\). By growing heterostructures of the type \(MnBi_2Te_4\) -- n layer \(Bi_2Te_3\) -- \(Fe_3GeTe_2\)for n between 0 and 2, it is shown, that a considerable magnetic coupling can be introduced between the \(MnBi_2Te_4\) top-layer and the substrate. Finally the interplay between topology and magnetism in the ferromagnetic extension is studied directly by angle-resolved photoemission spectroscopy. The heterostructure is found to host a linearly dispersing TSS at the centre of the Brillouin zone. Using low temperature and high-resolution ARPES a large magnetic gap opening of \(\sim\) 35 meV is found at the Dirac point of the TSS. By following its temperature evolution, it is apparent that the scaling behaviour coincides with the magnetic order parameter of the modified surface.},
  subject      = {Molekularstrahlepitaxie},
  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}
}
@article{DawoodBreuerStebanietal.2023,
  author    = {Dawood, Peter and Breuer, Felix and Stebani, Jannik and Burd, Paul and Homolya, Istv{\´a}n and Oberberger, Johannes and Jakob, Peter M. and Blaimer, Martin},
  title     = {Iterative training of robust k-space interpolation networks for improved image reconstruction with limited scan specific training samples},
  series = {Magnetic Resonance in Medicine},
  volume    = {89},
  journal   = {Magnetic Resonance in Medicine},
  number    = {2},
  doi       = {10.1002/mrm.29482},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-312306},
  pages     = {812 -- 827},
  year      = {2023},
  abstract  = {To evaluate an iterative learning approach for enhanced performance of robust artificial-neural-networks for k-space interpolation (RAKI), when only a limited amount of training data (auto-calibration signals [ACS]) are available for accelerated standard 2D imaging. Methods In a first step, the RAKI model was tailored for the case of limited training data amount. In the iterative learning approach (termed iterative RAKI [iRAKI]), the tailored RAKI model is initially trained using original and augmented ACS obtained from a linear parallel imaging reconstruction. Subsequently, the RAKI convolution filters are refined iteratively using original and augmented ACS extracted from the previous RAKI reconstruction. Evaluation was carried out on 200 retrospectively undersampled in vivo datasets from the fastMRI neuro database with different contrast settings. Results For limited training data (18 and 22 ACS lines for R = 4 and R = 5, respectively), iRAKI outperforms standard RAKI by reducing residual artifacts and yields better noise suppression when compared to standard parallel imaging, underlined by quantitative reconstruction quality metrics. Additionally, iRAKI shows better performance than both GRAPPA and standard RAKI in case of pre-scan calibration with varying contrast between training- and undersampled data. Conclusion RAKI benefits from the iterative learning approach, which preserves the noise suppression feature, but requires less original training data for the accurate reconstruction of standard 2D images thereby improving net acceleration.},
  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}
}
@article{GrueneLondiGillettetal.2023,
  author    = {Gr{\"u}ne, Jeannine and Londi, Giacomo and Gillett, Alexander J. and St{\"a}hly, Basil and Lulei, Sebastian and Kotova, Maria and Olivier, Yoann and Dyakonov, Vladimir and Sperlich, Andreas},
  title     = {Triplet Excitons and Associated Efficiency-Limiting Pathways in Organic Solar Cell Blends Based on (Non-) Halogenated PBDB-T and Y-Series},
  series = {Advanced Functional Materials},
  volume    = {33},
  journal   = {Advanced Functional Materials},
  number    = {12},
  doi       = {10.1002/adfm.202212640},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-312164},
  year      = {2023},
  abstract  = {The great progress in organic photovoltaics (OPV) over the past few years has been largely achieved by the development of non-fullerene acceptors (NFAs), with power conversion efficiencies now approaching 20\%. To further improve device performance, loss mechanisms must be identified and minimized. Triplet states are known to adversely affect device performance, since they can form energetically trapped excitons on low-lying states that are responsible for non-radiative losses or even device degradation. Halogenation of OPV materials has long been employed to tailor energy levels and to enhance open circuit voltage. Yet, the influence on recombination to triplet excitons has been largely unexplored. Using the complementary spin-sensitive methods of photoluminescence detected magnetic resonance and transient electron paramagnetic resonance corroborated by transient absorption and quantum-chemical calculations, exciton pathways in OPV blends are unravelled employing the polymer donors PBDB-T, PM6, and PM7 together with NFAs Y6 and Y7. All blends reveal triplet excitons on the NFA populated via non-geminate hole back transfer and, in blends with halogenated donors, also by spin-orbit coupling driven intersystem crossing. Identifying these triplet formation pathways in all tested solar cell absorber films highlights the untapped potential for improved charge generation to further increase plateauing OPV efficiencies.},
  language  = {en}
}