@phdthesis{Adler2021,
  author    = {Adler, Florian Rudolf},
  title     = {Electronic Correlations in Two-dimensional Triangular Adatom Lattices},
  doi       = {10.25972/OPUS-24175},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-241758},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {Two-dimensional triangular lattices of group IV adatoms on semiconductor substrates provide a rich playground for the investigation of Mott-Hubbard physics. The possibility to combine various types of adatoms and substrates makes members of this material class versatile model systems to study the influence of correlation strength, band filling and spin-orbit coupling on the electronic structure - both experimentally and with dedicated many-body calculation techniques. The latter predict exotic ground states such as chiral superconductivity or spin liquid behavior for these frustrated lattices, however, experimental confirmation is still lacking. In this work, three different systems, namely the \(\alpha\)-phases of Sn/SiC(0001), Pb/Si(111), and potassium-doped Sn/Si(111) are investigated with scanning tunneling microscopy and photoemission spectroscopy in this regard. The results are potentially relevant for spintronic applications or quantum computing. For the novel group IV triangular lattice Sn/SiC(0001), a combined experimental and theoretical study reveals that the system features surprisingly strong electronic correlations because they are boosted by the substrate through its partly ionic character and weak screening capabilities. Interestingly, the spectral function, measured for the first time via angle-resolved photoemission, does not show any additional superstructure beyond the intrinsic \(\sqrt{3} \times \sqrt{3} R30^{\circ}\) reconstruction, thereby raising curiosity regarding the ground-state spin pattern. For Pb/Si(111), preceding studies have noted a phase transition of the surface reconstruction from \(\sqrt{3} \times \sqrt{3} R30^{\circ}\) to \(3 \times 3\) at 86 K. In this thesis, investigations of the low-temperature phase with high-resolution scanning tunneling microscopy and spectroscopy unveil the formation of a charge-ordered ground state. It is disentangled from a concomitant structural rearrangement which is found to be 2-up/1-down, in contrast to previous predictions. Applying an extended variational cluster approach, a phase diagram of local and nonlocal Coulomb interactions is mapped out. Based on a comparison of theoretical spectral functions with scattering vectors found via quasiparticle interference, Pb/Si(111) is placed in said phase diagram and electronic correlations are found to be the driving force of the charge-ordered state. In order to realize a doped Mott insulator in a frustrated geometry, potassium was evaporated onto the well-known correlated Sn/Si(111) system. Instead of the expected insulator-to-metal transition, scanning tunneling spectroscopy data indicates that the electronic structure of Sn/Si(111) is only affected locally around potassium atoms while a metallization is suppressed. The potassium atoms were found to be adsorbed on empty \(T_4\) sites of the substrate which eventually leads to the formation of two types of K-Sn alloys with a relative potassium content of 1/3 and 1/2, respectively. Complementary measurements of the spectral function via angle-resolved photoemission reveal that the lower Hubbard band of Sn/Si(111) gradually changes its shape upon potassium deposition. Once the tin and potassium portion on the surface are equal, this evolution is complete and the system can be described as a band insulator without the need to include Coulomb interactions.},
  subject      = {Rastertunnelmikroskopie},
  language  = {en}
}
@article{AndelovicWinterJakobetal.2021,
  author    = {Andelovic, Kristina and Winter, Patrick and Jakob, Peter Michael and Bauer, Wolfgang Rudolf and Herold, Volker and Zernecke, Alma},
  title     = {Evaluation of plaque characteristics and inflammation using magnetic resonance imaging},
  series = {Biomedicines},
  volume    = {9},
  journal   = {Biomedicines},
  number    = {2},
  issn      = {2227-9059},
  doi       = {10.3390/biomedicines9020185},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-228839},
  year      = {2021},
  abstract  = {Atherosclerosis is an inflammatory disease of large and medium-sized arteries, characterized by the growth of atherosclerotic lesions (plaques). These plaques often develop at inner curvatures of arteries, branchpoints, and bifurcations, where the endothelial wall shear stress is low and oscillatory. In conjunction with other processes such as lipid deposition, biomechanical factors lead to local vascular inflammation and plaque growth. There is also evidence that low and oscillatory shear stress contribute to arterial remodeling, entailing a loss in arterial elasticity and, therefore, an increased pulse-wave velocity. Although altered shear stress profiles, elasticity and inflammation are closely intertwined and critical for plaque growth, preclinical and clinical investigations for atherosclerosis mostly focus on the investigation of one of these parameters only due to the experimental limitations. However, cardiovascular magnetic resonance imaging (MRI) has been demonstrated to be a potent tool which can be used to provide insights into a large range of biological parameters in one experimental session. It enables the evaluation of the dynamic process of atherosclerotic lesion formation without the need for harmful radiation. Flow-sensitive MRI provides the assessment of hemodynamic parameters such as wall shear stress and pulse wave velocity which may replace invasive and radiation-based techniques for imaging of the vascular function and the characterization of early plaque development. In combination with inflammation imaging, the analyses and correlations of these parameters could not only significantly advance basic preclinical investigations of atherosclerotic lesion formation and progression, but also the diagnostic clinical evaluation for early identification of high-risk plaques, which are prone to rupture. In this review, we summarize the key applications of magnetic resonance imaging for the evaluation of plaque characteristics through flow sensitive and morphological measurements. The simultaneous measurements of functional and structural parameters will further preclinical research on atherosclerosis and has the potential to fundamentally improve the detection of inflammation and vulnerable plaques in patients.},
  language  = {en}
}
@article{AndelovicWinterKampfetal.2021,
  author    = {Andelovic, Kristina and Winter, Patrick and Kampf, Thomas and Xu, Anton and Jakob, Peter Michael and Herold, Volker and Bauer, Wolfgang Rudolf and Zernecke, Alma},
  title     = {2D Projection Maps of WSS and OSI Reveal Distinct Spatiotemporal Changes in Hemodynamics in the Murine Aorta during Ageing and Atherosclerosis},
  series = {Biomedicines},
  volume    = {9},
  journal   = {Biomedicines},
  number    = {12},
  issn      = {2227-9059},
  doi       = {10.3390/biomedicines9121856},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-252164},
  year      = {2021},
  abstract  = {Growth, ageing and atherosclerotic plaque development alter the biomechanical forces acting on the vessel wall. However, monitoring the detailed local changes in wall shear stress (WSS) at distinct sites of the murine aortic arch over time has been challenging. Here, we studied the temporal and spatial changes in flow, WSS, oscillatory shear index (OSI) and elastic properties of healthy wildtype (WT, n = 5) and atherosclerotic apolipoprotein E-deficient (Apoe\(^{-/-}\), n = 6) mice during ageing and atherosclerosis using high-resolution 4D flow magnetic resonance imaging (MRI). Spatially resolved 2D projection maps of WSS and OSI of the complete aortic arch were generated, allowing the pixel-wise statistical analysis of inter- and intragroup hemodynamic changes over time and local correlations between WSS, pulse wave velocity (PWV), plaque and vessel wall characteristics. The study revealed converse differences of local hemodynamic profiles in healthy WT and atherosclerotic Apoe\(^{-/-}\) mice, and we identified the circumferential WSS as potential marker of plaque size and composition in advanced atherosclerosis and the radial strain as a potential marker for vascular elasticity. Two-dimensional (2D) projection maps of WSS and OSI, including statistical analysis provide a powerful tool to monitor local aortic hemodynamics during ageing and atherosclerosis. The correlation of spatially resolved hemodynamics and plaque characteristics could significantly improve our understanding of the impact of hemodynamics on atherosclerosis, which may be key to understand plaque progression towards vulnerability.},
  language  = {en}
}
@phdthesis{Balles2021,
  author    = {Balles, Andreas},
  title     = {In-line phase contrast and grating interferometry at a liquid-metal-jet source with micrometer resolution},
  doi       = {10.25972/OPUS-23591},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-235917},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {As a non-destructive testing method, X-ray imaging has proved to be suitable for the examination of a variety of objects. The measurement principle is based on the attenuation of X-rays caused by these objects. This attenuation can be recorded as shades of intensity using X-ray detectors and thus contains information about the inner structure of the investigated object. Since X-rays are electromagnetic waves, they also experience a change of phase in addition to their attenuation while penetrating an object. In general, imaging methods based on this effect are referred to as phase contrast imaging techniques. In the laboratory, the two mainly used methods are the propagation based phase contrast or in-line phase contrast and the grating interferometry. While in-line phase contrast - under certain conditions - shows edge enhancement at interfaces due to interference, phase contrast in the grating interferometry is only indirectly measurable by the use of several gratings. In addition to phase contrast, grating interferometry provides access to the so-called dark-field imaging contrast, which measures the scattering of X-rays caused by an object. These two imaging techniques, together with a novel concept of laboratory X-ray sources, the liquid-metal-jet, form the main part of this work. Compared to conventional X-ray sources, the liquid-metal-jet source offers higher brightness. The term brightness is defined by the number of X-ray photons per second, emitting area (area of the X-ray spot) and solid angle at which they are emitted. On the basis of this source, a high resolution in-line phase contrast setup was partially developed in the scope of this work. Several computed tomographies show the feasibility of in-line phase contrast and the improvement of image quality by applying phase retrieval algorithms. Moreover, the determination of optimized sample positions for in-line phase contrast imaging is treated at which the edge enhancement is maximized. Based on primitive fiber objects, this optimization has proven to be a good approximation. With its high brightness in combination with a high spatial coherence, the liquid-metal-jet source is also interesting for grating interferometry. The development of such a setup is also part of this work. The overall concept and the characterization of the setup is presented as well as the applicability and its limits for the investigation of various objects. Due to the very unique concept of this grating interferometer it was possible to realize a modified interferometer system by using a single grating only. Its concept and results are also presented in this work. Furthermore, a grating interferometer based on a microfocus X-ray tube was tested regarding its performance. Thereby, parameters like the anode material, acquisition geometry and gratings were altered in order to find the advantages and disadvantages of each configuration.},
  subject      = {Phasenkontrastverfahren},
  language  = {en}
}
@phdthesis{Bathon2021,
  author    = {Bathon, Thomas},
  title     = {Gezielte Manipulation Topologischer Isolatoren},
  doi       = {10.25972/OPUS-23920},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-239204},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {Neue physikalische Erkenntnisse vervollst{\"a}ndigen die Sicht auf die Welt und erschließen gleichzeitig Wege f{\"u}r Folgeexperimente und technische Anwendungen. Das letzte Jahrzehnt der Festk{\"o}rperforschung war vom zunehmenden Fokus der theoretischen und experimentellen Erkundung topologischer Materialien gepr{\"a}gt. Eine fundamentale Eigenschaft ist ihre Resistenz gegen{\"u}ber solchen St{\"o}rungen, welche spezielle physikalische Symmetrien nicht verletzen. Insbesondere die Topologischen Isolatoren - Halbleiter mit isolierenden Volumen- sowie gleichzeitig leitenden und spinpolarisierten Oberfl{\"a}chenzust{\"a}nden - sind vielversprechende Kandidaten zur Realisierung breitgef{\"a}cherter spintronischer Einsatzgebiete. Bis zur Verwirklichung von Quantencomputern und anderer, heute noch exotisch anmutender Konzepte bedarf es allerdings ein umfassenderes Verst{\"a}ndnis der grundlegenden, physikalischen Zusammenh{\"a}nge. Diese kommen vor allem an Grenzfl{\"a}chen zum Tragen, weshalb oberfl{\"a}chensensitive Methoden bei der Entdeckung der Topologischen Isolatoren eine wichtige Rolle spielten. Im Rahmen dieser Arbeit werden daher strukturelle, elektronische und magnetische Eigenschaften Topologischer Isolatoren mittels Tieftemperatur-Rastertunnelmikroskopie und -spektroskopie sowie begleitenden Methoden untersucht. Die Ver{\"a}nderung der Element-Ausgangskonzentration w{\"a}hrend dem Wachstum des prototypischen Topologischen Isolators Bi2Te3 f{\"u}hrt zur Realisierung eines topologischen p-n {\"U}bergangs innerhalb des Kristalls. Bei einem spezifischen Verh{\"a}ltnis von Bi zu Te in der Schmelze kommt es aufgrund unterschiedlicher Erstarrungstemperaturen der Komponenten zu einer Ansammlung von Bi- und Te-reichen Gegenden an den gegen{\"u}berliegenden Enden des Kristalls. In diesen bildet sich infolge des jeweiligen Element{\"u}berschusses durch Kristallersetzungen und -fehlstellen eine Dotierung des Materials aus. Daraus resultiert die Existenz eines {\"U}bergangsbereiches, welcher durch Transportmessungen verifiziert werden kann. Mit der r{\"a}umlich aufl{\"o}senden Rastertunnelmikroskopie wird diese Gegend lokalisiert und strukturell sowie elektronisch untersucht. Innerhalb des {\"U}bergangsbereiches treten charakteristische Kristalldefekte beider Arten auf - eine Defektunterdr{\"u}ckung bleibt folglich aus. Dennoch ist dort der Beitrag der Defekte zum Stromtransport aufgrund ihres gegens{\"a}tzlichen Dotiercharakters vernachl{\"a}ssigbar, sodass der topologische Oberfl{\"a}chenzustand die maßgeblichen physikalischen Eigenschaften bestimmt. Dar{\"u}ber hinaus tritt der {\"U}bergangsbereich in energetischen und r{\"a}umlichen Gr{\"o}ßenordnungen auf, die Anwendungen bei Raumtemperatur denkbar machen. Neben der Ver{\"a}nderung Topologischer Isolatoren durch den gezielten Einsatz intrinsischer Kristalldefekte bieten magnetische St{\"o}rungen die M{\"o}glichkeit zur Pr{\"u}fung des topologischen Oberfl{\"a}chenzustandes auf dessen Widerstandsf{\"a}higkeit sowie der gegenseitigen Wechselwirkungen. Die Zeitumkehrinvarianz ist urs{\"a}chlich f{\"u}r den topologischen Schutz des Oberfl{\"a}chenzustandes, weshalb magnetische Oberfl{\"a}chen- und Volumendotierung diese Symmetrie brechen und zu neuartigem Verhalten f{\"u}hren kann. Die Oberfl{\"a}chendotierung Topologischer Isolatoren kann zu einer starken Bandverbiegung und einer energetischen Verschiebung des Fermi-Niveaus f{\"u}hren. Bei einer wohldosierten Menge der Adatome auf p-dotiertem Bi2Te3 kommt die Fermi-Energie innerhalb der Volumenzustands-Bandl{\"u}cke zum Liegen. Folglich wird bei Energien rund um das Fermi-Niveau lediglich der topologische Oberfl{\"a}chenzustand bev{\"o}lkert, welcher eine Wechselwirkung zwischen den Adatomen vermitteln kann. F{\"u}r Mn-Adatome kann R{\"u}ckstreuung beobachtet werden, die aufgrund der Zeitumkehrinvarianz in undotierten Topologischen Isolatoren verboten ist. Die {\"u}berraschenderweise starken und fokussierten Streuintensit{\"a}ten {\"u}ber mesoskopische Distanzen hinweg resultieren aus der ferromagnetischen Kopplung nahegelegener Adsorbate, was durch theoretische Berechnungen und R{\"o}ntgendichroismus-Untersuchungen best{\"a}tigt wird. Gleichwohl wird f{\"u}r die Proben ein superparamagnetisches Verhalten beobachtet. Im Gegensatz dazu f{\"u}hrt die ausreichende Volumendotierung von Sb2Te3 mit V-Atomen zu einem weitreichend ferromagnetischen Verhalten. Erstaunlicherweise kann trotz der weitl{\"a}ufig verbreiteten Theorie Zeitumkehrinvarianz-gebrochener Dirac-Zust{\"a}nde und der experimentellen Entdeckung des Anormalen Quanten-Hall-Effektes in {\"a}hnlichen Probensystemen keinerlei Anzeichen einer spektroskopischen Bandl{\"u}cke beobachtet werden. Dies ist eine direkte Auswirkung der dualen Natur der magnetischen Adatome: W{\"a}hrend sie einerseits eine magnetisch induzierte Bandl{\"u}cke {\"o}ffnen, besetzen sie diese durch St{\"o}rstellenresonanzen wieder. Ihr stark lokaler Charakter kann durch die Aufnahme ihrer r{\"a}umlichen Verteilung aufgezeichnet werden und f{\"u}hrt zu einer Mobilit{\"a}ts-Bandl{\"u}cke, deren Indizien durch vergleichende Untersuchungen an undotiertem und dotiertem Sb2Te3 best{\"a}tigt werden.},
  subject      = {Rastertunnelmikroskopie},
  language  = {de}
}
@article{BeierleinEgorovHarderetal.2021,
  author    = {Beierlein, J. and Egorov, O. A. and Harder, T. H. and Gagel, P. and Emmerling, M. and Schneider, C. and H{\"o}fling, S. and Peschel, U. and Klembt, S.},
  title     = {Bloch Oscillations of Hybrid Light-Matter Particles in a Waveguide Array},
  series = {Advanced Optical Materials},
  volume    = {9},
  journal   = {Advanced Optical Materials},
  number    = {13},
  doi       = {10.1002/adom.202100126},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-239814},
  year      = {2021},
  abstract  = {Bloch oscillations are a phenomenon well known from quantum mechanics where electrons in a lattice experience an oscillatory motion in the presence of an electric field gradient. Here, the authors report on Bloch oscillations of hybrid light-matter particles, called exciton-polaritons (polaritons), being confined in an array of coupled microcavity waveguides. To this end, the waveguide widths and their mutual couplings are carefully designed such that a constant energy gradient is induced perpendicular to the direction of motion of the propagating polaritons. This technique allows us to directly observe and study Bloch oscillations in real- and momentum-space. Furthermore, the experimental findings are supported by numerical simulations based on a modified Gross-Pitaevskii approach. This work provides an important transfer of basic concepts of quantum mechanics to integrated solid state devices, using quantum fluids of light.},
  language  = {en}
}
@phdthesis{Bunzmann2021,
  author    = {Bunzmann, Nikolai Eberhard},
  title     = {Excited State Pathways in 3rd Generation Organic Light-Emitting Diodes},
  doi       = {10.25972/OPUS-22078},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-220786},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {This work revealed spin states that are involved in the light generation of organic light-emitting diodes (OLEDs) that are based on thermally activated delayed fluorescence (TADF). First, several donor:acceptor-based TADF systems forming exciplex states were investigated. Afterwards, a TADF emitter that shows intramolecular charge transfer states but also forms exciplex states with a proper donor molecule was studied. The primary experimental technique was electron paramagnetic resonance (EPR), in particular the advanced methods electroluminescence detected magnetic resonance (ELDMR), photoluminescence detected magnetic resonance (PLDMR) and electrically detected magnetic resonance (EDMR). Additional information was gathered from time-resolved and continuous wave photoluminescence measurements.},
  subject      = {Elektronenspinresonanz},
  language  = {en}
}
@article{BunzmannKrugmannWeissenseeletal.2021,
  author    = {Bunzmann, Nikolai and Krugmann, Benjamin and Weissenseel, Sebastian and Kudriashova, Liudmila and Ivaniuk, Khrystyna and Stakhira, Pavlo and Cherpak, Vladyslav and Chapran, Marian and Grybauskaite-Kaminskiene, Gintare and Grazulevicius, Juozas Vidas and Dyakonov, Vladimir and Sperlich, Andreas},
  title     = {Spin- and Voltage-Dependent Emission from Intra- and Intermolecular TADF OLEDs},
  series = {Advanced Electronic Materials},
  volume    = {7},
  journal   = {Advanced Electronic Materials},
  number    = {3},
  doi       = {10.1002/aelm.202000702},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-224434},
  year      = {2021},
  abstract  = {Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) utilize molecular systems with a small energy splitting between singlet and triplet states. This can either be realized in intramolecular charge transfer states of molecules with near-orthogonal donor and acceptor moieties or in intermolecular exciplex states formed between a suitable combination of individual donor and acceptor materials. Here, 4,4′-(9H,9′H-[3,3′-bicarbazole]-9,9′-diyl)bis(3-(trifluoromethyl) benzonitrile) (pCNBCzoCF\(_{3}\)) is investigated, which shows intramolecular TADF but can also form exciplex states in combination with 4,4′,4′′-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Orange emitting exciplex-based OLEDs additionally generate a sky-blue emission from the intramolecular emitter with an intensity that can be voltage-controlled. Electroluminescence detected magnetic resonance (ELDMR) is applied to study the thermally activated spin-dependent triplet to singlet up-conversion in operating devices. Thereby, intermediate excited states involved in OLED operation can be investigated and the corresponding activation energy for both, intra- and intermolecular based TADF can be derived. Furthermore, a lower estimate is given for the extent of the triplet wavefunction to be ≥ 1.2 nm. Photoluminescence detected magnetic resonance (PLDMR) reveals the population of molecular triplets in optically excited thin films. Overall, the findings allow to draw a comprehensive picture of the spin-dependent emission from intra- and intermolecular TADF OLEDs.},
  language  = {en}
}
@phdthesis{EliasdosSantos2021,
  author    = {Elias dos Santos, Graciely},
  title     = {Spin-Orbit Torques and Galvanomagnetic Effects Generated by the 3D Topological Insulator HgTe},
  doi       = {10.25972/OPUS-24797},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-247971},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {In meiner Dissertation besch{\"a}ftigte ich mich mit der Frage, ob der 3D topologische Isolator Quecksilbertellurid (3D TI HgTe) ein geeignetes Material f{\"u}r Spintronik-Anwendungen ist. Wir untersuchten Spin-Bahn-Drehmomente, die auf Elektronen beim Tunneln zwischen HgTe und einem angrenzenden Ferromagneten (Permalloy) einwirken. Zun{\"a}chst setzten wir die Methode der Ferromagnetresonanz (SOT-FMR) f{\"u}r diese Untersuchungen ein. Im ersten Teil der Dissertation werden die Leser in die mathematische Beschreibung von Spin- Bahn-Drehmomenten in einem Hybridsystem bestehend aus topologischem Isolator (TI) und Ferromagnet (FM) eingef{\"u}hrt. Des Weiteren werden die Probenherstellung und der Messaufbau f{\"u}r SOT-FMR Messungen besprochen. Unsere SOT-FMR Messungen ergaben, dass bei tiefen Temperaturen (T = 4.2 K) die Normalkomponente (bezogen auf der TI-Oberfl{\"a}che) des Drehmoments groß war. Bei Raumtemperatur konnten im Signal beide Komponenten (parallel und normal zur TI-Oberfl{\"a}che) beobachtet werden. Aus der Symmetrie der Mixing-Spannung (Abbildungen 3.14 und 3.15) schlossen wir, dass 3D TI HgTe ein Spin-Bahn-Drehmoment auf das Elektronensystem des Permalloys {\"u}bertr{\"a}gt. Unsere Untersuchungen zeigten dar{\"u}ber hinaus, dass die Effizienz dieser {\"U}bertragung mit der anderer vorhandener topologischen Isolatoren vergleichbar ist (siehe Abb. 3.17). Abschließend wurden parasit{\"a}re Effekte bei der Absch{\"a}tzung des Spin-Bahn-Drehmoments bzw. andere Interpretationen des Messsignals und seiner Komponenten (z.B., Thermospannungen) ausf{\"u}hrlich diskutiert. Obwohl die hier gezeigten Ergebnisse vermehrt darauf hinweisen, dass der 3D TI HgTe m{\"o}glicherweise effizient f{\"u}r die Anwendung von Spin-Drehmomenten in angrezenden Ferromagneten ist [1], wird dem Leser weiderholt klargemacht, dass parasit{\"a}re Effekte eventuelle das korrekte Schreiben und Lesen der Information in Ferromagneten verunreignigt. Diese sollten auch bei der Interpretation von publizierten Resultaten besonders hohen Spin-Bahn-Drehmoment{\"u}bertragungen in der Literatur ber{\"u}cksichtigt werden [1-3]. Die Nachteile der SOT-FMR-Messmethode f{\"u}hrten zu einerWeiterentwicklung unseres Messkonzepts, bei dem der Ferromagnet durch eine Spin-Valve-Struktur ersetzt wurde. In dieser Messanordnung ist der Stromfluss durch den 3D TI im Gegensatz zu den vorangegangenen Messungen bekannt und die Widerstands{\"a}nderung der Spin-Valve-Struktur kann durch den GMR-Effekt ausgelesen werden. Die Ausrichtung der Magnetisierung des Ferromagneten in den SOT-FMR-Experimenten erforderte es, ein magnetisches Feld von bis zu 300 mT parallel zur TI-Oberfl{\"a}che anzulegen. Motiviert durch diesen Umstand, untersuchten wir den Einfluss eines parallelen Magnetfelds auf den Magnetowiderstand in 3D TI HgTe. Die {\"u}berraschenden Resultate dieser Messungen werden im zweiten Teil der Dissertation beschrieben. Obwohl nichtmagnetisches Quecksilbertellurid untersucht wurde, oszillierte der transversale Magnetowiderstand (Rxy) mit dem Winkel � zwischen der Magnetfeldrichtung (parallel zur Oberfl{\"a}che) und der elektrischen Stromflussrichtung im topologischen Isolator. Dieser Effekt ist eine typische Eigenschaft von ferromagnetischen Materialien und wird planarer Hall-Effekt (PHE) genannt[4, 5]. Magnetowiderstands- (MR-)Oszillationen wurden ebenfalls sowohl im L{\"a}ngswiderstand (Rxx) und im transversalen Widerstand (Rxy) {\"u}ber einen weiten Bereich von magnetischen Feldst{\"a}rken und Ladungstr{\"a}gerdichten des topologischen Isolators beobachtet. Der PHE wurde bereits zuvor in einem anderen TI-Material (Bi2-xSbxTe3) beschrieben [6]. Als physikalischer Mechanismus wurde von den Autoren Elektronenstreuung an magnetisch polarisierten Streuzentren vorgeschlagen. Wir diskutierten sowohl diesen Erkl{\"a}rungsansatz als auch andere Theorievorschl{\"a}ge in der Literatur [7, 8] kritisch. In dieser Doktorarbeit haben wir versucht, der PHE des 3D TI HgTe durch die Asymmetrie in der Bandstruktur dieses Materials zu erkl{\"a}ren. In k.p Bandstrukturrechnungen mit einer 6-Orbital-Basis zeigten wir, dass das Zwischenspiel von Rashba- und Dresselhaus-Spin-Bahn- Wechselwirkung mit dem magnetischen Feld parallel zur TI-Oberfl{\"a}che zu einer Verformung der Fermikontur des Valenzbands von 3D TI-HgTe f{\"u}hrt, welche ihrerseits eine Anisotropie des Leitf{\"a}higkeit bedingt. Die ben{\"o}tigten Magnetfeldst{\"a}rken in diesem Modell waren mit bis zu 40 T jedoch etwa eine Gr{\"o}ßenordnung gr{\"o}ßer als jene in unseren Experimenten. Des Weiteren lieferte eine direkte Berechnung der Zustandsdichten f{\"u}r Bin k I und Bin ? I bisher keine klaren Resultate. Die komplizierte Abh{\"a}ngigkeit der Rashba-Spin-Bahn-Kopplung f{\"u}r p-leitendes HgTe [9] machte es außerdem schwierig, diesen Term in die Bandstrukturrechnung zu inkludieren. Trotz umfangreicher Bem{\"u}hungen, den Ursprung der galvanomagnetischen Effekte im 3D TI HgTe zu verstehen, konnte in dieser Arbeit der Mechanismus des PHE und der MR-Oszillationen nicht eindeutig bestimmt werden. Es gelang jedoch, einige aus der Literatur bekannte Theorien f{\"u}r den PHE und die MR-Oszillationseffekte in topologischen Isolatoren auszuschließen. Die Herausforderung, eine vollst{\"a}ndige theoretische Beschreibung zu entwickeln, die allen experimentellen Aspekten (PHE, Gatespannungsabh{\"a}ngigkeit und MR-Oszillationen) gerecht wird, bleibt weiter bestehen. Abschließend m{\"o}chte die Autorin ihre Hoffnung ausdr{\"u}cken, den Lesern die Komplexit{\"a}t der Fragestellung n{\"a}her gebracht zu haben und sie in die Kunst elektrischer Messungen an topologischen Isolatoren bei angelegtem parallelem Magnetfeld initiiert zu haben.},
  language  = {en}
}
@article{GottschollDiezSoltamovetal.2021,
  author    = {Gottscholl, Andreas and Diez, Matthias and Soltamov, Victor and Kasper, Christian and Krauße, Dominik and Sperlich, Andreas and Kianinia, Mehran and Bradac, Carlo and Aharonovich, Igor and Dyakonov, Vladimir},
  title     = {Spin defects in hBN as promising temperature, pressure and magnetic field quantum sensors},
  series = {Nature Communications},
  volume    = {12},
  journal   = {Nature Communications},
  number    = {1},
  doi       = {10.1038/s41467-021-24725-1},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-261581},
  year      = {2021},
  abstract  = {Spin defects in solid-state materials are strong candidate systems for quantum information technology and sensing applications. Here we explore in details the recently discovered negatively charged boron vacancies (V\(_B\)\(^-\)) in hexagonal boron nitride (hBN) and demonstrate their use as atomic scale sensors for temperature, magnetic fields and externally applied pressure. These applications are possible due to the high-spin triplet ground state and bright spin-dependent photoluminescence of the V\(_B\)\(^-\). Specifically, we find that the frequency shift in optically detected magnetic resonance measurements is not only sensitive to static magnetic fields, but also to temperature and pressure changes which we relate to crystal lattice parameters. We show that spin-rich hBN films are potentially applicable as intrinsic sensors in heterostructures made of functionalized 2D materials.},
  language  = {en}
}
@article{Graetz2021,
  author    = {Graetz, Jonas},
  title     = {Simulation study towards quantitative X-ray and neutron tensor tomography regarding the validity of linear approximations of dark-field anisotropy},
  series = {Scientific Reports},
  volume    = {11},
  journal   = {Scientific Reports},
  doi       = {10.1038/s41598-021-97389-y},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-261844},
  year      = {2021},
  abstract  = {Tensor tomography is fundamentally based on the assumption of a both anisotropic and linear contrast mechanism. While the X-ray or neutron dark-field contrast obtained with Talbot(-Lau) interferometers features the required anisotropy, a preceding detailed study of dark-field signal origination however found its specific orientation dependence to be a non-linear function of the underlying anisotropic mass distribution and its orientation, especially challenging the common assumption that dark-field signals are describable by a function over the unit sphere. Here, two approximative linear tensor models with reduced orientation dependence are investigated in a simulation study with regard to their applicability to grating based X-ray or neutron dark-field tensor tomography. By systematically simulating and reconstructing a large sample of isolated volume elements covering the full range of feasible anisotropies and orientations, direct correspondences are drawn between the respective tensors characterizing the physically based dark-field model used for signal synthesization and the mathematically motivated simplified models used for reconstruction. The anisotropy of freely rotating volume elements is thereby confirmed to be, for practical reconstruction purposes, approximable both as a function of the optical axis' orientation or as a function of the interferometer's grating orientation. The eigenvalues of the surrogate models' tensors are found to exhibit fuzzy, yet almost linear relations to those of the synthesization model. Dominant orientations are found to be recoverable with a margin of error on the order of magnitude of 1 degrees. Although the input data must adequately address the full orientation dependence of dark-field anisotropy, the present results clearly support the general feasibility of quantitative X-ray dark-field tensor tomography within an inherent yet acceptable statistical margin of uncertainty.},
  language  = {en}
}
@phdthesis{Hammer2021,
  author    = {Hammer, Sebastian Tobias},
  title     = {Influence of Crystal Structure on Excited States in Crystalline Organic Semiconductors},
  doi       = {10.25972/OPUS-24401},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-244019},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {This thesis focused on the influence of the underlying crystal structure and hence, of the mutual molecular orientation, on the excited states in ordered molecular aggregates. For this purpose, two model systems have been investigated. In the prototypical donor-acceptor complex pentacene-perfluoropentacene (PEN-PFP) the optical accessibility of the charge transfer state and the possibility to fabricate highly defined interfaces by means of single crystal templates enabled a deep understanding of the spatial anisotropy of the charge transfer state formation. Transferring the obtained insights to the design of prototypical donor-acceptor devices, the importance of interface control to minimize the occurrence of charge transfer traps and thereby, to improve the device performance, could be demonstrated. The use of zinc phthalocyanine (ZnPc) allowed for the examination of the influence of molecular packing on the excited electronic states without a change in molecular species by virtue of its inherent polymorphism. Combining structural investigations, optical absorption and emission spectroscopy, as well as Franck-Condon modeling of emission spectra revealed the nature of the optical excited state emission in relation to the structural \(\alpha \) and \(\beta \) phase over a wide temperature range from 4 K to 300 K. As a results, the phase transition kinetics of the first order \(\alpha \rightarrow \beta\) phase transition were characterized in depth and applied to the fabrication of prototypical dual luminescent OLEDs.},
  subject      = {Organischer Halbleiter},
  language  = {en}
}
@phdthesis{Kasper2021,
  author    = {Kasper, Christian Andreas},
  title     = {Engineering of Highly Coherent Silicon Vacancy Defects in Silicon Carbide},
  doi       = {10.25972/OPUS-23779},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-237797},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {In this work the creation of silicon vacancy spin defects in silicon carbide with predictable properties is demonstrated. Neutron and electron irradiation was used to create silicon vacancy ensembles and proton beam writing to create isolated vacancies at a desired position. The coherence properties of the created silicon vacancies as a function of the emitter density were investigated and a power-law function established. Sample annealing was implemented to increase the coherence properties of existing silicon vacancies. Further, spectral hole burning was used to implement absolute dc-magnetometry.},
  subject      = {St{\"o}rstelle},
  language  = {en}
}
@article{KiermaschFischerGilEscrigetal.2021,
  author    = {Kiermasch, David and Fischer, Mathias and Gil-Escrig, Lid{\´o}n and Baumann, Andreas and Bolink, Henk J. and Dyakonov, Vladimir and Tvingstedt, Kristofer},
  title     = {Reduced Recombination Losses in Evaporated Perovskite Solar Cells by Postfabrication Treatment},
  series = {Solar RRL},
  volume    = {5},
  journal   = {Solar RRL},
  number    = {11},
  doi       = {10.1002/solr.202100400},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-258003},
  year      = {2021},
  abstract  = {The photovoltaic perovskite research community has now developed a large set of tools and techniques to improve the power conversion efficiency (PCE). One such arcane trick is to allow the finished devices to dwell in time, and the PCE often improves. Herein, a mild postannealing procedure is implemented on coevaporated perovskite solar cells confirming a substantial PCE improvement, mainly attributed to an increased open-circuit voltage (V\(_{OC}\)). From a V\(_{OC}\) of around 1.11 V directly after preparation, the voltage improves to more than 1.18 V by temporal and thermal annealing. To clarify the origin of this annealing effect, an in-depth device experimental and simulation characterization is conducted. A simultaneous reduction of the dark saturation current, the ideality factor (n\(_{id}\)), and the leakage current is revealed, signifying a substantial impact of the postannealing procedure on recombination losses. To investigate the carrier dynamics in more detail, a set of transient optoelectrical methods is first evaluated, ascertaining that the bulk carrier lifetime is increased with device annealing. Second, a drift-diffusion simulation is used, confirming that the beneficial effect of the annealing has its origin in effective bulk trap passivation that accordingly leads to a reduction of Shockley-Read-Hall recombination rates.},
  language  = {en}
}
@phdthesis{Leisegang2021,
  author    = {Leisegang, Markus},
  title     = {Eine neue Methode zur Detektion ballistischen Transports im Rastertunnelmikroskop: Die Molekulare Nanosonde},
  doi       = {10.25972/OPUS-25076},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-250762},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {Verlustarmer Ladungstr{\"a}gertransport ist f{\"u}r die Realisierung effizienter und kleiner elektronischer Bauteile von großem Interesse. Dies hilft entstehende W{\"a}rme zu minimieren und den Energieverbrauch gleichzeitig zu reduzieren. Einzelne Streuprozesse, die den Verlust bei Ladungstr{\"a}gertransport bestimmen, laufen jedoch auf L{\"a}ngenskalen von Nano- bis Mikrometern ab. Um diese detailliert untersuchen zu k{\"o}nnen, bedarf es Messmethoden mit hoher zeitlicher oder {\"o}rtlicher Aufl{\"o}sung. F{\"u}r Letztere gibt es wenige etablierte Experimente, h{\"a}ufig basierend auf der Rastertunnelmikroskopie, welche jedoch verschiedenen Einschr{\"a}nkungen unterliegen. Um die M{\"o}glichkeiten der Detektion von Ladungstr{\"a}gertransport auf Distanzen der mittleren freien Wegl{\"a}nge und damit im ballistischen Regime zu verbessern, wurde im Rahmen dieser Dissertation die Molekulare Nanosonde charakterisiert und etabliert. Diese Messmethode nutzt ein einzelnes Molek{\"u}l als Detektor f{\"u}r Ladungstr{\"a}ger, welche mit der Sondenspitze des Rastertunnelmikroskops (RTM) wenige Nanometer entfernt vom Molek{\"u}l in das untersuchte Substrat injiziert werden. Die hohe Aufl{\"o}sung des RTM in Kombination mit der geringen Ausdehnung des molekularen Detektors erm{\"o}glicht dabei atomare Kontrolle von Transportpfaden {\"u}ber wenige Nanometer. Der erste Teil dieser Arbeit widmet sich der Charakterisierung der Molekularen Nanosonde. Hierf{\"u}r werden zun{\"a}chst die elektronischen Eigenschaften dreier Phthalocyanine mittels Rastertunnelspektroskpie untersucht, welche im Folgenden zur Charakterisierung des Molek{\"u}ls als Detektor Anwendung finden. Die anschließende Analyse der Potentiallandschaft der Tautomerisation von H2Pc und HPc zeigt, dass die NH- Streckschwinung einem effizienten Schaltprozess zu Grunde liegt. Darauf basierend wird der Einfluss der Umgebung anhand von einzelnen Adatomen sowie des Substrats selbst auf den molekularen Schalter analysiert. In beiden F{\"a}llen zeigt sich eine signifikante {\"A}nderung der Potentiallandschaft der Tautomerisation. Anschließend wird der Einfluss geometrischer Eigenschaften des Molek{\"u}ls selbst untersucht, wobei sich eine Entkopplung vom Substrat auf Grund von dreidimensionalen tert-Butyl-Substituenten ergibt. Zus{\"a}tzlich zeigt sich bei dem Vergleich von Naphthalocyanin zu Phthalocyanin der Einfluss lateraler Ausdehnung auf die Detektionsfl{\"a}che, was einen nicht-punktf{\"o}rmigen Detektor best{\"a}tigt. Im letzten Abschnitt werden zwei Anwendungen der Molekularen Nanosonde pr{\"a}sentiert. Zun{\"a}chst wird mit Phthalocyanin auf Ag(111) demonstriert, dass die Interferenz von ballistischen Ladungstr{\"a}gern auf Distanzen von wenigen Nanometern mit dieser Technik detektierbar ist. Im zweiten Teil zeigt sich, dass der ballistische Transport auf einer Pd(110)-Oberfl{\"a}che durch die anisotrope Reihenstruktur auf atomarer Skala moduliert wird.},
  subject      = {Rastertunnelmikroskopie},
  language  = {de}
}
@phdthesis{Martin2021,
  author    = {Martin, Konstantin},
  title     = {Current-induced Magnetization Switching by a generated Spin-Orbit Torque in the 3D Topological Insulator Material HgTe},
  doi       = {10.25972/OPUS-24049},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-240490},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {Magnetic random access memory (MRAM) technology aims to replace dynamic RAM (DRAM) due to its significantly lower power consumption and non-volatility [Dong08]. During the last couple of years the commercial focus was set on spin-transfer torque MRAM (STT-MRAM) systems, where a current is pushed through a ferromagnetic (FM) free layer and a reference layer which are separated by an insulator. The free layer can be set to parallel or anti-parallel depending on the current direction [Kim11]. Unfortunately these currents have to be quite high which could lead to damages of the tunnel barrier of the magnetic tunnel junction resulting in higher power consumption as well as reliability issues. At this point a new effect, where the current is passed below the ferromagnetic layer stack, can be exploited to change the direction of the free layer magnetization. The effect is known as spin-orbit torque (SOT) and describes the transfer of angular momentum onto an adjacent magnetization either by the spin Hall effect (SHE) or inverse spin galvanic effect (iSGE) [Manchon19]. The latter describes a spin accumulation due to a current. This is similar to the process of spin accumulation in TIs, where a current corresponds to an effective spin due to spin-momentum locking [Qi11]. Thus TIs exhibit a high current-to-spin conversion rate, which makes them a promising material system for SOT experiments. Among all TIs it is HgTe, which can be reliably grown as an insulator. This thesis covers the development of a working device for SOT measurements (SOT-device) in a CdTe/CdHgTe/HgTe/CdHgTe heterostructure. It involves the development of a tunnel barrier (ZrOx) as well as the investigation of the behavior of a ferromagnetic layer stack on top of etched HgTe. The main result of this work is the successful construction and evaluation of a working SOT-device, which exhibits the up to date most efficient switching of in-plane magnetized ferromagnetic layer stacks. In order to avoid hybridization between HgTe and the adjacent ferromagnetic atoms, which would cause a breakdown of the topological surface state, it is necessary to implement a thin tunnel barrier in between the TI and free layer [Zhang16]. Aside from hybridization a tunnel barrier avoids shunting of the current, that is pushed on the surface of the HgTe/CdHgTe interface. Thus a bigger part of the current can be used for spin accumulation and, at the same time, the resistance measurement of the ferromagnetic layer stack is not perturbed. In chapter 3 the focus is set on investigating the tunneling characteristics of ZrOx on top of dry etched HgTe. Thin barriers are used as the interaction of the current generated spin and the adjacent magnetization decreases with distance. On the other hand too small insulator thicknesses lead to leakage currents which disturb heavily the measurement of the resistance of the ferromagnetic layer stack. Thus an optimum thickness of 10 ALD cycles (\(d\approx 1.6\rm\, nm\)) is determined which yields a resistance area product of \(R\cdot A \approx 3\rm\, k\Omega\mu m^{2}\). This corresponds to a tunneling resistance of \(R_{T}\approx 20\rm\, k\Omega\) over a structure surface of \(A_{T} = 0.12\rm\, \mu m^2\). Multiple samples with different thicknesses have been produced. All samples have been examined on their tunneling behavior. The resistance area product as a function of thickness shows a linear behavior on a logarithmic scale. Furthermore all working samples show non-linear I-V curves as well as parabolic dI/dV-curves. Additionally the tunneling resistance \(R_{T}\) increases with decreasing temperature. All above mentioned properties are typical for tunnel barriers which do not include pinholes [Jonsson00]. The last part of chapter 3 deals with thermal properties of HgTe. By measuring the second harmonic of a biasing AC current in the channel below the tunnel barrier it is attempted to extract the diffusion thermopower of the heated electrons. Unfortunately the measured signal showed a far superior contribution of the first harmonic. According to electric circuit simulations a small asymmetry in the barrier (penetration and leaving point of electrons) could be responsible for this behavior. A ferromagnetic layer stack, consisting of PY/Cu/CoFe, serves as a sensor for magnetization changes due to external fields and current induced spin accumulations. The layer stack exhibits a giant magnetoresistance (GMR) which has been measured by a resistance bridge. The biggest peculiarity in depositing a GMR stack on top of HgTe is that its easy axis forms along only one of the crystal axes (\((110)\) or \((1\overline{1}0)\)). The reason for this anisotropy is still unclear. Sources such as an influence of the terminating material, miscut, furrows during IBE or sputter ripples have been ruled out. It can be speculated that the surface states due to HgTe might have an influence on the development of this easy axis but this would need further investigation. A consequence of this unexpected anisotropy is that every CdTe/CdHgTe/HgTe/CdHgTe wafer has first to be characterized in SQUID in order to find the easy axis. A ferromagnetic resonance (FMR) measurement confirmed this observation. The shape of the ferromagnetic layer stack is chosen to be an ellipse in order to support the easy axis direction by shape anisotropy. Over 8 million ellipses are used to generate a SQUID signal of \(m > 10^{-5}\rm\, emu\). This is sufficient to extract the main characteristics of an average nano pillar under the influence of an external magnetic field. As in the case of bigger structures the ellipse shaped structure shows a step-like behavior. A measured minor loop confirms the existence of the irreversible anti-parallel stable magnetic state. Furthermore this state persists for both directions at \(m=0\) resulting in an anti-ferromagnetic coupling between Py and CoFe. The geometry of the SOT-device is chosen in such a way that the current induced spin aligns either parallel or anti-parallel to the effective magnetic field \(\vec{B}_{eff}=\vec{B}_{ext}+\vec{B}_{aniso}+\vec{B}_{shape}\), which acts on the pillar. Due to interaction of the spin with the adjacent magnetization of Py the magnetization direction gets changed by a torque \(\vec{T}\). In general this torque can be decomposed into two components a field-like torque \(\vec{\tau}_{FL}\) and a damping-like torque \(\vec{\tau}_{DL}\) [Manchon19]. In the case of TIs \(\vec{T}\) is additionally depending on the z-component of \(\vec{m}\) [Ndiaye17]. In our case the magnetization is lying in the sample plane (\(m_{z}=0\)) which results in \(\vec{\tau}_{DL}=0\). Thus, in the case of \(\vec{S}\parallel\left(\vec{\hat{z}}\times\vec{j}\right)\) and \(\vec{j}\parallel\vec{\hat{y}}\), the only spin dependent effective magnetic field is \(\vec{B}_{FL}=\tau_{FL}\cdot\vec{\hat{x}}\) which is lying parallel or anti-parallel to \(\vec{B}_{eff}\). The evaluation of \(\vec{B}_{FL}\) can therefore be done in the following manner. First a high \(B_{ext}\) has to be set along the easy axis of the pillar. Then \(B_{ext}\) has to be reduced just a few \(\rm\, Oe\) before the switching occurs at the magnetic field \(B_{ext,0}\). At the magnetic field \(\Delta B = B_{ext}-B_{ext,0}\approx 0.5\rm\, Oe\) the lower resistive state should be stable over a longer time range (\(10-30\rm\, min\)) in order to exclude switching due to fluctuations. Now a positive or negative current can be pushed through the channel below the pillar. For one of the two current directions the magnetization of Py switches. It is therefore not a thermal effect that drives the change of \(\vec{m}\). Current densities that are able to switch \(\vec{m}\) at small \(\Delta B\neq 0\) lie in the range of \(j\approx 10^{4}\rm\, A/cm^{2}\). In all experiments the switching efficiency \(\Delta B/j\) decreases with rising \(j\). Furthermore the efficiency as a function of \(j\) depends on the temperature as \(\Delta B/j\) values tend to be up to 20 times higher at \(T=1.8\rm\, K\) and \(j\approx 0\) than at \(T=4.2\rm\, K\). This temperature dependence suggests that switching occurs not due to Oersted fields. Furthermore the Biot-Savart fields had been calculated for four different models: an infinite long rectangular wire, two infinite planes, a full volume and two thin volume planes. Every model shows an efficiency, which is at least three times lower than the observation. The highest efficiencies in our samples show up to 10 times higher values than in heavy-metal/ferromagnets heterostructures. In contrast to measurement procedures of most other groups our method leads to direct determination of SOT parameters like the effective magnetic field \(\vec{B}_{FL}\). Other groups make use of spin-transfer FMR (ST-FMR) where they AC bias their structure and extract SOT parameters (like \(\tau_{FL}\) and \(\tau_{DL}\)) from second harmonics by fitting theoretical models. Material systems consisting of TIs and magnetic insulators (MIs) on the other hand show 10 times higher efficiencies [Khang18,Li19]. In those cases the magnetization points out of the sample plane which is conceptually different from in-plane magnetic anisotropy geometries like in our case. The greatest benefit in-plane magnetic anisotropy systems is its easy realisation [Bhatti17]. Here only an elliptical shape has to be lithographically implemented instead of conducting research on the appropriate combination of material systems that result in perpendicular magnetic anisotropies [Apalkov16]. Despite the fact that in our case only \(\vec{\tau}_{FL}\) acts as the driving force for changing \(m\) our device still exhibits the up to date highest efficiencies in the class of in-plane magnetized anisotropies of all material classes ever recorded.},
  language  = {en}
}
@phdthesis{Metzger2021,
  author    = {Metzger, Christian Thomas Peter},
  title     = {Development of photoemission spectroscopy techniques for the determination of the electronic and geometric structure of organic adsorbates},
  doi       = {10.25972/OPUS-22952},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-229525},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {The projects presented in this thesis cover the examination of the electronic and structural properties of organic thin films at noble metal-organic interfaces. Angle-resolved photoemission spectroscopy is used as the primary investigative tool due to the connection of the emitted photoelectrons to the electronic structure of the sample. The surveyed materials are of relevance for fundamental research and practical applications on their own, but also serve as archetypes for the photoemission techniques presented throughout the four main chapters of this thesis. The techniques are therefore outlined with their adaptation to other systems in mind and a special focus on the proper description of the final state. The most basic description of the final state that is still adequate for the evaluation of photoemission data is a plane wave. Its simplicity enables a relatively intuitive interpretation of photoemission data, since the initial and final state are related to one another by a Fourier transform and a geometric factor in this approximation. Moreover, the initial states of some systems can be reconstructed in three dimensions by combining photoemission measurements at various excitation energies. This reconstruction can even be carried out solely based on experimental data by using suitable iterative algorithms. Since the approximation of the final state in the photoemission process by a plane wave is not valid in all instances, knowledge on the limitations of its applicability is indispensable. This can be gained by a comparison to experimental data as well as calculations with a more detailed description of the photoemission final state. One possible appraoch is based on independently emitting atoms where the coherent superposition of partial, atomic final states produces the total final state. This approach can also be used for more intricate studies on organic thin films. To this end, experimental data can be related to theoretical calculations to gain extensive insights into the structural and electronic properties of molecules in organic thin films.},
  subject      = {ARPES},
  language  = {en}
}
@article{MuellerGraetzBallesetal.2021,
  author    = {M{\"u}ller, Dominik and Graetz, Jonas and Balles, Andreas and Stier, Simon and Hanke, Randolf and Fella, Christian},
  title     = {Laboratory-Based Nano-Computed Tomography and Examples of Its Application in the Field of Materials Research},
  series = {Crystals},
  volume    = {11},
  journal   = {Crystals},
  number    = {6},
  issn      = {2073-4352},
  doi       = {10.3390/cryst11060677},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-241048},
  year      = {2021},
  abstract  = {In a comprehensive study, we demonstrate the performance and typical application scenarios for laboratory-based nano-computed tomography in materials research on various samples. Specifically, we focus on a projection magnification system with a nano focus source. The imaging resolution is quantified with common 2D test structures and validated in 3D applications by means of the Fourier Shell Correlation. As representative application examples from nowadays material research, we show metallization processes in multilayer integrated circuits, aging in lithium battery electrodes, and volumetric of metallic sub-micrometer fillers of composites. Thus, the laboratory system provides the unique possibility to image non-destructively structures in the range of 170-190 nanometers, even for high-density materials.},
  language  = {en}
}
@article{OpolkaMuellerFellaetal.2021,
  author    = {Opolka, Alexander and M{\"u}ller, Dominik and Fella, Christian and Balles, Andreas and Mohr, J{\"u}rgen and Last, Arndt},
  title     = {Multi-lens array full-field X-ray microscopy},
  series = {Applied Sciences},
  volume    = {11},
  journal   = {Applied Sciences},
  number    = {16},
  issn      = {2076-3417},
  doi       = {10.3390/app11167234},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-244974},
  year      = {2021},
  abstract  = {X-ray full-field microscopy at laboratory sources for photon energies above 10 keV suffers from either long exposure times or low resolution. The photon flux is mainly limited by the objectives used, having a limited numerical aperture NA. We show that this can be overcome by making use of the cone-beam illumination of laboratory sources by imaging the same field of view (FoV) several times under slightly different angles using an array of X-ray lenses. Using this technique, the exposure time can be reduced drastically without any loss in terms of resolution. A proof-of-principle is given using an existing laboratory metal-jet source at the 9.25 keV Ga K\(_α\)-line and compared to a ray-tracing simulation of the setup.},
  language  = {en}
}
@phdthesis{Scheuermann2021,
  author    = {Scheuermann, Julian},
  title     = {Interbandkaskadenlaser f{\"u}r Anwendungen in der Absorptionsspektroskopie},
  doi       = {10.25972/OPUS-25179},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-251797},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {Das Ziel dieser Arbeit war die Entwicklung und Weiterentwicklung von Laserlichtquellen basierend auf der Interbandkaskadentechnologie in einem Wellenl{\"a}ngenbereich von ca. 3 bis 6 µm. Der Fokus lag dabei auf der Entwicklung von Kantenemitter-Halbleiterlasern, welche bei verschiedensten Emissionswellenl{\"a}ngen erfolgreich hergestellt werden konnten. Dabei wurde auf jeweilige Herausforderungen eingegangen, welche entweder durch die Herstellung selbst oder der anwendungstechnischen Zielsetzung bedingt war. Im Rahmen dieser Arbeit wurden verschiedene, spektral einzelmodige Halbleiterlaser im angesprochenen Wellenl{\"a}ngenbereich entwickelt und hergestellt. Basierend auf dem jeweiligen Epitaxiematerial und der angestrebten Emissionswellenl{\"a}nge wurden Simulationen der optischen Lasermode durchgef{\"u}hrt und die grundlegenden f{\"u}r die Herstellung notwendigen Parameter bestimmt und experimentell umgesetzt. Des Weiteren wurden die verwendeten Verfahren f{\"u}r den jeweiligen Herstellungsprozess angepasst und optimiert. Das umfasst die in den ersten Kapiteln beschriebenen Schritte wie optische Lithografie, Elektronenstrahllithografie, reaktives Trocken{\"a}tzen und verschiedene Arten der Materialdeposition. Mit einer Emissionswellenl{\"a}nge von 2,8 µm wurde beispielsweise der bislang kurzwelligste bei Raumtemperatur im Dauerstrichbetrieb betriebene einzelmodige Interbandkaskadenlaser hergestellt. Dessen Leistungsmerkmale sind mit Diodenlasern im entsprechenden Emissionsbereich vergleichbar. Somit erg{\"a}nzt die Interbandkaskadentechnologie bestehende Technologien nahtlos und es ist eine l{\"u}ckenlose Wellenl{\"a}ngenabdeckung bis in den mittleren Infrarotbereich m{\"o}glich. Je nach Herstellungsprozess wurde außerdem auf die verteilte R{\"u}ckkopplung eingegangen und die Leistungsf{\"a}higkeit des verwendeten Metallgitterkonzeptes anhand von Messungen an spektral einzelmodigen Bauteile aufgezeigt. Es wurden aber auch die je nach Zielsetzung unterschiedlichen Herausforderungen aufgezeigt und diskutiert. F{\"u}r eine Anwendung wurden spezielle Laserchips mit zwei einzelmodigen Emissionswellenl{\"a}ngen bei 3928 nm und 4009 nm entwickelt. Die beiden Wellenl{\"a}ngen sind f{\"u}r die Detektion von Schwefeldioxid und Schwefelwasserstoff geeignet, welche zur {\"U}berwachung und Optimierung der Schwefelgewinnung durch das Claus-Verfahren notwendig sind. Bei der Umsetzung wurden auf einzelnen Chips zwei Laseremitter in einem Abstand von 70 µm platziert und mit je einem Metallgitter versehen. Das verwendete Epitaxiematerial war so konzipiert, dass es optimal f{\"u}r beide Zielwellenl{\"a}ngen verwendet werden kann. Die geforderten Eigenschaften wurden erf{\"u}llt und die Bauteile konnten erfolgreich hergestellt werden. Die Emissionseigenschaften und das spektrale Verhalten wurde bei beiden Zielwellenl{\"a}ngen bestimmt. Einzeln betrachtet erf{\"u}llen beide Emitter die notwendigen Eigenschaften um f{\"u}r spektroskopische Anwendungen eingesetzt werden zu k{\"o}nnen. Erg{\"a}nzend wurde zum einen das Abstimmverhalten der Emissionswellenl{\"a}nge in Abh{\"a}ngigkeit der Modulationsfrequenz des Betriebsstromes untersucht und zus{\"a}tzlich die thermische Abh{\"a}ngigkeit der Betriebsparameter beider Kan{\"a}le zueinander bestimmt. Diese Abh{\"a}ngigkeit ist f{\"u}r eine simultane Messung mit beiden Kan{\"a}len notwendig. Das Konzept mit mehreren Stegwellenleitern pro Laserchip wurde in einem weiteren Fall noch st{\"a}rker ausgearbeitet. Denn je nach Komplexit{\"a}t eines Gasgemisches sind zur Bestimmung der einzelnen Komponenten mehr Messpunkte bzw. Wellenl{\"a}ngen notwendig. Im zweiten Fall ist die Analyse der Kohlenwasserstoffe Methan, Ethan, Propan, Butan, Iso-Butan, Pentan und Iso-Pentan von Interesse, welche als Hauptbestandteile von Erdgas z.B. in Erdgasaufbereitungsanlagen oder zur Bestimmung des Heizwertes analysiert werden m{\"u}ssen. Die genannten Kohlenwasserstoffe zeigen ein starkes Absorptionsverhalten im Wellenl{\"a}ngenbereich von 3,3 bis 3,5 µm. Auf dem entsprechend angepassten Interbandkaskadenmaterial wurden Bauteile mit neun Wellenleitern pro Laserchip hergestellt. Mithilfe der neun einzelmodigen Emissionskan{\"a}le konnte ein Bereich von bis zu 190 nm (21 meV, 167 cm-1) adressiert werden. Außerdem wurde der sich mit zunehmender Wellenl{\"a}nge {\"a}ndernde Schichtaufbau und dessen Einfluss auf die Bauteileigenschaften diskutiert. Die Leistungsdaten der langwelligsten Epitaxie waren im Vergleich deutlich schw{\"a}cher. Um diesen Nachteil zu kompensieren, wurde eine spezielle Wellenleitergeometrie mit doppeltem Steg genutzt. Die Eigenschaften des Konzeptes wurden zuerst mittels Simulation untersucht und ein entsprechendes Herstellungsverfahren entwickelt. Mit der Simulation als Grundlage wurden die verschiedenen Prozessparameter {\"u}ber mehrere Prozessl{\"a}ufe iterativ optimiert und somit die Performance der Laser verbessert. Auch mit diesem Verfahren konnte ausreichende Kopplung an das Metallgitter erzielt werden. Abschließend wurden mit diesem Herstellungsverfahren einzelmodige Laser im Wellenl{\"a}ngenbereich von 5,9 bis {\"u}ber 6 Mikrometern realisiert. Diese Laser emittierten im Dauerstrichbetrieb bei einer maximalen Betriebstemperatur von -2 °C. Insgesamt wurde anhand der im Rahmen dieser Arbeit entwickelten Bauteilen und de ren Charakterisierung gezeigt, dass diese die Anforderungen von TLAS Anwendungen erf{\"u}llen. Jedoch konnte nur auf einen Teil der M{\"o}glichkeiten eingegangen werden, den die Interbandkaskadentechnologie bietet, denn die angesprochenen Einsatzgebiete stellen nur einzelne grundlegende M{\"o}glichkeiten dieser Technologie mit Schwerpunkt auf laserbasierte Lichtquellen dar. Zusammenfassend kann allerdings gesagt werden, dass sich die Interbandkaskadentechnologie etabliert hat. Gerade durch die gezeigten Leistungsdaten bei den Wellenl{\"a}ngen um 2,9 µm, 3,4 µm und 4,0 µm im Dauerstrichbetrieb bei Raumtemperatur wird ersichtlich, dass im Bereich der Sensorik die ICL Technologie in Bezug auf niedriger Strom- bzw. Leistungsaufnahme quasi konkurrenzlos ist. Sicherlich werden die Anwendungsgebiete in Zukunft noch vielf{\"a}ltiger. Denn es sind auf jeden Fall weitere Fortschritte in Richtung h{\"o}herer Emissionswellenl{\"a}ngen, deutlich h{\"o}herer Betriebstemperaturen, verbreiterte Emissionsbereiche oder g{\"a}nzlich andere Bauteil Konzepte wie z.B. f{\"u}r Frequenzk{\"a}mme bzw. Terahertz Anwendungen zu erwarten. Diese Entwicklung betrifft nicht nur den Einsatz als Lichtquelle, denn auch Interbandkaskadendetektoren bzw. Solarzellen wurden schon realisiert und werden weiterentwickelt.},
  subject      = {Halbleiterlaser},
  language  = {de}
}