@phdthesis{Winnerlein2020, author = {Winnerlein, Martin}, title = {Molecular Beam Epitaxy and Characterization of the Magnetic Topological Insulator (V,Bi,Sb)\(_2\)Te\(_3\)}, doi = {10.25972/OPUS-21166}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-211666}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2020}, abstract = {The subject of this thesis is the fabrication and characterization of magnetic topological insulator layers of (V,Bi,Sb)\(_2\)Te\(_3\) exhibiting the quantum anomalous Hall effect. A major task was the experimental realization of the quantum anomalous Hall effect, which is only observed in layers with very specific structural, electronic and magnetic properties. These properties and their influence on the quantum anomalous Hall effect are analyzed in detail. First, the optimal conditions for the growth of pure Bi\(_2\)Te\(_3\) and Sb\(_2\)Te\(_3\) crystal layers and the resulting structural quality are studied. The crystalline quality of Bi\(_2\)Te\(_3\) improves significantly at higher growth temperatures resulting in a small mosaicity-tilt and reduced twinning defects. The optimal growth temperature is determined as 260\(^{\circ}\)C, low enough to avoid desorption while maintaining a high crystalline quality. The crystalline quality of Sb\(_2\)Te\(_3\) is less dependent on the growth temperature. Temperatures below 230\(^{\circ}\)C are necessary to avoid significant material desorption, though. Especially for the nucleation on Si(111)-H, a low sticking coefficient is observed preventing the coalescence of islands into a homogeneous layer. The influence of the substrate type, miscut and annealing sequence on the growth of Bi\(_2\)Te\(_3\) layers is investigated. The alignment of the layer changes depending on the miscut angle and annealing sequence: Typically, layer planes align parallel to the Si(111) planes. This can enhance the twin suppression due to transfer of the stacking order from the substrate to the layer at step edges, but results in a step bunched layer morphology. For specific substrate preparations, however, the layer planes are observed to align parallel to the surface plane. This alignment avoids displacement at the step edges, which would cause anti-phase domains. This results in narrow Bragg peaks in XRD rocking curve scans due to long-range order in the absence of anti-phase domains. Furthermore, the use of rough Fe:InP(111):B substrates leads to a strong reduction of twinning defects and a significantly reduced mosaicity-twist due to the smaller lattice mismatch. Next, the magnetically doped mixed compound V\(_z\)(Bi\(_{1-x}\)Sb\(_x\))\(_{2-z}\)Te\(_3\) is studied in order to realize the quantum anomalous Hall effect. The addition of V and Bi to Sb\(_2\)Te\(_3\) leads to efficient nucleation on the Si(111)-H surface and a closed, homogeneous layer. Magneto-transport measurements of layers reveal a finite anomalous Hall resistivity significantly below the von Klitzing constant. The observation of the quantum anomalous Hall effect requires the complete suppression of parasitic bulklike conduction due to defect induced carriers. This can be achieved by optimizing the thickness, composition and growth conditions of the layers. The growth temperature is observed to strongly influence the structural quality. Elevated temperatures result in bigger islands, improved crystallographic orientation and reduced twinning. On the other hand, desorption of primarily Sb is observed, affecting the thickness, composition and reproducibility of the layers. At 190\(^{\circ}\)C, desorption is avoided enabling precise control of layer thickness and composition of the quaternary compound while maintaining a high structural quality. It is especially important to optimize the Bi/Sb ratio in the (V,Bi,Sb)\(_2\)Te\(_3\) layers, since by alloying n-type Bi\(_2\)Te\(_3\) and p-type Sb\(_2\)Te\(_3\) charge neutrality is achieved at a specific mixing ratio. This is necessary to shift the Fermi level into the magnetic exchange gap and fully suppress the bulk conduction. The Sb content x furthermore influences the in-plane lattice constant a significantly. This is utilized to accurately determine x even for thin films below 10 nm thickness required for the quantum anomalous Hall effect. Furthermore, x strongly influences the surface morphology: with increasing x the island size decreases and the RMS roughness increases by up to a factor of 4 between x = 0 and x = 1. A series of samples with x varied between 0.56-0.95 is grown, while carefully maintaining a constant thickness of 9 nm and a doping concentration of 2 at.\% V. Magneto-transport measurements reveal the charge neutral point around x = 0.86 at 4.2 K. The maximum of the anomalous Hall resistivity of 0.44 h/e\(^2\) is observed at x = 0.77 close to charge neutrality. Reducing the measurement temperature to 50 mK significantly increases the anomalous Hall resistivity. Several samples in a narrow range of x between 0.76-0.79 show the quantum anomalous Hall effect with the Hall resistivity reaching the von Klitzing constant and a vanishing longitudinal resistivity. Having realized the quantum anomalous Hall effect as the first group in Europe, this breakthrough enabled us to study the electronic and magnetic properties of the samples in close collaborations with other groups. In collaboration with the Physikalisch-Technische Bundesanstalt high-precision measurements were conducted with detailed error analysis yielding a relative de- viation from the von Klitzing constant of (0.17 \(\pm\) 0.25) * 10\(^{-6}\). This is published as the smallest, most precise value at that time, proving the high quality of the provided samples. This result paves the way for the application of magnetic topological insulators as zero-field resistance standards. Non-local magneto-transport measurements were conducted at 15 mK in close collaboration with the transport group in EP3. The results prove that transport happens through chiral edge channels. The detailed analysis of small anomalies in transport measurements reveals instabilities in the magnetic phase even at 15 mK. Their time dependent nature indicates the presence of superparamagnetic contributions in the nominally ferromagnetic phase. Next, the influence of the capping layer and the substrate type on structural properties and the impact on the quantum anomalous Hall effect is investigated. To this end, a layer was grown on a semi-insulating Fe:InP(111)B substrate using the previously optimized growth conditions. The crystalline quality is improved significantly with the mosaicity twist reduced from 5.4\(^{\circ}\) to 1.0\(^{\circ}\). Furthermore, a layer without protective capping layer was grown on Si and studied after providing sufficient time for degradation. The uncapped layer on Si shows perfect quantization, while the layer on InP deviates by about 5\%. This may be caused by the higher crystalline quality, but variations in e.g. Sb content cannot be ruled out as the cause. Overall, the quantum anomalous Hall effect seems robust against changes in substrate and capping layer with only little deviations. Furthermore, the dependence of the quantum anomalous Hall effect on the thickness of the layers is investigated. Between 5-8 nm thickness the material typically transitions from a 2D topological insulator with hybridized top and bottom surface states to a 3D topological insulator. A set of samples with 6 nm, 8 nm, and 9 nm thickness exhibits the quantum anomalous Hall effect, while 5 nm and 15 nm thick layers show significant bulk contributions. The analysis of the longitudinal and Hall conductivity during the reversal of magnetization reveals distinct differences between different thicknesses. The 6 nm thick layer shows scaling consistent with the integer quantum Hall effect, while the 9 nm thick layer shows scaling expected for the topological surface states of a 3D topological insulator. The unique scaling of the 9 nm thick layer is of particular interest as it may be a result of axion electrodynamics in a 3D topological insulator. Subsequently, the influence of V doping on the structural and magnetic properties of the host material is studied systematically. Similarly to Bi alloying, increased V doping seems to flatten the layer surface significantly. With increasing V content, Te bonding partners are observed to increase simultaneously in a 2:3 ratio as expected for V incorporation on group-V sites. The linear contraction of the in-plane and out-of-plane lattice constants with increasing V doping is quantitatively consistent with the incorporation of V\(^{3+}\) ions, possibly mixed with V\(^{4+}\) ions, at the group-V sites. This is consistent with SQUID measurements showing a magnetization of 1.3 \(\mu_B\) per V ion. Finally, magnetically doped topological insulator heterostructures are fabricated and studied in magneto-transport. Trilayer heterostructures with a non-magnetic (Bi,Sb)\(_2\)Te\(_3\) layer sandwiched between two magnetically doped layers are predicted to host the axion insulator state if the two magnetic layers are decoupled and in antiparallel configuration. Magneto-transport measurements of such a trilayer heterostructure with 7 nm undoped (Bi,Sb)\(_2\)Te\(_3\) between 2 nm thick layers doped with 1.5 at.\% V exhibit a zero Hall plateau representing an insulating state. Similar results in the literature were interpreted as axion insulator state, but in the absence of a measurement showing the antiparallel magnetic orientation other explanations for the insulating state cannot be ruled out. Furthermore, heterostructures including a 2 nm thin, highly V doped layer region show an anomalous Hall effect of opposite sign compared to previous samples. A dependency on the thickness and position of the doped layer region is observed, which indicates that scattering at the interfaces causes contributions to the anomalous Hall effect of opposite sign compared to bulk scattering effects. Many interesting phenomena in quantum anomalous Hall insulators as well as axion insulators are still not unambiguously observed. This includes Majorana bound states in quantum anomalous Hall insulator/superconductor hybrid systems and the topological magneto-electric effect in axion insulators. The limited observation temperature of the quantum anomalous Hall effect of below 1 K could be increased in 3D topological insulator/magnetic insulator heterostructures which utilize the magnetic proximity effect. The main achievement of this thesis is the reproducible growth and characterization of (V,Bi,Sb)2Te3 layers exhibiting the quantum anomalous Hall effect. The detailed study of the structural requirements of the quantum anomalous Hall effect and the observation of the unique axionic scaling behavior in 3D magnetic topological insulator layers leads to a better understanding of the nature of this new quantum state. The high-precision measurements of the quantum anomalous Hall effect reporting the smallest deviation from the von Klitzing constant are an important step towards the realization of a zero-field quantum resistance standard.}, subject = {Bismutverbindungen}, language = {en} } @phdthesis{Knapp2019, author = {Knapp, Alexander Gerhard}, title = {Resonant Spin Flip Raman-Spectroscopy of Electrons and Manganese-Ions in the n-doped Diluted Magnetic Semiconductor (Zn,Mn)Se:Cl}, doi = {10.25972/OPUS-18609}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-186099}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {Main focus of the present dissertation was to gain new insight about the interaction between magnetic ions and the conduction band of diluted magnetic semiconductors. This interaction in magnetic semiconductors with carrier concentrations near the metal-insulator transition (MIT) in an external magnetic field is barely researched. Hence, n-doped Zn1-xMnxSe:Cl samples were studied. Resonant Raman spectroscopy was employed at an external magnetic field between 1T and 7T and a temperature of 1.5K. The resulting magnetization of the material amplifies the splitting of states with opposite spins both in the valence and the conduction band. This is known as the "giant-Zeeman-effect". In this thesis, the resonance of the electron spin flip process, i.e. the enhancement of the signal depending on the excitation energy, was used as an indicator to determine the density of states of the charge carriers. The measured resonance profiles of each sample showed a structure, which consist of two partially overlapping Gaussian curves. The analysis of the Gaussian curves revealed that their respective maxima are separated independent of the magnetic field strenght by about 5 meV, which matches the binding energy of the donor bound exciton (D0, X). A widening of the full width at half maximum of the resonance profile was observed with increasing magnetic field. A detailed analysis of this behavior showed that the donor bound exciton spin flip resonance primarily accounts for the widening for all samples with doping concentrations below the metal insulator transition. A model was proposed for the interpretation of this observation. This is based on the fundamental assumptions of a spatially random distribution of the manganese ions on the group-II sublattice of the ZnSe crystal and the finite extension of the excitons. Thus, each exciton covers an individual quantity of manganese ions, which manifest as a local manganese concentration. This local manganese concentration is normally distributed for a set of excitons and hence, the evaluation of the distribution allows the determination of exciton radii Two trends were identified for the (D0, X) radii. The radius of the bound exciton decreases with increasing carrier concentration as well as with increasing manganese concentration. The determination of the (D0, X) radii by the use of resonant spin flip Raman spectroscopy and also the observation of the behavior of the (D0, X) radius depending on the carrier concentration, was achieved for the first time. For all samples with carrier concentrations below the metal-insulator transition, the obtained (X0) radii are up to a factor of 5.9 larger than the respective (D0, X) radii. This observation is explained by the unbound character of the (X0). For the first time, such an observation could be made by Raman spectroscopy.Beside the resonance studies, the shape of the Raman signal of the electron spin flip was analyzed. Thereby an obvious asymmetry of the signal, with a clear flank to lower Raman shifts, was observed. This asymmetry is most pronounced, when the spin flip process is excited near the (D0, X) resonance. To explain this observation, a theoretical model was introduced in this thesis. Based on the asymmetry of the resonantly excited spin flip signal, it was possible to estimate the (D0, X) radii, too. At external magnetic fields between 1.25T and 7T, the obtained radii lie between 2.38nm and 2.75nm. Additionally, the asymmetry of the electron spin flip signal was observed at different excitation energies. Here it is striking that the asymmetry vanishes with increasing excitation energy. At the highest excitation energy, where the electron spin flip was still detectable, the estimated radius of the exciton is 3.92nm. Beside the observations on the electron spin flip, the resonance behavior of the spin flip processes in the d-shell of the incorporated Mn ions was studied in this thesis. This was performed for the direct Mn spin flip process as well as for the sum process of the longitudinal optical phonon with the Mn spin flip. For the Stokes and anti-Stokes direct spin flip process and for the Stokes sum process, each the resonance curve is described by considering only one resonance mechanism. In contrast, resonance for the sum process in which an anti-Stokes Mn spin flip is involved, consists of two partially overlapping resonances due to different mechanisms. A detailed analysis of this resonance profile showed that for (Zn,Mn)Se at the chosen experimental parameters, an incoming and outgoing resonance can be achieved, separated by a few meV. Hereby, at a specific excitation energy range and a high excitation power, it was possible to achieve an inversion of the anti-Stokes to Stokes intensity, because only the anti-Stokes Mn spin flip process was enhanced resonantly.}, subject = {Raman-Spektroskopie}, language = {en} } @phdthesis{Geissler2017, author = {Geißler, Florian}, title = {Transport properties of helical Luttinger liquids}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-153450}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {The prediction and the experimental discovery of topological insulators has set the stage for a novel type of electronic devices. In contrast to conventional metals or semiconductors, this new class of materials exhibits peculiar transport properties at the sample surface, as conduction channels emerge at the topological boundaries of the system. In specific materials with strong spin-orbit coupling, a particular form of a two-dimensional topological insulator, the quantum spin Hall state, can be observed. Here, the respective one-dimensional edge channels are helical in nature, meaning that there is a locking of the spin orientation of an electron and its direction of motion. Due to the symmetry of time-reversal, elastic backscattering off interspersed impurities is suppressed in such a helical system, and transport is approximately ballistic. This allows in principle for the realization of novel energy-efficient devices, ``spintronic`` applications, or the formation of exotic bound states with non-Abelian statistics, which could be used for quantum computing. The present work is concerned with the general transport properties of one-dimensional helical states. Beyond the topological protection mentioned above, inelastic backscattering can arise from various microscopic sources, of which the most prominent ones will be discussed in this Thesis. As it is characteristic for one-dimensional systems, the role of electron-electron interactions can be of major importance in this context. First, we review well-established techniques of many-body physics in one dimension such as perturbative renormalization group analysis, (Abelian) bosonization, and Luttinger liquid theory. The latter allow us to treat electron interactions in an exact way. Those methods then are employed to derive the corrections to the conductance in a helical transport channel, that arise from various types of perturbations. Particularly, we focus on the interplay of Rashba spin-orbit coupling and electron interactions as a source of inelastic single-particle and two-particle backscattering. It is demonstrated, that microscopic details of the system, such as the existence of a momentum cutoff, that restricts the energy spectrum, or the presence of non-interacting leads attached to the system, can fundamentally alter the transport signature. By comparison of the predicted corrections to the conductance to a transport experiment, one can gain insight about the microscopic processes and the structure of a quantum spin Hall sample. Another important mechanism we analyze is backscattering induced by magnetic moments. Those findings provide an alternative interpretation of recent transport measurements in InAs/GaSb quantum wells.}, subject = {Topologischer Isolator}, language = {en} } @phdthesis{Pfenning2018, author = {Pfenning, Andreas Theo}, title = {Optoelektronische Transportspektroskopie an Resonanztunneldioden-Fotodetektoren}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-163205}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {Die vorliegende Arbeit besch{\"a}ftigt sich mit optoelektronischer Transportspektroskopie verschiedener Resonanztunneldioden (RTDs). Die Arbeit ist thematisch in zwei Schwerpunktee untergliedert. Im ersten Schwerpunkt werden anhand GaAs-basierter RTD-Fotosensoren f{\"u}r den Telekommunikationswellenl{\"a}ngenbereich um 1,3 µm die Akkumulationsdynamiken photogenerierter Minorit{\"a}tsladungstr{\"a}ger und deren Wirkung auf den RTD-Tunnelstrom untersucht. Im zweiten Schwerpunkt werden GaSb-basierte Al(As)Sb/GaSb-Doppelbarrieren-Quantentrog-RTDs in Hinblick auf ihren Raumtemperaturbetrieb entwickelt und erforscht. Diese legen den Grundstein f{\"u}r die sp{\"a}tere Realisation von RTD-Fotodetektoren im mittleren infraroten (MIR) Spektralbereich. Im Folgenden ist eine kurze inhaltliche Zusammenfassung der einzelnen Kapitel gegeben. Kapitel 1 leitet vor dem Hintergrund eines stark steigenden Bedarfs an verl{\"a}sslichen und sensitiven Fotodetektoren f{\"u}r Telekommunikationsanwendungen sowie f{\"u}r die optische Molek{\"u}l- und Gasspektroskopie in das {\"u}bergeordnete Thema der RTD-Fotodetektoren ein. Kapitel 2 erl{\"a}utert ausgew{\"a}hlte physikalische und technische Grundlagen zu RTD-Fotodetektoren. Ausgehend von einem kurzem {\"U}berblick zu RTDs, werden aktuelle Anwendungsgebiete aufgezeigt und die physikalischen Grundlagen elektrischen Transports in RTDs diskutiert. Anschließend werden Grundlagen, Definitionen und charakteristische Kenngr{\"o}ßen optischer Detektoren und Sensoren definiert. Abschließend werden die physikalischen Grundlagen zum Fotostrom in RTDs beschrieben. In Kapitel 3 RTD-Fotosensor zur Lichtdetektion bei 1,3 µm werden AlGaAs/GaAs-Doppelbarrieren-Quantentrog-Resonanztunneldioden (DBQW-RTDs) mit gitterangepasster, quatern{\"a}rer GaInNAs-Absorptionsschicht als Raumtemperatur-Fotodetektoren f{\"u}r den nahen infraroten (NIR) Spektralbereich bei der Telekommunikationswellenl{\"a}nge von λ=1,3 µm untersucht. RTDs sind photosensitive Halbleiterbauteile, die innerhalb der vergangenen Jahre aufgrund ihrer hohen Fotosensitivit{\"a}t und F{\"a}higkeit selbst einzelne Photonen zu detektieren, ein beachtliches Interesse geweckt haben. Die RTD-Fotosensitivit{\"a}t basiert auf einer Coulomb-Wechselwirkung photogenerierter und akkumulierter Ladungstr{\"a}ger. Diese ver{\"a}ndern das lokale elektrostatische Potential und steuern so einen empfindlichen Resonanztunnelstrom. Die Kenntnis der zugrundeliegenden physikalischen Parameter und deren Spannungsabh{\"a}ngigkeit ist essentiell, um optimale Arbeitspunkte und Bauelementdesigns zu identifizieren. Unterkapitel 3.1 gibt einen {\"U}berblick {\"u}ber das Probendesign der untersuchten RTD-Fotodetektoren, deren Fabrikationsprozess sowie eine Erl{\"a}uterung des Fotodetektionsmechanismus. {\"U}ber Tieftemperatur-Elektrolumineszenz-Spektroskopie wird die effektive RTD-Quantentrog-Breite zu d_DBQW≃3,4 nm bestimmt. Die Quantisierungsenergien der Elektron- und Schwerloch-Grundzust{\"a}nde ergeben sich zu E_Γ1≈144 meV und E_hh1≈39 meV. Abschließend wird der in der Arbeit verwendeten Messaufbau skizziert. In Unterkapitel 3.2 werden die physikalischen Parameter, die die RTD-Fotosensitivit{\"a}t bestimmen, auf ihre Spannungsabh{\"a}ngigkeit untersucht. Die Fotostrom-Spannungs-Kennlinie des RTD-Fotodetektors ist nichtlinear und {\"u}ber drei spannungsabh{\"a}ngige Parametern gegeben: der RTD-Quanteneffizienz η(V), der mittleren Lebensdauer photogenerierter und akkumulierter Minorit{\"a}tsladungstr{\"a}ger (L{\"o}cher) τ(V) und der RTD-I(V)-Kennlinie im Dunkeln I_dark (V). Die RTD Quanteneffizienz η(V) kann {\"u}ber eine Gaußsche-Fehlerfunktion modelliert werden, welche beschreibt, dass Lochakkumulation erst nach {\"U}berschreiten einer Schwellspannung stattfindet. Die mittlere Lebensdauer τ(V) f{\"a}llt exponentiell mit zunehmender Spannung V ab. {\"U}ber einen Vergleich mit thermisch limitierten Lebensdauern in Quantentr{\"o}gen k{\"o}nnen Leitungsband- und Valenzband-Offset zu Q_C \≈0,55 und Q_V≈0,45 abgesch{\"a}tzt werden. Basierend auf diesen Ergebnissen wird ein Modell f{\"u}r die Fotostrom-Spannungs-Kennlinie erstellt, das eine elementare Grundlage f{\"u}r die Charakterisierung von RTD-Photodetektoren bildet. In Unterkapitel 3.3 werden die physikalischen Parameter, die die RTD-Fotosensitivit{\"a}t beschr{\"a}nken, detailliert auf ihre Abh{\"a}ngigkeit gegen{\"u}ber der einfallenden Lichtleistung untersucht. Nur f{\"u}r kleine Lichtleistungen wird eine konstante Sensitivit{\"a}t von S_I=5,82×〖10〗^3 A W-1 beobachtet, was einem Multiplikationsfaktor von M=3,30×〖10〗^5 entspricht. F{\"u}r steigende Lichtleistungen f{\"a}llt die Sensitivit{\"a}t um mehrere Gr{\"o}ßenordnungen ab. Die abfallende, nichtkonstante Sensitivit{\"a}t ist maßgeblich einer Reduktion der mittleren Lebensdauer τ zuzuschreiben, die mit steigender Lochpopulation exponentiell abf{\"a}llt. In Kombination mit den Ergebnissen aus Unterkapitel 3.2 wird ein Modell der RTD-Fotosensitivit{\"a}t vorgestellt, das die Grundlage einer Charakterisierung von RTD-Fotodetektoren bildet. Die Ergebnisse k{\"o}nnen genutzt werden, um die kritische Lichtleistung zu bestimmen, bis zu der der RTD-Fotodetektor mit konstanter Sensitivit{\"a}t betrieben werden kann, oder um den idealen Arbeitspunkt f{\"u}r eine minimale rausch{\"a}quivalente Leistung (NEP) zu identifizieren. Dieser liegt f{\"u}r eine durch theoretisches Schrotrauschen limitierte RTD bei einem Wert von NEP=1,41×〖10〗^(-16) W Hz-1/2 bei V=1,5 V. In Kapitel 4 GaSb-basierte Doppelbarrieren-RTDs werden unterschiedliche Al(As)Sb/GaSb-DBQW-RTDs auf ihre elektrische Transporteigenschaften untersucht und erstmalig resonantes Tunneln von Elektronen bei Raumtemperatur in solchen Resonanztunnelstrukturen demonstriert. Unterkapitel 4.1 beschreibt den Wachstums- und der Fabrikationsprozess der untersuchten AlAsSb/GaSb-DBQW-RTDs. In Unterkapitel 4.2 wird Elektronentransport durch eine AlSb/GaSb-DBQW-Resonanztunnelstruktur untersucht. Bei einer Temperatur von T=4,2 K konnte resonantes Tunneln mit bisher unerreicht hohen Resonanz-zu-Talstrom-Verh{\"a}ltnisse von PVCR=20,4 beobachtet werden. Dies wird auf die exzellente Qualit{\"a}t des Halbleiterkristallwachstums und des Fabrikationsprozesses zur{\"u}ckgef{\"u}hrt. Resonantes Tunneln bei Raumtemperatur konnte hingegen nicht beobachtet werden. Dies wird einer Besonderheit des Halbleiters GaSb zugeschrieben, welche daf{\"u}r sorgt, dass bei Raumtemperatur die Mehrheit der Elektronen Zust{\"a}nde am L-Punkt anstelle des Γ Punktes besetzt. Resonantes Tunneln {\"u}ber den klassischen Γ Γ Γ-Tunnelpfad ist so unterbunden. In Unterkapitel 4.3 werden die elektrischen Transporteigenschaften von AlAsSb/GaSb DBQW RTDs mit pseudomorph gewachsenen tern{\"a}ren Vorquantentopfemittern untersucht. Der prim{\"a}re Zweck der Vorquantentopfstrukturen liegt in der Erh{\"o}hung der Energieseparation zwischen Γ- und L-Punkt. So kann Elektronentransport {\"u}ber L- Kan{\"a}le unterdr{\"u}ckt und Elektronenzust{\"a}nde am Γ-Punkt wiederbev{\"o}lkert werden. Zudem ist bei gen{\"u}gend tiefen Vorquantentopfstrukturen aufgrund von Quantisierungseffekten eine Verbesserung der RTD-Transporteigenschaften m{\"o}glich. Strukturen ohne Vorquantentopf-Emitter zeigen ein Tieftemperatur- (T=77 K) Resonanz-zu-Talstrom-Verh{\"a}ltnis von PVCR=8,2, w{\"a}hrend bei Raumtemperatur kein resonantes Tunneln beobachtet werden kann. Die Integration von Ga0,84In0,16Sb- beziehungsweise GaAs0,05Sb0,95-Vorquantentopfstrukturen f{\"u}hrt zu resonantem Tunneln bei Raumtemperatur mit Resonanz-zu-Talstrom-Verh{\"a}ltnissen von PVCR=1,45 und 1,36. In Unterkapitel 4.4 wird die Abh{\"a}ngigkeit der elektrischen Transporteigenschaften von AlAsSb/GaSb RTDs vom As-Stoffmengenanteil des GaAsSb-Emitter-Vorquantentopfs und der AlAsSb-Tunnelbarriere untersucht. Eine Erh{\"o}hung der As-Stoffmengenkonzentration f{\"u}hrt zu einem erh{\"o}hten Raumtemperatur-PVCR mit Werten von bis zu 2,36 bei gleichzeitig reduziertem Tieftemperatur-PVCR. Das reduzierte Tieftemperatur-Transportverm{\"o}gen wird auf eine mit steigendem As-Stoffmengenanteil zunehmend degradierende Kristallqualit{\"a}t zur{\"u}ckgef{\"u}hrt. In Kapitel 5 AlAsSb/GaSb-RTD-Fotosensoren zur MIR-Lichtdetektion werden erstmalig RTD-Fotodetektoren f{\"u}r den MIR-Spektralbereich vorgestellt und auf ihre optoelektronischen Transporteigenschaften hin untersucht. Zudem wird erstmalig ein p-dotierter RTD-Fotodetektor demonstriert. In Unterkapitel 5.1 wird das Probendesign GaSb-basierter RTD-Fotodetektoren f{\"u}r den mittleren infraroten Spektralbereich vorgestellt. Im Speziellen werden Strukturen mit umgekehrter Ladungstr{\"a}gerpolarit{\"a}t (p- statt n-Dotierung, L{\"o}cher als Majorit{\"a}tsladungstr{\"a}ger) vorgestellt. In Unterkapitel 5.2 werden die optischen Eigenschaften der gitterangepassten quatern{\"a}ren GaInAsSb-Absorptionsschicht mittels Fourier-Transformations-Infrarot-Spektroskopie untersucht. {\"U}ber das Photolumineszenz-Spektrum wird die Bandl{\"u}ckenenergie zu E_Gap≅(447±5) meV bestimmt. Das entspricht einer Grenzwellenl{\"a}nge von λ_G≅(2,77±0,04) µm. Aus dem niederenergetischen monoexponentiellem Abfall der Linienform wird eine Urbach-Energie von E_U=10 meV bestimmt. Der hochenergetische Abfall folgt der Boltzmann-Verteilungsfunktion mit einem Abfall von k_B T=25 meV. In Unterkapitel 5.3 werden die elektrischen Transporteigenschaften der RTD-Fotodetektoren untersucht und mit denen einer n-dotierten Referenzprobe verglichen. Erstmalig wird resonantes Tunneln von L{\"o}chern in AlAsSb/GaSb-DBQW-RTDs bei Raumtemperatur demonstriert. Dabei ist PVCR=1,58. Bei T=4,2 K zeigen resonantes Loch- und Elektrontunneln vergleichbare Kenngr{\"o}ßen mit PVCR=10,1 und PVCR=11,4. Die symmetrische I(V)-Kennlinie der p-dotierten RTD-Fotodetektoren deutet auf eine geringe Valenzbanddiskontinuit{\"a}t zwischen GaSb und der GaInAsSb-Absorptionsschicht hin. Zudem sind die p-dotierten RTDs besonders geeignet f{\"u}r eine sp{\"a}tere Integration mit Typ-II-{\"U}bergittern. In Unterkapitel 5.4 werden die optoelektronischen Transporteigenschaften p-dotierter RTD-Fotodetektoren untersucht. Das vorgestellte neuartige RTD-Fotodetektorkonzept, welches auf resonanten Lochtransport als Majorit{\"a}tsladungstr{\"a}ger setzt, bietet speziell im f{\"u}r den MIR-Spektralbereich verwendeten GaSb-Materialsystem Vorteile, l{\"a}sst sich aber auch auf das InP- oder GaAs- Materialsystem {\"u}bertragen. Die untersuchten p-dotierten Fotodetektoren zeigen eine ausgepr{\"a}gte Fotosensitivit{\"a}t im MIR-Spektralbereich. Fotostromuntersuchungen werden f{\"u}r optische Anregung mittels eines Halbleiterlasers der Wellenl{\"a}nge λ=2,61 µm durchgef{\"u}hrt. Bei dieser Wellenl{\"a}nge liegen fundamentale Absorptionslinien atmosph{\"a}rischen Wasserdampfs. Die Fotostrom-Spannungs-Charakteristik best{\"a}tigt, dass die Fotosensitivit{\"a}t auf einer Modulation des resonanten Lochstroms {\"u}ber Coulomb-Wechselwirkung akkumulierter photogenerierter Minorit{\"a}tsladungstr{\"a}ger (Elektronen) beruht. Es werden Sensitivit{\"a}ten von S_I=0,13 A W-1 ermittelt. Durch eine verbesserte RTD-Quanteneffizienz aufgrund eines optimierten Dotierprofils der Absorptionsschicht l{\"a}sst sich die Sensitivit{\"a}t auf S_I=2,71 A W-1 erh{\"o}hen, was einem Multiplikationsfaktor von in etwa M\≈8,6 entspricht. Gleichzeitig wird jedoch der RTD-Hebelfaktor verringert, sodass n_(RTD p2)=0,42⋅n_(RTD p1). Erstmalig wurde damit erfolgreich Gas-Absorptionsspektroskopie anhand von H2O-Dampf mittels MIR-RTD-Fotodetektor an drei beieinanderliegenden Absorptionslinien demonstriert.}, subject = {Resonanz-Tunneldiode}, language = {de} } @phdthesis{Wiedenmann2018, author = {Wiedenmann, Jonas}, title = {Induced topological superconductivity in HgTe based nanostructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-162782}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {This thesis describes the studies of topological superconductivity, which is predicted to emerge when pair correlations are induced into the surface states of 2D and 3D topolog- ical insulators (TIs). In this regard, experiments have been designed to investigate the theoretical ideas first pioneered by Fu and Kane that in such system Majorana bound states occur at vortices or edges of the system [Phys. Rev. Lett. 100, 096407 (2008), Phys. Rev. B 79, 161408 (2009)]. These states are of great interest as they constitute a new quasiparticle which is its own antiparticle and can be used as building blocks for fault tolerant topological quantum computing. After an introduction in chapter 1, chapter 2 of the thesis lays the foundation for the understanding of the field of topology in the context of condensed matter physics with a focus on topological band insulators and topological superconductors. Starting from a Chern insulator, the concepts of topological band theory and the bulk boundary corre- spondence are explained. It is then shown that the low energy Hamiltonian of mercury telluride (HgTe) quantum wells of an appropriate thickness can be written as two time reversal symmetric copies of a Chern insulator. This leads to the quantum spin Hall effect. In such a system, spin-polarized one dimensional conducting states form at the edges of the material, while the bulk is insulating. This concept is extended to 3D topological insulators with conducting 2D surface states. As a preliminary step to treating topological superconductivity, a short review of the microscopic theory of superconductivity, i.e. the theory of Bardeen, Cooper, and Shrieffer (BCS theory) is presented. The presence of Majorana end modes in a one dimensional superconducting chain is explained using the Kitaev model. Finally, topological band insulators and conventional superconductivity are combined to effectively engineer p-wave superconductivity. One way to investigate these states is by measuring the periodicity of the phase of the Josephson supercurrent in a topological Josephson junction. The signature is a 4π-periodicity compared to the 2π-periodicity in conventional Josephson junctions. The proof of the presence of this effect in HgTe based Josephson junction is the main goal of this thesis and is discussed in chapters 3 to 6. Chapter 3 describes in detail the transport of a 3D topological insulator based weak link under radio-frequency radiation. The chapter starts with a review of the state of research of (i) strained HgTe as 3D topological insulator and (ii) the progress of induc- ing superconducting correlations into the topological surface states and the theoretical predictions of 3D TI based Josephson junctions. Josephson junctions based on strained HgTe are successfully fabricated. Before studying the ac driven Josephson junctions, the dc transport of the devices is analysed. The critical current as a function of temperature is measured and it is possible to determine the induced superconducting gap. Under rf illumination Shapiro steps form in the current voltage characteristic. A missing first step at low frequencies and low powers is found in our devices. This is a signature of a 4π-periodic supercurrent. By studying the device in a wide parameter range - as a 147148 SUMMARY function of frequency, power, device geometry and magnetic field - it is shown that the results are in agreement with the presence of a single gapless Andreev doublet and several conventional modes. Chapter 4 gives results of the numerical modelling of the I -V dynamics in a Josephson junction where both a 2π- and a 4π-periodic supercurrents are present. This is done in the framework of an equivalent circuit representation, namely the resistively shunted Josephson junction model (RSJ-model). The numerical modelling is in agreement with the experimental results in chapter 3. First, the missing of odd Shapiro steps can be understood by a small 4π-periodic supercurrent contribution and a large number of modes which have a conventional 2π-periodicity. Second, the missing of odd Shapiro steps occurs at low frequency and low rf power. Third, it is shown that stochastic processes like Landau Zener tunnelling are most probably not responsible for the 4π contribution. In a next step the periodicity of Josephson junctions based on quantum spin Hall insulators using are investigated in chapter 5. A fabrication process of Josephson junctions based on inverted HgTe quantum wells was successfully developed. In order to achieve a good proximity effect the barrier material was removed and the superconductor deposited without exposing the structure to air. In a next step a gate electrode was fabricated which allows the chemical potential of the quantum well to be tuned. The measurement of the diffraction pattern of the critical current Ic due to a magnetic field applied perpendicular to the sample plane was conducted. In the vicinity to the expected quantum spin Hall phase, the pattern resembles that of a superconducting quantum interference device (SQUID). This shows that the current flows predominantly on the edges of the mesa. This observation is taken as a proof of the presence of edge currents. By irradiating the sample with rf, missing odd Shapiro steps up to step index n = 9 have been observed. This evidences the presence of a 4π-periodic contribution to the supercurrent. The experiment is repeated using a weak link based on a non-inverted HgTe quantum well. This material is expected to be a normal band insulator without helical edge channels. In this device, all the expected Shapiro steps are observed even at low frequencies and over the whole gate voltage range. This shows that the observed phenomena are directly connected to the topological band structure. Both features, namely the missing of odd Shapiro steps and the SQUID like diffraction pattern, appear strongest towards the quantum spin Hall regime, and thus provide evidence for induced topological superconductivity in the helical edge states. A more direct way to probe the periodicity of the Josephson supercurrent than using Shapiro steps is the measurement of the emitted radiation of a weak link. This experiment is presented in chapter 6. A conventional Josephson junction converts a dc bias V to an ac current with a characteristic Josephson frequency fJ = eV /h. In a topological Josephson junction a frequency at half the Josephson frequency fJ /2 is expected. A new measurement setup was developed in order to measure the emitted spectrum of a single Josephson junction. With this setup the spectrum of a HgTe quantum well based Josephson junction was measured and the emission at half the Josephson frequency fJ /2 was detected. In addition, fJ emission is also detected depending on the gate voltage and detection frequency. The spectrum is again dominated by half the Josephson emission at low voltages while the conventional emission is determines the spectrum at high voltages. A non-inverted quantum well shows only conventional emission over the whole gateSUMMARY 149 voltage and frequency range. The linewidth of the detected frequencies gives a measure on the lifetime of the bound states: From there, a coherence time of 0.3-4ns for the fJ /2 line has been deduced. This is generally shorter than for the fJ line (3-4ns). The last part of the thesis, chapter 7, reports on the induced superconducting state in a strained HgTe layer investigated by point-contact Andreev reflection spectroscopy. For the experiment, a HgTe mesa was fabricated with a small constriction. The diameter of the orifice was chosen to be smaller than the mean free path estimated from magne- totransport measurements. Thus one gets a ballistic point-contact which allows energy resolved spectroscopy. One part of the mesa is covered with a superconductor which induces superconducting correlations into the surface states of the topological insulator. This experiment therefore probes a single superconductor normal interface. In contrast to the Josephson junctions studied previously, the geometry allows the acquisition of energy resolved information of the induced superconducting state through the measurement of the differential conductance dI/dV as a function of applied dc bias for various gate voltages, temperatures and magnetic fields. An induced superconducting order parame- ter of about 70µeV was extracted but also signatures of the niobium gap at the expected value around Δ Nb ≈ 1.1meV have been found. Simulations using the theory developed by Blonder, Tinkham and Klapwijk and an extended model taking the topological surface states into account were used to fit the data. The simulations are in agreement with a small barrier at the topological insulator-induced topological superconductor interface and a high barrier at the Nb to topological insulator interface. To understand the full con- ductance curve as a function of applied voltage, a non-equilibrium driven transformation is suggested. The induced superconductivity is suppressed at a certain bias value due to local electron population. In accordance with this suppression, the relevant scattering regions change spatially as a function of applied bias. To conclude, it is emphasized that the experiments conducted in this thesis found clear signatures of induced topological superconductivity in HgTe based quantum well and bulk devices and opens up the avenue to many experiments. It would be interesting to apply the developed concepts to other topological matter-superconductor hybrid systems. The direct spectroscopy and manipulation of the Andreev bound states using circuit quantum electrodynamic techniques should be the next steps for HgTe based samples. This was already achieved in superconducting atomic break junctions by the group in Saclay [Science 2015, 349, 1199-1202 (2015)]. Another possible development would be the on-chip detection of the emitted spectrum as a function of the phase φ through the junction. In this connection, the topological junction needs to be shunted by a parallel ancillary junction. Such a setup would allow the current phase relation I(φ) directly and the lifetime of the bound states to be measured directly. By coupling this system to a spectrometer, which can be another Josephson junction, the energy dependence of the Andreev bound states E(φ) could be obtained. The experiments on the Andreev reflection spectroscopy described in this thesis could easily be extended to two dimensional topological insulators and to more complex geometries, like a phase bias loop or a tunable barrier at the point-contact. This work might also be useful for answering the question how and why Majorana bound states can be localized in quantum spin Hall systems.}, subject = {Quecksilbertellurid}, language = {en} } @phdthesis{Kramer2017, author = {Kramer, Christian}, title = {Investigation of Nanostructure-Induced Localized Light Phenomena Using Ultrafast Laser Spectroscopy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-150681}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {In recent years, the interaction of light with subwavelength structures, i.e., structures that are smaller than the optical wavelength, became more and more interesting to scientific research, since it provides the opportunity to manipulate light-induced dynamics below the optical diffraction limit. Specifically designed nanomaterials can be utilized to tailor the temporal evolution of electromagnetic fields at the nanoscale. For the investigation of strongly localized processes, it is essential to resolve both their spatial and their temporal behavior. The aim of this thesis was to study and/or control the temporal evolution of three nanostructure-induced localized light phenomena by using ultrafast laser spectroscopy with high spatial resolution. In Chapter 4, the absorption of near-infrared light in thin-film a-Si:H solar cells was investigated. Using nanotextured instead of smooth interfaces for such devices leads to an increase of absorption from < 20\% to more than 50\% in the near-infrared regime. Time-resolved experiments with femtosecond laser pulses were performed to clarify the reason for this enhancement. The coherent backscattered radiation from nanotextured solar cell devices was measured as a function of the sample position and evaluated via spectral interferometry. Spatially varying resonance peaks in the recorded spectra indicated the formation of localized photonic modes within the nanotextured absorber layers. In order to identify the modes separately from each other, coherent two-dimensional (2D) nanoscopy was utilized, providing a high spatial resolution < 40 nm. In a nanoscopy measurement on a modified device with an exposed nanotextured a-Si:H absorber layer, hot-spot electron emission was observed and confirmed the presence of localized modes. Fitting the local 2D nanospectra at the hot-spot positions enabled the determination of the resonance frequencies and coherence lifetimes of the modes. The obtained lifetime values varied between 50 fs and 130 fs. Using a thermionic emission model allowed the calculation of the locally absorbed energy density and, with this, an estimation of the localization length of the photonic modes (≈1 μm). The localization could be classified by means of the estimated localization length and additional data evaluation of the backscattered spectra as strong localization ─ the so-called Anderson localization. Based on the experimental results, it was concluded that the enhanced absorption of near-infrared light in thin-film silicon solar cells with nanotextured interfaces is caused by the formation of strongly localized photonic modes within the disordered absorber layers. The incoming near-infrared light is trapped in these long-living modes until absorption occurs. In Chapter 5, a novel hybridized plasmonic device was introduced and investigated in both theory and experiment. It consists of two widely separated whispering gallery mode (WGM) nanoantennas located in an elliptical plasmonic cavity. The goal was to realize a periodic long-range energy transfer between the nanoantennas. In finite-difference time-domain (FDTD) simulations, the device was first optimized with respect to strong coupling between the localized antenna modes and the spatially-extended cavity mode. The geometrical parameters of the antennas and the cavity were adjusted separately so that the m="0" antenna mode and the cavity mode were resonant at λ="800 nm" . A high spatial overlap of the modes was achieved by positioning the two antennas in the focal spots of the cavity, leading to a distance between the antenna centers of more than twice the resonant wavelength of the modes. The spectral response of the optimized device revealed an energy splitting of the antenna and the cavity mode into three separated hybridized eigenmodes within an energy range of about 90 meV due to strong coupling. It could be well reproduced by a simple model of three coupled Lorentzian oscillators. In the time domain, an oscillatory energy transfer between both antennas with a period of 86 fs and an energy transfer efficiency of about 7\% was observed for single-pulse excitation. For the experiments, devices with cavities and antennas of varying size were fabricated by means of focused-ion-beam (FIB) milling. Time-resolved correlation measurements were performed with high spatial and temporal resolution by using sequences of two femtosecond laser pulses for excitation and photoemission electron microscopy (PEEM) for detection. Local correlation traces at antennas in resonant devices, i.e., devices with enhanced electron emission at both antenna positions, were investigated and reconstructed by means of the coupled-oscillator model. The corresponding spectral response revealed separated peaks, confirming the formation of hybridized eigenmodes due to strong coupling. In a subsequent simulation for single-pulse excitation, one back-and-forth energy transfer between both antennas with an energy transfer efficiency of about 10\% was observed. Based on the theoretical and experimental results, it was demonstrated that in the presented plasmonic device a periodic long-range energy transfer between the two nanoantennas is possible. Furthermore, the coupled-oscillator model enables one to study in depth how specific device properties impact the temporal electric-field dynamics within the device. This can be exploited to further optimize energy transfer efficiency of the device. Future applications are envisioned in ultrafast plasmonic nanocircuitry. Moreover, the presented device can be employed to realize efficient SPP-mediated strong coupling between widely separated quantum emitters. In Chapter 6, it was investigated in theory how the local optical chirality enhancement in the near field of plasmonic nanostructures can be optimized by tuning the far-field polarization of the incident light. An analytic expression was derived that enables the calculation of the optimal far-field polarizations, i.e., the two far-field polarizations which lead to the highest positive and negative local optical chirality, for any given nanostructure geometry. The two optimal far-field polarizations depend on the local optical response of the respective nanostructure and thus are functions of both the frequency ω and the position r. Their ellipticities differ only in their sign, i.e., in their direction of rotation in the time domain, and the angle between their orientations, i.e., the angle between the principal axes of their ellipses, is ±π/"2" . The handedness of optimal local optical chirality can be switched by switching between the optimal far-field polarizations. In numerical simulations, it was exemplarily shown for two specific nanostructure assemblies that the optimal local optical chirality can significantly exceed the optical chirality values of circularly polarized light in free space ─ the highest possible values in free space. The corresponding optimal far-field polarizations were different from linear and circular and varied with frequency. Using femtosecond polarization pulse shaping provides the opportunity to coherently control local optical chirality over a continuous frequency range. Furthermore, symmetry properties of nanostructures can be exploited to determine which far-field polarization is optimal. The theoretical findings can have impact on future experimental studies about local optical chirality enhancement. Tuning the far-field polarization of the incident light offers a promising tool to enhance chirally specific interactions of local electromagnetic fields with molecular and other quantum systems in the vicinity of plasmonic nanostructures. The presented approach can be utilized for applications in chiral sensing of adsorbed molecules, time-resolved chirality-sensitive spectroscopy, and chiral quantum control. In conclusion, each of the localized light phenomena that were investigated in this thesis ─ the enhanced local absorption of near-infrared light due to the formation of localized photonic modes, the periodic long-range energy transfer between two nanoantennas within an elliptical plasmonic cavity, and the optimization of local optical chirality enhancement by tuning the far-field polarization of the incident light ─ can open up new perspectives for a variety of future applications. .}, subject = {Ultrakurzzeitspektroskopie}, language = {en} } @phdthesis{Kessel2016, author = {Kessel, Maximilian}, title = {HgTe shells on CdTe nanowires: A low-dimensional topological insulator from crystal growth to quantum transport}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-149069}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {A novel growth method has been developed, allowing for the growth of strained HgTe shells on CdTe nanowires (NWs). The growth of CdTe-HgTe core-shell NWs required high attention in controlling basic parameters like substrate temperature and the intensity of supplied material fluxes. The difficulties in finding optimized growth conditions have been successfully overcome in this work. We found the lateral redistribution of liquid growth seeds with a ZnTe growth start to be crucial to trigger vertical CdTe NW growth. Single crystalline zinc blende CdTe NWs grew, oriented along [111]B. The substrate temperature was the most critical parameter to achieve straight and long wires. In order to adjust it, the growth was monitored by reflection high-energy electron diffraction, which was used for fine tuning of the temperature over time in each growth run individually. For optimized growth conditions, a periodic diffraction pattern allowed for the detailed analysis of atomic arrangement on the surfaces and in the bulk. The ability to do so reflected the high crystal quality and ensemble uniformity of our CdTe NWs. The NW sides were formed by twelve stable, low-index crystalline facets. We observed two types stepped and polar sides, separated by in total six flat and non-polar facets. The high crystalline quality of the cores allowed to grow epitaxial HgTe shells around. We reported on two different heterostructure geometries. In the first one, the CdTe NWs exhibit a closed HgTe shell, while for the second one, the CdTe NWs are overgrown mainly on one side. Scanning electron microscopy and scanning transmission electron microscopy confirmed, that many of the core-shell NWs are single crystalline zinc blende and have a high uniformity. The symmetry of the zinc blende unit cell was reduced by residual lattice strain. We used high-resolution X-ray diffraction to reveal the strain level caused by the small lattice mismatch in the heterostructures. Shear strain has been induced by the stepped hetero-interface, thereby stretching the lattice of the HgTe shell by 0.06 \% along a direction oriented with an angle of 35 ° to the interface. The different heterostructures obtained, were the base for further investigation of quasi-one-dimensional crystallites of HgTe. We therefore developed methods to reliably manipulate, align, localize and contact individual NWs, in order to characterize the charge transport in our samples. Bare CdTe cores were insulating, while the HgTe shells were conducting. At low temperature we found the mean free path of charge carriers to be smaller, but the phase coherence length to be larger than the sample size of several hundred nanometers. We observed universal conductance fluctuations and therefore drew the conclusion, that the trajectories of charge carriers are defined by elastic backscattering at randomly distributed scattering sites. When contacted with superconducting leads, we saw induced superconductivity, multiple Andreev reflections and the associated excess current. Thus, we achieved HgTe/superconductor interfaces with high interfacial transparency. In addition, we reported on the appearance of peaks in differential resistance at Delta/e for HgTe-NW/superconductor and 2*Delta/e for superconductor/HgTe-NW/superconductor junctions, which is possibly related to unconventional pairing at the HgTe/superconductor interface. We noticed that the great advantage of our self-organized growth is the possibility to employ the metallic droplet, formerly seeding the NW growth, as a superconducting contact. The insulating wire cores with a metallic droplet at the tip have been overgrown with HgTe in a fully in-situ process. A very high interface quality was achieved in this case.}, subject = {Quecksilbertellurid}, language = {en} } @phdthesis{Hansen2017, author = {Hansen, Nis Hauke}, title = {Mikroskopische Ladungstransportmechanismen und Exzitonen Annihilation in organischen Einkristallen und D{\"u}nnschichten}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-143972}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {Um die Natur der Transportdynamik von Ladungstr{\"a}gern auch auf mikroskopischen L{\"a}ngenskalen nicht-invasiv untersuchen zu k{\"o}nnen, wurde im ersten Schwerpunkt dieser Arbeit das PL- (Photolumineszenz-) Quenching (engl.: to quench: l{\"o}schen; hier: strahlungslose Rekombination von Exzitonen) in einer organischen D{\"u}nnschicht durch die injizierten und akkumulierten L{\"o}cher in einer Transistorgeometrie analysiert. Diese Zusammenf{\"u}hrung zweier Methoden - der elektrischen Charakterisierung von D{\"u}nnschichttransistoren und der Photolumineszenzspektroskopie - erfasst die {\"A}nderung des strahlenden Zerfalls von Exzitonen infolge der Wechselwirkung mit Ladungstr{\"a}gern. Dadurch werden r{\"a}umlich aufgel{\"o}ste Informationen {\"u}ber die Ladungsverteilung und deren Spannungsabh{\"a}ngigkeit im Transistorkanal zug{\"a}nglich. Durch den Vergleich mit den makroskopischen elektrischen Kenngr{\"o}ßen wie der Schwell- oder der Turn-On-Spannung kann die Funktionsweise der Transistoren damit detaillierter beschrieben werden, als es die Kenngr{\"o}ßen alleine erm{\"o}glichen. Außerdem wird die Quantifizierung dieser mikroskopischen Interaktionen m{\"o}glich, welche beispielsweise als Verlustkanal in organischen Photovoltaikzellen und organicshen Leuchtdioden auftreten k{\"o}nnen. Die Abgrenzung zu anderen dissipativen Prozessen, wie beispielsweise der Exziton-Exziton Annihilation, Ladungstr{\"a}gerrekombination, Triplett-{\"U}berg{\"a}nge oder Rekombination an St{\"o}rstellen oder metallischen Grenzfl{\"a}chen, erlaubt die detaillierte Analyse der Wechselwirkung von optisch angeregten Zust{\"a}nden mit Elektronen und L{\"o}chern. Im zweiten Schwerpunkt dieser Arbeit werden die Transporteigenschaften des Naphthalindiimids Cl2-NDI betrachtet, bei dem der molekulare {\"U}berlapp sowie die Reorganisationsenergie in derselben Gr{\"o}ßenordnung von etwa 0,1 eV liegen. Um experimentell auf den mikroskopischen Transport zu schließen, werden nach der Optimierung des Kristallwachstums Einkristalltransistoren hergestellt, mit Hilfe derer die Beweglichkeit entlang verschiedener kristallographischer Richtungen als Funktion der Temperatur gemessen werden kann. Die einkristalline Natur der Proben und die spezielle Transistorgeometrie erm{\"o}glichen die Analyse der r{\"a}umlichen Anisotropie des Stromflusses. Der gemessene Beweglichkeitstensor wird daraufhin mit simulierten Tensoren auf der Basis von Levich-Jortner Raten verglichen, um auf den zentralen Ladungstransfermechanismus zu schließen.}, subject = {Organischer Halbleiter}, language = {de} } @phdthesis{Schmitt2019, author = {Schmitt, Martin}, title = {Strukturanalyse und magnetische Eigenschaften von Ketten aus 3d-{\"U}bergangsmetalloxiden auf Ir(001) und Pt(001)}, doi = {10.25972/OPUS-19182}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-191823}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {In der vorliegenden Arbeit werden die strukturellen und magnetischen Eigenschaften verschiedener 3d-{\"U}bergangsmetalloxidketten (TMO-Ketten) auf Ir(001) und Pt(001) untersucht. Diese weisen eine (3 × 1) Struktur mit periodisch angeordneten Ketten auf, die nur {\"u}ber die Sauerstoffbindung an das Substrat gekoppelt sind. W{\"a}hrend die Struktur durch experimentelle und theoretische Untersuchungen best{\"a}tigt ist, liegen f{\"u}r die magnetischen Eigenschaften ausschließlich Rechnungen vor. Zur {\"U}berpr{\"u}fung dieser theoretischen Vorhersagen wird die Methode der spinpolarisierten Rastertunnelmikroskopie (SP-STM) verwendet, die die Abbildung der magnetischen Ordnung mit atomarer Aufl{\"o}sung erlaubt. Die Untersuchungen beginnen mit der Vorstellung der Ir(001) Oberfl{\"a}che, die eine (5 × 1) Rekonstruktion aufweist. Eine Aufhebung dieser Rekonstruktion erreicht man durch das Heizen des Ir-Substrats in Sauerstoffatmosph{\"a}re unter Bildung einer (2 × 1) Sauerstoffrekonstruktion. Die Qualit{\"a}t der Oberfl{\"a}che h{\"a}ngt dabei von der Wachstumstemperatur T und dem verwendeten Sauerstoffdruck pOx ab. Die bei T = 550°C und pOx = 1 × 10^-8 mbar hergestellte Sauerstoffrektonstruktion dient als Ausgangspunkt f{\"u}r die folgenden Pr{\"a}parationen von CoO2, FeO2 und MnO2-Ketten. Dazu wird jeweils eine drittel Monolage (ML) des {\"U}bergangsmetalls auf die Oberfl{\"a}che des Substrates gedampft und die Probe unter Sauerstoffatmosph{\"a}re ein weiteres Mal geheizt. Auf diese Weise kann die (3 × 1) Struktur der bekannten Ketten best{\"a}tigt und die Gruppe der TMO-Ketten um die CrO2-Ketten erweitert werden. In der einschl{\"a}gigen Fachliteratur wurden Vorhersagen bez{\"u}glich der magnetischen Struktur der TMO-Ketten publiziert, wonach entlang und zwischen CoO2-Ketten eine ferromagnetische (FM) und f{\"u}r FeO2 und MnO2-Ketten eine antiferromagnetische (AFM-) Kopplung vorliegt.W{\"a}hrend die {\"U}berpr{\"u}fung dieser Vorhersagen mit SP-STM f{\"u}r CoO2 und CrO2-Ketten keine Hinweise auf magnetische Strukturen liefert, liegen bei FeO2 und MnO2-Ketten unterschiedliche magnetische Phasen vor. In der Tat kann mit den experimentell gefundenen Einheitszellen die AFM-Kopplung entlang beider Ketten best{\"a}tigt werden. Im Gegensatz widersprechen die Kopplungen zwischen den Ketten den Berechnungen. Bei FeO2-Ketten liegt eine stabile FM Ordnung vor, die zu einer magnetischen (3 × 2) Einheitszelle mit einer leichten Magnetisierung in Richtung der Oberfl{\"a}chennormalen f{\"u}hrt (out-of-plane). Die MnO2-Ketten weichen ebenfalls von der berechneten magnetischen kollinearen Ordnung zwischen benachbarten Ketten ab und zeigen eine chirale Struktur. Durch die Rotation der Mn-Spins um 120° in der Probenebenen (in-plane) entsteht eine magnetische (9 × 2) Einheitszelle, deren Periode durch neue DFT-Rechnungen best{\"a}tigt wird. Nach diesen Berechnungen handelt es sich um eine Spinspirale, die durch die Dzyaloshinskii-Moriya (DM-) Wechselwirkung bei einem Energiegewinn von 0,3 meV pro Mn-Atom gegen{\"u}ber den kollinearen FM Zustand stabilisiert wird. Diese wird {\"a}hnlich wie bei bereits publizierten Clustern und Adatomen auf Pt(111) durch die Rudermann-Kittel-Kasuya-Yosida (RKKY-) Wechselwirkung vermittelt und erkl{\"a}rt den experimentell gefundenen einheitlichen Drehsinn der Spiralen. Die RKKY-Wechselwirkung zeigt eine starke Abh{\"a}ngigkeit von der Fermi-Oberfl{\"a}che des Substrats. Im folgenden Kapitel werden deshalb mit TMO-Ketten auf Pt(001) die strukturellen und magnetischen Eigenschaften auf einem weiteren Substrat analysiert, wobei zum Zeitpunkt der Arbeit nur die Existenz der CoO2-Ketten aus der Literatur bekannt war. Vergleichbar mit Ir(001) besitzt auch Pt(001) eine rekonstruierte Oberfl{\"a}che, die sich aber stabil gegen{\"u}ber Oxidation zeigt. Dadurch muss die drittel ML des {\"U}bergangsmetalls direkt auf die Rekonstruktion aufgedampft werden. Das Wachstum des {\"U}bergangsmetalls ist dabei von der Temperatur des Substrats abh{\"a}ngig und beeinflusst das Ergebnis der nachfolgenden Oxidation. Diese erfolgt analog zum Wachstum der Ketten auf Ir(001) durch das Heizen der Probe in Sauerstoffatmosph{\"a}re und resultiert nur f{\"u}r das Aufdampfen des {\"U}bergangsmetalls auf kalte Pt(001) Oberfl{\"a}chen in Ketten mit der Periode von 3aPt. Auf diese Weise kann nicht nur die (3 × 1) Struktur der CoO2-Ketten best{\"a}tigt werden, sondern auch durch atomare Aufl{\"o}sung die Gruppe der TMO-Ketten um MnO2-Ketten auf Pt(001) erweitert werden. Im Gegensatz dazu sind die nicht magnetischen Messungen im Fall von Fe nicht eindeutig. Zwar liegen auch hier Ketten im Abstand des dreifachen Pt Gittervektors vor, trotzdem ist die (3 × 1) Struktur nicht nachweisbar. Dies liegt an einer Korrugation mit einer Periode von 2aPt entlang der Ketten, was ein Hinweis auf eine Peierls Instabilit{\"a}t sein kann. Entsprechend dem Vorgehen f{\"u}r Ir(001) werden f{\"u}r die TMO-Ketten auf Pt(001) SP-STM Messungen durchgef{\"u}hrt und die Vorhersage einer AFM-Kopplung f{\"u}r CoO2-Ketten {\"u}berpr{\"u}ft. Auch hier k{\"o}nnen, wie im Fall von CoO2-Ketten und im Widerspruch zur Vorhersage, f{\"u}r beide Polarisationsrichtungen der Spitze keine magnetischen Strukturen gefunden werden. Dar{\"u}ber hinaus verhalten sich die MnO2-Ketten auf Pt(001) mit ihrer chiralen magnetischen Struktur {\"a}hnlich zu denen auf Ir(001). Dies best{\"a}tigt die Annahme einer indirekten DM-Wechselwirkung, wobei durch die 72° Rotation der Mn-Spins eine l{\"a}ngere Periode der zykloidalen Spinspirale festgestellt wird. Die Erkl{\"a}rung daf{\"u}r liegt in der Abh{\"a}ngigkeit der RKKY-Wechselwirkung vom Fermi-Wellenvektor des Substrats, w{\"a}hrend sich die DM-Wechselwirkung beim {\"U}bergang von Ir zu Pt nur wenig {\"a}ndert.}, subject = {Rastertunnelmikroskopie}, language = {de} } @unpublished{HuberPresWittmannetal.2019, author = {Huber, Bernhard and Pres, Sebastian and Wittmann, Emanuel and Dietrich, Lysanne and L{\"u}ttig, Julian and Fersch, Daniel and Krauss, Enno and Friedrich, Daniel and Kern, Johannes and Lisinetskii, Victor and Hensen, Matthias and Hecht, Bert and Bratschitsch, Rudolf and Riedle, Eberhard and Brixner, Tobias}, title = {Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate}, issn = {0034-6748}, doi = {10.1063/1.5115322}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-191906}, year = {2019}, abstract = {We describe a setup for time-resolved photoemission electron microscopy (TRPEEM) with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215-970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale ("2D nanoscopy"). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump-probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity (LEER) measurements. Finally, we demonstrate the in-situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.}, language = {en} } @phdthesis{Knebl2019, author = {Knebl, Georg}, title = {Epitaktisches Wachstum und Transportuntersuchung topologisch isolierender Materialien: GaSb/InAs Doppelquantenfilme und Bi\(_2\)Se\(_3\) Nanostrukturen}, doi = {10.25972/OPUS-19147}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-191471}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {Topologische Isolatoren geh{\"o}ren zu einer Klasse von Materialien, an deren Realisation im Rahmen der zweiten quantenmechanischen Revolution gearbeitet wird. Einerseits sind zahlreiche Fragestellungen zu diesen Materialen und deren Nutzbarmachung noch nicht beantwortet, andererseits treiben vielversprechende Anwendungen im Feld der Quantencomputer und Spintronik die L{\"o}sung dieser Fragen voran. Topologische Rand- bzw. Oberfl{\"a}chenzust{\"a}nde wurden f{\"u}r unterschiedlichste Materialien und Strukturen theoretisch vorhergesagt, so auch f{\"u}r GaSb/InAs Doppelquantenfilme und Bi2Se3. Trotz intensiver Forschungsarbeiten und großer Fortschritte bed{\"u}rfen viele Prozesse v. a. im Bereich der Probenherstellung und Verarbeitung noch der Optimierung. Die vorliegende Arbeit pr{\"a}sentiert Ergebnisse zur Molekularstahlepitaxie, zur Probenfertigung sowie zu elektro-optisch modulierter Transportuntersuchung von GaSb/InAs Doppelquantenfilmen und der epitaktischen Fertigung von Bi2Se3 Nanostrukturen. Im ersten Teil dieser Arbeit werden die Parameter zur Molekularstrahlepitaxie sowie die Anpassung der Probenfertigung von GaSb/InAs Doppelquantenfilmen an material- und untersuchungsbedingte Notwendigkeiten beschrieben. Dieser verbesserte Prozess erm{\"o}glicht die Fertigung quantitativ vergleichbarer Probenserien. Anschließend werden Ergebnisse f{\"u}r Strukturen mit variabler InAs Schichtdicke unter elektrostatischer Kontrolle mit einem Frontgate pr{\"a}sentiert. Auch mit verbessertem Prozess zeigten sich Leckstr{\"o}me zum Substrat. Diese erschweren eine elektrostatische Kontrolle {\"u}ber Backgates. Die erstmals durch optische Anregung pr{\"a}sentierte Manipulation der Ladungstr{\"a}gerart sowie des Phasenzustandes in GaSb/InAs Doppelquantenfilmen bietet eine Alternative zu problembehafteten elektrostatisch betriebenen Gates. Im zweiten Teil wird die epitaktische Herstellung von Bi2Se3 Nanostrukturen gezeigt. Mit dem Ziel, Vorteile aus dem erh{\"o}hten Oberfl{\"a}che-zu-Volumen Verh{\"a}ltnis zu ziehen, wurden im Rahmen dieser Arbeit erstmals Bi2Se3 Nanodr{\"a}hte und -flocken mittels Molekularstrahlepitaxie f{\"u}r die Verwendung als topologischer Isolator hergestellt. Ein Quantensprung - Kapitel 1 f{\"u}hrt {\"u}ber die umgangssprachliche Wortbedeutung des Quantensprungs und des damit verbundenen Modells der Quantenmechanik in das Thema. Die Anwendung dieses Modells auf Quanten-Ensembles und dessen technische Realisation wird heute als erste Quantenmechanische Revolution bezeichnet und ist aus unserem Alltag nicht mehr wegzudenken. Im Rahmen der zweiten Quantenmechanischen Revolution soll nun die Anwendung auf einzelne Zust{\"a}nde realisiert und technisch nutzbar gemacht werden. Hierbei sind topologische Isolatoren ein vielversprechender Baustein. Es werden das Konzept des topologischen Isolators sowie die Eigenschaften der beiden in dieser Arbeit betrachteten Systeme beschrieben: GaSb/InAs Doppelquantenfilme und Bi2Se3 Nanostrukturen. GaSb/InAs Doppelquantenfilme Kapitel 2 beschreibt die notwendigen physikalischen und technischen Grundlagen. Ausgehend von der Entdeckung des Hall-Effekts 1879 werden die Quanten-Hall-Effekte eingef{\"u}hrt. Quanten-Spin-Hall-Isolatoren oder allgemeiner topologische Isolatoren sind Materialien mit einem isolierenden Inneren, weisen an der Oberfl{\"a}che aber topologisch gesch{\"u}tzte Zust{\"a}nde auf. Doppelquantenfilme aus GaSb/InAs, die in AlSb gebettet werden, weisen - abh{\"a}ngig vom Aufbau der Heterostruktur - eine typische invertierte Bandstruktur auf und sind ein vielversprechender Kandidat f{\"u}r die Nutzbarmachung der topologischen Isolatoren. GaSb, InAs und AlSb geh{\"o}ren zur 6,1 {\AA}ngstr{\"o}m-Familie, welche f{\"u}r ihre opto-elektronischen Eigenschaften bekannt ist und h{\"a}ufig verwendet wird. Die Eigenschaften sowie die technologischen Grundlagen der epitaktischen Fertigung von Heterostrukturen aus den Materialien der 6,1 {\AA}ngstr{\"o}m-Familie mittels Molekularstrahlepitaxie werden besprochen. Abschließend folgen die Charakterisierungs- und Messmethoden. Ein {\"U}berblick {\"u}ber die Literatur zu GaSb/InAs Doppelquantenfilmen in Bezug auf topologische Isolatoren rundet dieses Kapitel ab. Zu Beginn dieser Arbeit stellten Kurzschlusskan{\"a}le eine Herausforderung f{\"u}r die Detektion der topologischen Randkan{\"a}le dar. Kapitel 3 behandelt L{\"o}sungsans{\"a}tze hierf{\"u}r und beschreibt die Verbesserung der Herstellung von GaSb/InAs Doppelquantenfilm-Strukturen mit Blick auf die zuk{\"u}nftige Realisation topologischer Randkan{\"a}le. In Abschnitt 3.1 werden numerische Simulationen pr{\"a}sentiert, die sich mit der Inversion der elektronischen Niveaus in Abh{\"a}ngigkeit der GaSb und InAs Schichtdicken dGaSb und dInAs besch{\"a}ftigen. Ein geeigneter Schichtaufbau f{\"u}r Strukturen mit invertierter Bandordnung liegt im Parameterraum von 8 nm ≾ dInAs ≾ 12 nm und 8 nm ≾ dGaSb ≾ 10 nm. Abschnitt 3.2 beschreibt die epitaktische Herstellung von GaSb/InAs Doppelquantenfilmen mittels Molekularstrahlepitaxie. Die Fertigung eines GaSb Quasisubstrats auf ein GaAs Substrat wird pr{\"a}sentiert und anschließend der Wechsel auf native GaSb Substrate mit einer reduzierten Defektdichte sowie reproduzierbar hoher Probenqualit{\"a}t begr{\"u}ndet. Ein Wechseln von bin{\"a}rem AlSb auf gitterangepasstes AlAsSb erlaubt die Verwendung dickerer Barrieren. Versuche, eine hinl{\"a}ngliche Isolation des Backgates durch das Einbringen einer dickeren unteren Barriere zu erreichen, werden in diesem Abschnitt diskutiert. In Abschnitt 3.3 wird die Optimierung der Probenprozessierung gezeigt. Die Kombination zweier angepasster {\"A}tzprozesse - eines trockenchemischen und eines sukzessive folgenden nasschemischen Schrittes - liefert zusammen mit der Entfernung von Oberfl{\"a}chenoxiden reproduzierbar gute Ergebnisse. Ein materialselektiver {\"A}tzprozess mit darauffolgender direkter Kontaktierung des InAs Quantenfilmes liefert gute Kontaktwiderst{\"a}nde, ohne Kurzschlusskan{\"a}le zu erzeugen. Abschnitt 3.4 gibt einen kompakten {\"U}berblick, {\"u}ber den im weiteren Verlauf der Arbeit verwendeten „best practice" Prozess. Mit diesem verbesserten Prozess wurden Proben mit variabler InAs Schichtdicke gefertigt und bei 4,2 K auf ihre Transporteigenschaften hin untersucht. Dies ist in Kapitel 4 pr{\"a}sentiert und diskutiert. Abschnitt 4.1 beschreibt die Serie aus drei Proben mit GaSb/InAs Doppelquantenfilm in AlSb Matrix mit einer variablen InAs Schichtdicke. Die InAs Schichtdicke wurde {\"u}ber numerische Simulationen so gew{\"a}hlt, dass je eine Probe im trivialen Regime, eine im invertierten Regime und eine am {\"U}bergang liegt. Gezeigt werden in Kapitel 4.2 Magnetotransportmessungen f{\"u}r konstante Frontgatespannungen sowie Messungen mit konstantem Magnetfeld gegen die Frontgatespannung. Die Messungen best{\"a}tigen eine Fertigung quantitativ vergleichbarer Proben, zeigen aber auch, dass keine der Proben im topologischen Regime liegt. Hierf{\"u}r kommen mehrere Ursachen in Betracht: Eine {\"U}bersch{\"a}tzung der Hybridisierung durch die numerische Simulation, zu geringe InAs Schichtdicken in der Fertigung oder ein asymmetrisches Verschieben mit nur einem Gate (Kapitel 4.3). Zur Reduktion der Volumenleitf{\"a}higkeit wurden Al-haltigen Schichten am GaSb/InAs {\"U}bergang eingebracht. Die erwartete Widerstandssteigerung konnte in ersten Versuchen nicht gezeigt werde. Die in Kapitel 5 gezeigte optische Manipulation des dominanten Ladungstr{\"a}gertyps der InAs/GaSb-Doppelquantent{\"o}pfe gibt eine zus{\"a}tzliche Kontrollm{\"o}glichkeit im Phasendiagramm. Optische Anregung erm{\"o}glicht den Wechsel der Majorit{\"a}tsladungstr{\"a}ger von Elektronen zu L{\"o}chern. Dabei wird ein Regime durchlaufen, in dem beide Ladungstr{\"a}ger koexistieren. Dies weist stark auf eine Elektron-Loch-Hybridisierung mit nichttrivialer topologischer Phase hin. Dabei spielen zwei unterschiedliche physikalische Prozesse eine Rolle, die analog eines Frontgates bzw. eines Backgates wirken. Der Frontgate Effekt beruht auf der negativ persistenten Photoleitf{\"a}higkeit, der Backgate Effekt fußt auf der Akkumulation von Elektronen auf der Substratseite. Das hier gezeigte optisch kontrollierte Verschieben der Zust{\"a}nde belegt die Realisation von opto-elektronischem Schalten zwischen unterschiedlichen topologischen Phasen. Dies zeigt die M{\"o}glichkeit einer optischen Kontrolle des Phasendiagramms der topologischen Zust{\"a}nde in GaSb/InAs Doppelquantenfilmen. In Abschnitt 5.1 wird die optische Verstimmung von GaSb/InAs Quantenfilmen gezeigt und erkl{\"a}rt. Sie wird in Abh{\"a}ngigkeit von der Temperatur, der Anregungswellenl{\"a}nge sowie der Anregungsintensit{\"a}t untersucht. Kontrollversuche an Proben mit einem unterschiedlichen Strukturaufbau zeigen, dass das Vorhandensein eines {\"U}bergitters auf der Substratseite der Quantenfilmstruktur essentiell f{\"u}r die Entstehung der Backgate-Wirkung ist (Abschnitt 5.2). Abschließend werden in Abschnitt 5.3 die Erkenntnisse zur optischen Kontrolle zusammengefasst und deren M{\"o}glichkeiten, wie optisch definierte topologischen Phasen-Grenzfl{\"a}chen, diskutiert. Bi2Se3 Nanostrukturen Mit Blick auf die Vorteile eines erh{\"o}hten Oberfl{\"a}che-zu-Volumen Verh{\"a}ltnisses ist die Verwendung von Nanostrukturen f{\"u}r das Anwendungsgebiet der dreidimensionalen topologischen Isolatoren effizient. Mit dem Ziel, diesen Effekt f{\"u}r die Realisation des topologischen Isolators in Bi2Se3 auszunutzen, wurde im Rahmen dieser Arbeit erstmalig das Wachstum von Bi2Se3 Nanodr{\"a}hten und -flocken mit Molekularstrahlepitaxie realisiert. In Kapitel 6 werden technische und physikalische Grundlagen hierzu erl{\"a}utert (Abschnitt 6.1). Ausgehend von einer Einf{\"u}hrung in dreidimensionale topologische Isolatoren werden die Eigenschaften des topologischen Zustandes in Bi2Se3 gezeigt. Darauf folgen die Kristalleigenschaften von Bi2Se3 sowie die Erkl{\"a}rung des epitaktischen Wachstums von Nanostrukturen mit Molekularstrahlepitaxie. In Abschnitt 6.2 schließt sich die Beschreibung der epitaktischen Herstellung an. Die Kristallstruktur wurde mittels hochaufl{\"o}sender R{\"o}ntgendiffraktometrie und Transmissionselektronenmikroskopie als Bi2Se3 identifiziert. Rasterelektronenmikroskopie-Aufnahmen zeigen Nanodr{\"a}hte und Nanoflocken auf mit Gold vorbehandelten bzw. nicht mit Gold vorbehandelten Proben. Der Wachstumsmechanismus f{\"u}r Nanodr{\"a}hte kann nicht zweifelsfrei definiert werden. Das Fehlen von Goldtr{\"o}pfchen an der Drahtspitze legt einen wurzelbasierten Wachstumsmechanismus nahe (Abschnitt 6.3).}, language = {de} } @phdthesis{Schnells2019, author = {Schnells, Vera}, title = {Fractional Insulators and their Parent Hamiltonians}, doi = {10.25972/OPUS-18561}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-185616}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2019}, abstract = {In the past few years, two-dimensional quantum liquids with fractional excitations have been a topic of high interest due to their possible application in the emerging field of quantum computation and cryptography. This thesis is devoted to a deeper understanding of known and new fractional quantum Hall states and their stabilization in local models. We pursue two different paths, namely chiral spin liquids and fractionally quantized, topological phases. The chiral spin liquid is one of the few examples of spin liquids with fractional statistics. Despite its numerous promising properties, the microscopic models for this state proposed so far are all based on non-local interactions, making the experimental realization challenging. In the first part of this thesis, we present the first local parent Hamiltonians, for which the Abelian and non-Abelian chiral spin liquids are the exact and, modulo a topological degeneracy, unique ground states. We have developed a systematic approach to find an annihilation operator of the chiral spin liquid and construct from it a many-body interaction which establishes locality. For various system sizes and lattice geometries, we numerically find largely gapped eigenspectra and confirm to an accuracy of machine precision the uniqueness of the chiral spin liquid as ground state of the respective system. Our results provide an exact spin model in which fractional quantization can be studied. Topological insulators are one of the most actively studied topics in current condensed matter physics research. With the discovery of the topological insulator, one question emerged: Is there an interaction-driven set of fractionalized phases with time reversal symmetry? One intuitive approach to the theoretical construction of such a fractional topological insulator is to take the direct product of a fractional quantum Hall state and its time reversal conjugate. However, such states are well studied conceptually and do not lead to new physics, as the idea of taking a state and its mirror image together without any entanglement between the states has been well understood in the context of topological insulators. Therefore, the community has been looking for ways to implement some topological interlocking between different spin species. Yet, for all practical purposes so far, time reversal symmetry has appeared to limit the set of possible fractional states to those with no interlocking between the two spin species. In the second part of this thesis, we propose a new universality class of fractionally quantized, topologically ordered insulators, which we name "fractional insulator". Inspired by the fractional quantum Hall effect, spin liquids, and fractional Chern insulators, we develop a wave function approach to a new class of topological order in a two-dimensional crystal of spin-orbit coupled electrons. The idea is simply to allow the topological order to violate time reversal symmetry, while all locally observable quantities remain time reversal invariant. We refer to this situation as "topological time reversal symmetry breaking". Our state is based on the Halperin double layer states and can be viewed as a two-layer system of an ↑-spin and a ↓-spin sphere. The construction starts off with Laughlin states for the ↑-spin and ↓-spin electrons and an interflavor term, which creates correlations between the two layers. With a careful parameter choice, we obtain a state preserving time reversal symmetry locally, and label it the "311-state". For systems of up to six ↑-spin and six ↓-spin electrons, we manage to construct an approximate parent Hamiltonian with a physically realistic, local interaction.}, subject = {Spinfl{\"u}ssigkeit}, language = {en} } @phdthesis{Huewe2017, author = {H{\"u}we, Florian}, title = {Electrothermal Investigation on Charge and Heat Transport in the Low-Dimensional Organic Conductor (DCNQI)\(_2\)Cu}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-153492}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {This thesis aimed at the coherent investigation of the electrical and thermal transport properties of the low-dimensional organic conductor (DCNQI)2M (DCNQI: dicyanoquinonediimine; M: metallic counterion). These radical anion salts present a promising, new material class for thermoelectric applications and hence, a consistent characterization of the key parameters is required to evaluate and to optimize their performance. For this purpose, a novel experimental measurement setup enabling the determination of the electrical conductivity, the Seebeck coefficient and the thermal conductivity on a single crystalline specimen has been designed and implemented in this work. The novel measurement setup brought to operation within this thesis enabled a thorough investigation of the thermal transport properties in the (DCNQI)2M system. The thermal conductivity of (DCNQI-h8)2Cu at RT was determined to κ=1.73 W m^(-1) K^(-1). By reducing of the copper content in isostructural, crystalline (DMe-DCNQI)2CuxLi1-x alloys, the electrical conductivity has been lowered by one order of magnitude and the correlated changes in the thermal conductivity allowed for a verification of the Wiedemann-Franz (WF) law at RT. A room temperature Lorenz number of L=(2.48±0.45)⋅〖10〗^(-8) WΩK^(-2) was obtained in agreement with the standard Lorenz number L_0=2,44⋅〖10〗^(-8) WΩK^(-2) for 3D bulk metals. This value appears to be significantly reduced upon cooling below RT, even far above the Debye temperature of θ_D≈82 K, below which a breakdown of the WF law is caused by different relaxation times in response to thermal and to electric field perturbations. The experimental data enabled the first consistent evaluation of the thermoelectric performance of (DCNQI)\$_2\$Cu. The RT power factor of 110 μWm^(-1) K^(-2) is comparable to values obtained on PEDOT-based thermoelectric polymers. The RT figure of merit amounts to zT=0.02 which falls short by a factor of ten compared to the best values of zT=0.42 claimed for conducting polymers. It originates from the larger thermal conductivity in the organic crystals of about 1.73 W m^(-1) K^(-1) in (DCNQI)2Cu. Yet, more elaborate studies on the anisotropy of the thermal conductivity in PEDOT polymers assume their figure of merit to be zT=0.15 at most, recently. Therefore, (DCNQI)2Cu can be regarded as thermoelectric material of similar performance to polymer-based ones. Moreover, it represents one of the best organic n-type thermoelectric materials to date and as such, may also become important in hybrid thermoelectrics in combination with conducting polymers. Upon cooling below room temperature, (DCNQI)2Cu reveals its full potential attaining power factors of 50 mW K^(-2) m^(-1) and exceeding values of zT>0.15 below 40 K. These values represent the best thermoelectric performance in this low-temperature regime for organic as well as inorganic compounds and thus, low-dimensional organic conductors might pave the way toward new applications in cryogenic thermoelectrics. Further improvements may be expected from optimizing the charge carrier concentration by taking control over the CT process via the counterion stack of the crystal lattice. The concept has also been demonstrated in this work. Moreover, the thermoelectric performance in the vicinity of the CDW transition in (MeBr-DCNQI)2Cu was found to be increased by a factor of 5. Accordingly, the diversity of electronic ground states accessible in organic conductors provides scope for further improvements. Finally, the prototype of an all-organic thermoelectric generator has been built in combination with the p-type organic metal TTT2I3. While it only converts about 0.02\% of the provided heat into electrical energy, the specific power output per active area attains values of up to 5 mW cm^(-2). This power output, defining the cost-limiting factor in the recovery of waste heat, is three orders of magnitude larger than in conducting polymer devices and as such, unrivaled in organic thermoelectrics. While the thermoelectric key parameters of (DCNQI)2Cu still lack behind conventional thermoelectrics made of e.g. Bi2Te3, the promising performance together with its potential for improvements make this novel material class an interesting candidate for further exploration. Particularly, the low-cost and energy-efficient synthesis routes of organic materials highlight their relevance for technological applications.}, subject = {Radikalanionensalz}, language = {en} } @phdthesis{Herrmann2016, author = {Herrmann, Oliver}, title = {Graphene-based single-electron and hybrid devices, their lithography, and their transport properties}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-146924}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2016}, abstract = {This work explores three different aspects of graphene, a single-layer of carbon atoms arranged in a hexagonal lattice, with regards to its usage in future electronic devices; for instance in the context of quantum information processing. For a long time graphene was believed to be thermodynamically unstable. The discovery of this strictly two-dimensional material completed the family of carbon based structures, which had already been subject of intensive research with focus on zero-dimensional fullerenes and one-dimensional carbon nanotubes. Within only a few years of its discovery, the field of graphene related research has grown into one of today's most diverse and prolific areas in condensed matter physics, highlighted by the award of the 2010 Nobel Prize in Physics to A.K. Geim and K. Noveselov for "their groundbreaking experiments regarding the two-dimensional material graphene". From the point of view of an experimental physicist interested in the electronic properties of a material system, the most intriguing characteristic of graphene is found in the Dirac-like nature of its charge carriers, a peculiar fact that distinguishes graphene from all other known standard semiconductors. The dynamics of charge carriers close to zero energy are described by a linear energy dispersion relation, as opposed to a parabolic one, which can be understood as a result of the underlying lattice symmetry causing them to behave like massless relativistic particles. This fundamentally different behavior can be expected to lead to the observation of completely new phenomena or the occurrence of deviations in well-known effects. Following a brief introduction of the material system in chapter 2, we present our work studying the effect of induced superconductivity in mesoscopic graphene Josephson junctions by proximity to superconducting contacts in chapter 3. We explore the use of Nb as the superconducting material driven by the lack of high critical temperature and high critical magnetic field superconductor technology in graphene devices at that time. Characterization of sputter-deposited Nb films yield a critical transition temperature of \(T_{C}\sim 8{\rm \,mK}\). A prerequisite for successful device operation is a high interface quality between graphene and the superconductor. In this context we identify the use of an Ti as interfacial layer and incorporate its use by default in our lithography process. Overall we are able to increase the interface transparency to values as high as \(85\\%\). With the prospect of interesting effects in the ballistic regime we try to enhance the electronic quality of our Josephson junction devices by substrate engineering, yet with limited success. We achieve moderate charge carrier mobilities of up to \(7000{\rm \,cm^2/Vs}\) on a graphene/Boron-nitride heterostructure (fabrication details are covered in chapter 5) putting the junction in the diffusive regime (\(L_{device} d_c = 6.3 nm aufweisen m{\"u}ssen. Im Anschluss wird das Konzept eines QPCs allgemein eingef{\"u}hrt sowie das zugeh{\"o}rige Transportverhalten analytisch beschrieben. {\"U}berdies werden die Einschr{\"a}nkungen und Randbedingungen diskutiert, welche bei der Realisierung eines QPCs in einem QSH-System Ber{\"u}cksichtigung finden m{\"u}ssen. Darauf folgt die Pr{\"a}sentation des eigens zur QPC-Herstellung entwickelten Lithographieprozesses, welcher auf einer mehrstufigen Anwendung eines f{\"u}r HgTe-Quantentrogstrukturen geeigneten nasschemischen {\"A}tzverfahrens beruht. Die im Nachgang diskutierten Transportmessungen exemplarischer Proben zeigen die erwartete Leitwertquantisierung in Schritten von ΔG ≈ 2e^2/h im Bereich des Leitungsbandes -- sowohl f{\"u}r eine topologische als auch f{\"u}r eine triviale (d_QW < d_c) QPC-Probe. Mit dem Erreichen der Bandl{\"u}cke saturiert der Leitwert f{\"u}r den topologischen QPC um G_QSH ≈ 2e^2/h, wohingegen ebenjener f{\"u}r den Fall des trivialen Bauteils auf G ≈ 0 abf{\"a}llt. Dar{\"u}ber hinaus belegen durchgef{\"u}hrte Messungen des differentiellen Leitwertes einer invertierten QPC-Probe in Abh{\"a}ngigkeit einer Biasspannung die stabile Koexistenz von topologischen und trivialen Transportmoden. Gegenstand von Kapitel 3 ist die Beschreibung der Ausbildung eines QSH-Interferometers in QPCs mit geringer Weite, welche unter Verwendung von Quantentr{\"o}gen mit einer Trogdicke von d_QW = 7 nm hergestellt werden. Die Diskussion von Bandstrukturrechnungen legt dar, dass die r{\"a}umliche Ausdehnung der Randkan{\"a}le von der jeweiligen Position der Fermi-Energie im Bereich der Bandl{\"u}cke abh{\"a}ngt. Hieraus resultiert eine Transportsituation, in welcher -- unter bestimmten Voraussetzungen -- Reservoir-Elektronen mit randomisiertem Spin an beide QSH-Randkan{\"a}le mit gleicher Wahrscheinlichkeit koppeln, was in der Ausbildung eines QSH-Rings resultiert. Diese Ringbildung wird im Rahmen eines durch Plausibilit{\"a}ts{\"u}berpr{\"u}fung getesteten Modells erkl{\"a}rt und spezifiziert. Danach erfolgt eine theoretische Einf{\"u}hrung von drei relevanten Quantenphasen, deren Akkumulation in der Folge f{\"u}r mehrere geeignete QPC-Proben nachgewiesen wird. Es handelt sich hierbei um die Aharonov-Bohm-Phase, um die dynamische Aharonov-Casher-Phase sowie um eine Spin-Bahn-Berry-Phase mit einem Wert von π. Diese experimentellen Ergebnisse stehen dar{\"u}ber hinaus im Einklang mit analytischen Modellbetrachtungen. Das anschließende Kapitel 4 stellt den letzten Teil der Arbeit dar und besch{\"a}ftigt sich mit der Beobachtung einer anomalen Leitwertsignatur, welche f{\"u}r QPC-Proben basierend auf einer Quantentrogdicke von d_QW = 10.5 nm auftritt. Diese Proben zeigen neben der durch die QSH-Phase bedingten Leitwertquantisierung von G_QSH ≈ 2e^2/h ein weiteres Leitwertplateau mit einem Wert von G ≈ e^2/h = 0.5 x G_QSH. Diese sogenannte 0.5-Anomalie ist nur f{\"u}r ein kleines Intervall von QPC-Weiten beobachtbar und wird mit zunehmender Bauteilweite abgeschw{\"a}cht. Weiterf{\"u}hrende Untersuchungen in Abh{\"a}ngigkeit der Temperatur sowie einer angelegten Biasspannung deuten dar{\"u}ber hinaus darauf hin, dass das Auftreten der 0.5-Anomalie mit einem modifizierten topologischen Zustand einhergeht. {\"U}berdies wird eine zus{\"a}tzliche sowie vervollst{\"a}ndigende Charakterisierung dieses Transportregimes durch die Realisierung eines neuartigen Bauteilkonzeptes m{\"o}glich, welches einen QPC in eine standardisierte Hall-Bar-Geometrie integriert. Das Ergebnis der experimentellen Analyse einer solchen Probe verkn{\"u}pft das Auftreten der 0.5-Anomalie mit der R{\"u}ckstreuung eines QSH-Randkanals. Demgem{\"a}ß wird aus Sicht des Einteilchenbildes geschlussfolgert, dass im Kontext der 0.5-Anomalie lediglich ein Randkanal transmittiert wird. Zudem werden zwei theoretische Modelle basierend auf Elektron-Elektron-Wechselwirkungen diskutiert, welche beide jeweils als urs{\"a}chlicher Mechanismus f{\"u}r das Auftreten der 0.5-Anomalie in Frage kommen. Abschließend ist zu deduzieren, dass die Implementierung einer QPC-Technologie in einem QSH-System eine bedeutende Entwicklung im Bereich der Erforschung von zweidimensionalen topologischen Isolatoren darstellt, welche eine Vielzahl zuk{\"u}nftiger Experimente erm{\"o}glicht. So existieren beispielsweise theoretische Vorhersagen, dass QPCs in einem QSH-System die Detektion von Majorana- sowie Para-Fermionen erm{\"o}glichen. {\"U}berdies ist die nachgewiesene Ausbildung eines QSH-Interferometers in geeigneten QPC-Proben eine Beobachtung von großer Folgewirkung. So erm{\"o}glicht die beobachtete dynamische Aharonov-Casher-Phase im QSH-Regime die kontrollierbare Modulation des topologischen Leitwertes, was die konzeptionelle Grundlage eines topologischen Transistors darstellt. Eine weitere Anwendungsm{\"o}glichkeit wird durch die Widerstandsf{\"a}higkeit geometrischer Phasen gegen{\"u}ber Dephasierung er{\"o}ffnet, wodurch die nachgewiesene Spin-Bahn-Berry-Phase mit einem Wert von π im Kontext potentieller Quantencomputerkonzepte von Interesse ist. Dar{\"u}ber hinaus ist die Transmission von nur einem QSH-Randkanal im Zuge des Auftretens der 0.5-Anomalie {\"a}quivalent zu 100 \% Spinpolarisierung, was einen Faktor essentieller Relevanz f{\"u}r die Realisierung spintronischer Anwendungen darstellt. Demgem{\"a}ß beinhaltet die vorliegende Arbeit den experimentellen Nachweis von drei unterschiedlichen Effekten, von welchen jedem einzelnen eine fundamentale Rolle im Rahmen der Entwicklung neuer Generationen logischer Bauelemente zukommen kann -- erm{\"o}glicht durch die Realisierung von QPCs in topologischen HgTe-Quantentr{\"o}gen.}, subject = {Topologischer Isolator}, language = {en} } @phdthesis{Harder2022, author = {Harder, Tristan H.}, title = {Topological Modes and Flatbands in Microcavity Exciton-Polariton Lattices}, doi = {10.25972/OPUS-25900}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-259008}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {The fascination of microcavity exciton-polaritons (polaritons) rests upon the combination of advanced technological control over both the III-V semiconductor material platform as well as the precise spectroscopic access to polaritonic states, which provide access to the investigation of open questions and complex phenomena due to the inherent nonlinearity and direct spectroscopic observables such as energy-resolved real and Fourier space information, pseudospin and coherence. The focus of this work was to advance the research area of polariton lattice simulators with a particular emphasis on their lasing properties. Following the brief introduction into the fundamental physics of polariton lattices in chapter 2, important aspects of the sample fabrication as well as the Fourier spectroscopy techniques used to investigate various features of these lattices were summarized in chapter 3. Here, the implementation of a spatial light modulator for advanced excitation schemes was presented. At the foundation of this work is the capability to confine polaritons into micropillars or microtraps resulting in discrete energy levels. By arranging these pillars or traps into various lattice geometries and ensuring coupling between neighbouring sites, polaritonic band structures were engineered. In chapter 4, the formation of a band structure was visualised in detail by investigating ribbons of honeycomb lattices. Here, the transition of the discrete energy levels of a single chain of microtraps to the fully developed band structure of a honeycomb lattice was observed. This study allows to design the size of individual domains in more complicated lattice geometries such that a description using band structures becomes feasible, as it revealed that a width of just six unit cells is sufficient to reproduce all characteristic features of the S band of a honeycomb lattice. In particular in the context of potential technological applications in the realms of lasing, the laser-like, coherent emission from polariton microcavities that can be achieved through the excitation of polariton condensates is intriguing. The condensation process is significantly altered in a lattice potential environment when compared to a planar microcavity. Therefore, an investigation of the polariton condensation process in a lattice with respect to the characteristics of the excitation laser, the exciton-photon detuning as well as the reduced trap distance that represents a key design parameter for polaritonic lattices was performed. Based on the demonstration of polariton condensation into multiple bands, the preferred condensation into a desired band was achieved by selecting the appropriate detuning. Additionally, a decreased condensation threshold in confined systems compared to a planar microcavity was revealed. In chapter 5, the influence of the peculiar feature of flatbands arising in certain lattice geometries, such as the Lieb and Kagome lattices, on polaritons and polariton condensates was investigated. Deviations from a lattice simulator described by a tight binding model that is solely based on nearest neighbour coupling cause a remaining dispersiveness of the flatbands along certain directions of the Brillouin zone. Therefore, the influence of the reduced trap distance on the dispersiveness of the flatbands was investigated and precise technological control over the flatbands was demonstrated. As next-nearest neighbour coupling is reduced drastically by increasing the distance between the corresponding traps, increasing the reduced trap distance enables to tune the S flatbands of both Lieb and Kagome lattices from dispersive bands to flatbands with a bandwidth on the order of the polariton linewidth. Additionally to technological control over the band structures, the controlled excitation of large condensates, single compact localized state (CLS) condensates as well as the resonant excitation of polaritons in a Lieb flatband were demonstrated. Furthermore, selective condensation into flatbands was realised. This combination of technological and spectroscopic control illustrates the capabilities of polariton lattice simulators and was used to study the coherence of flatband polariton condensates. Here, the ability to tune the dispersiveness from a dispersive band to an almost perfect flatband in combination with the selectivity of the excitation is particularly valuable. By exciting large flatband condensates, the increasing degree of localisation to a CLS with decreasing dispersiveness was demonstrated by measurements of first order spatial coherence. Furthermore, the first order temporal coherence of CLS condensates was increased from τ = 68 ps for a dispersive flatband, a value typically achieved in high-quality microcavity samples, to a remarkable τ = 459 ps in a flatband with a dispersiveness below the polarion linewidth. Corresponding to this drastic increase of the first order coherence time, a decrease of the second order temporal coherence function from g(2)(τ =0) = 1.062 to g(2)(0) = 1.035 was observed. Next to laser-like, coherent emission, polariton condensates can form vortex lattices. In this work, two distinct vortex lattices that can form in polariton condensates in Kagome flatbands were revealed. Furthermore, chiral, superfluid edge transport was realised by breaking the spatial symmetry through a localised excitation spot. This chirality was related to a change in the vortex orientation at the edge of the lattice and thus opens the path towards further investigations of symmetry breaking and chiral superfluid transport in Kagome lattices. Arguably the most influential concept in solid-state physics of the recent decades is the idea of topological order that has also provided a new degree of freedom to control the propagation of light. Therefore, in chapter 6, the interplay of topologically non-trivial band structures with polaritons, polariton condensates and lasing was emphasised. Firstly, a two-dimensional exciton-polariton topological insulator based on a honeycomb lattice was realised. Here, a topologically non-trivial band gap was opened at the Dirac points through a combination of TE-TM splitting of the photonic mode and Zeeman splitting of the excitonic mode. While the band gap is too small compared to the linewidth to be observed in the linear regime, the excitation of polariton condensates allowed to observe the characteristic, topologically protected, chiral edge modes that are robust against scattering at defects as well as lattice corners. This result represents a valuable step towards the investigation of non-linear and non-Hermitian topological physics, based on the inherent gain and loss of microcavities as well as the ability of polaritons to interact with each other. Apart from fundamental interest, the field of topological photonics is driven by the search of potential technological applications, where one direction is to advance the development of lasers. In this work, the starting point towards studying topological lasing was the Su-Schrieffer-Heeger (SSH) model, since it combines a simple and well-understood geometry with a large topological gap. The coherence properties of the topological edge defect of an SSH chain was studied in detail, revealing a promising degree of second order temporal coherence of g(2)(0) = 1.07 for a microlaser with a diameter of only d = 3.5 µm. In the context of topological lasing, the idea of using a propagating, topologically protected mode to ensure coherent coupling of laser arrays is particularly promising. Here, a topologically non-trivial interface mode between the two distinct domains of the crystalline topological insulator (CTI) was realised. After establishing selective lasing from this mode, the coherence properties were studied and coherence of a full, hexagonal interface comprised of 30 vertical-cavity surface-emitting lasers (VCSELs) was demonstrated. This result thus represents the first demonstration of a topological insulator VCSEL array, combining the compact size and convenient light collection of vertically emitting lasers with an in-plane topological protection. Finally, in chapter 7, an approach towards engineering the band structures of Lieb and honeycomb lattices by unbalancing the eigenenergies of the sites within each unit cell was presented. For Lieb lattices, this technique opens up a path towards controlling the coupling of a flatband to dispersive bands and could enable a detailed study of the influence of this coupling on the polariton flatband states. In an unbalanced honeycomb lattice, a quantum valley Hall boundary mode between two distinct, unbalanced honeycomb domains with permuted sites in the unit cells was demonstrated. This boundary mode could serve as the foundation for the realisation of a polariton quantum valley Hall effect with a truly topologically protected spin based on vortex charges. Modifying polariton lattices by unbalancing the eigenenergies of the sites that comprise a unit cell was thus identified as an additional, promising path for the future development of polariton lattice simulators.}, subject = {Exziton-Polariton}, language = {en} } @phdthesis{Tcakaev2023, author = {Tcakaev, Abdul-Vakhab}, title = {Soft X-ray Spectroscopic Study of Electronic and Magnetic Properties of Magnetic Topological Insulators}, doi = {10.25972/OPUS-30378}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-303786}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {After the discovery of three-dimensional topological insulators (TIs), such as tetradymite chalcogenides Bi\$_2\$Se\$_3\$, Bi\$_2\$Te\$_3\$ and Sb\$_2\$Te\$_3\$ - a new class of quantum materials characterized by their unique surface electronic properties - the solid state community got focused on topological states that are driven by strong electronic correlations and magnetism. An important material class is the magnetic TI (MTI) exhibiting the quantum anomalous Hall (QAH) effect, i.e. a dissipationless quantized edge-state transport in the absence of external magnetic field, originating from the interplay between ferromagnetism and a topologically non-trivial band structure. The unprecedented opportunities offered by these new exotic materials open a new avenue for the development of low-dissipation electronics, spintronics, and quantum computation. However, the major concern with QAH effect is its extremely low onset temperature, limiting its practical application. To resolve this problem, a comprehensive understanding of the microscopic origin of the underlying ferromagnetism is necessary. V- and Cr-doped (Bi,Sb)\$_2\$Te\$_3\$ are the two prototypical systems that have been widely studied as realizations of the QAH state. Finding microscopic differences between the strongly correlated V and Cr impurities would help finding a relevant model of ferromagnetic coupling and eventually provide better control of the QAH effect in these systems. Therefore, this thesis first focuses on the V- and Cr-doped (Bi,Sb)\$_2\$Te\$_3\$ systems, to better understand these differences. Exploiting the unique capabilities of x-ray absorption spectroscopy and magnetic circular dichroism (XAS/XMCD), combined with advanced modeling based on multiplet ligand-field theory (MLFT), we provide a detailed microscopic insight into the local electronic and magnetic properties of these systems and determine microscopic parameters crucial for the comparison with theoretical models, which include the \$d\$-shell filling, spin and orbital magnetic moments. We find a strongly covalent ground state, dominated by the superposition of one and two Te-ligand-hole configurations, with a negligible contribution from a purely ionic 3+ configuration. Our findings indicate the importance of the Te \$5p\$ states for the ferromagnetism in (Bi, Sb)\$_2\$Te\$_3\$ and favor magnetic coupling mechanisms involving \$pd\$-exchange. Using state-of-the-art density functional theory (DFT) calculations in combination with XMCD and resonant photoelectron spectroscopy (resPES), we reveal the important role of the \$3d\$ impurity states in mediating magnetic exchange coupling. Our calculations illustrate that the kind and strength of the exchange coupling varies with the impurity \$3d\$-shell occupation. We find a weakening of ferromagnetic properties upon the increase of doping concentration, as well as with the substitution of Bi at the Sb site. Finally, we qualitatively describe the origin of the induced magnetic moments at the Te and Sb sites in the host lattice and discuss their role in mediating a robust ferromagnetism based on a \$pd\$-exchange interaction scenario. Our findings reveal important clues to designing higher \$T_{\text{C}}\$ MTIs. Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of TIs, which is favorable for the realization of the high-temperature QAH effect. Therefore, we have further focused on Eu-doped Bi\$_2\$Te\$_3\$ and scrutinized whether the conditions for formation of a substantial gap in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the \$M_{4,5}\$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu\$^{2+}\$ valence, unlike most other rare earth elements, and confirms a large magnetic moment. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic ordering. This is in good agreement with DFT results, which predict AFM interactions between the Eu impurities due to the direct overlap of the impurity wave functions. Our results support the notion of antiferromagnetism coexisting with topological surface states in rare-earth doped Bi\$_2\$Te\$_3\$ and corroborate the potential of such doping to result in an antiferromagnetic TI with exotic quantum properties. The doping with impurities introduces disorder detrimental for the QAH effect, which may be avoided in stoichiometric, well-ordered magnetic compounds. In the last part of the thesis we have investigated the recently discovered intrinsic magnetic TI (IMTI) MnBi\$_6\$Te\$_{10}\$, where we have uncovered robust ferromagnetism with \$T_{\text{C}} \approx 12\$ K and connected its origin to the Mn/Bi intermixing. Our measurements reveal a magnetically intact surface with a large moment, and with FM properties similar to the bulk, which makes MnBi\$_6\$Te\$_{10}\$ a promising candidate for the QAH effect at elevated temperatures. Moreover, using an advanced ab initio MLFT approach we have determined the ground-state properties of Mn and revealed a predominant contribution of the \$d^5\$ configuration to the ground state, resulting in a \$d\$-shell electron occupation \$n_d = 5.31\$ and a large magnetic moment, in excellent agreement with our DFT calculations and the bulk magnetometry data. Our results together with first principle calculations based on the DFT-GGA\$+U\$, performed by our collaborators, suggest that carefully engineered intermixing plays a crucial role in achieving a robust long-range FM order and therefore could be the key for achieving enhanced QAH effect properties. We expect our findings to aid better understanding of MTIs, which is essential to help increasing the temperature of the QAH effect, thus facilitating the realization of low-power electronics in the future.}, subject = {Topologischer Isolator}, language = {en} } @phdthesis{Kissner2022, author = {Kißner, Katharina}, title = {Manipulation of electronic properties in strongly correlated Cerium-based surface alloys}, doi = {10.25972/OPUS-27306}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-273067}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Photoelectron spectroscopy proves as a versatile tool for investigating various aspects of the electronic structure in strongly correlated electron systems. Influencing the manifestation of strong correlation in Ce-based surface alloys is the main task of this work. It is shown, that the manifestation of the Kondo ground state is influenced by a multitude of parameters such as the choice of the metal binding partner in binary Ce compounds, the surface alloy layer thickness and accompanying variations in the lattice structure as well as the interfaces to substrate or vacuum. Gaining access to these parameters allows to directly influence essential state variables, such as the f level occupancy nf or the Kondo temperature TK. The center of this work are the intermetallic thin films of CePt5/Pt(111) and CeAgx/Ag(111). By utilizing different excitation energies, photoemission spectroscopy provides access to characteristic features of Kondo physics in the valence band, such as the Kondo resonance and its spin-orbit partner at the Fermi level, as well as the multiplet structure of the Ce 3d core levels. In this work both approaches are applied to CePt5/Pt(111) to determine nf and TK for a variety of surface alloy layer thicknesses. A temperature dependent study of the Ce 3d core levels allows to determine the systems TK for the different layer thicknesses. This leads to TK ≈200-270K in the thin layer thickness regime and TK >280K for larger layer thicknesses. These results are confirmed by fitting the Ce 3d multiplet based on the Gunnarsson-Sch{\"o}nhammer formalism for core level spectroscopy and additionally by valence band photoemission spectra of the respective Kondo resonances. The influence of varying layer thickness on the manifestation of strong correlation is subsequently studied for the surface alloy CeAgx/Ag(111). Furthermore, the heavy element Bi is added, to investigate the effects of strong spin-orbit coupling on the electronic structure of the surface alloy.}, subject = {Korrelation}, language = {en} } @phdthesis{Matthaiakakis2021, author = {Matthaiakakis, Ioannis}, title = {Hydrodynamics in Solid State Systems and the AdS/CFT correspondence}, doi = {10.25972/OPUS-24439}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-244390}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {We employ the AdS/CFT correspondence and hydrodynamics to analyze the transport properties of \(2+1\) dimensional electron fluids. In this way, we use theoretical methods from both condensed matter and high-energy physics to derive tangible predictions that are directly verifiable in experiment. The first research topic we consider is strongly-coupled electron fluids. Motivated by early results by Gurzhi on the transport properties of weakly coupled fluids, we consider whether similar properties are manifest in strongly coupled fluids. More specifically, we focus on the hydrodynamic tail of the Gurzhi effect: A decrease in fluid resistance with increasing temperature due to the formation of a Poiseuille flow of electrons in the sample. We show that the hydrodynamic tail of the Gurzhi effect is also realized in strongly coupled and fully relativistic fluids, but with modified quantitative features. Namely, strongly-coupled fluids always exhibit a smaller resistance than weakly coupled ones and are, thus, far more efficient conductors. We also suggest that the coupling dependence of the resistance can be used to measure the coupling strength of the fluid. In view of these measurements, we provide analytical results for the resistance as a function of the shear viscosity over entropy density \(\eta/s\) of the fluid. \(\eta/s\) is itself a known function of the coupling strength in the weak and infinite coupling limits. In further analysis for strongly-coupled fluids, we propose a novel strongly coupled Dirac material based on a kagome lattice, Scandium-substituted Herbertsmithite (ScHb). The large coupling strength of this material, as well as its Dirac nature, provides us with theoretical and experimental access to non-perturbative relativistic and quantum critical physics. A highly suitable method for analyzing such a material's transport properties is the AdS/CFT correspondence. Concretely, using AdS/CFT we derive an estimate for ScHb's \(\eta/s\) and show that it takes a value much smaller than that observed in weakly coupled materials. In turn, the smallness of \(\eta/s\) implies that ScHb's Reynolds number, \(Re\), is large. In fact, \(Re\) is large enough for turbulence, the most prevalent feature of fluids in nature, to make its appearance for the first time in electronic fluids. Switching gears, we proceed to the second research topic considered in this thesis: Weakly coupled parity-breaking electron fluids. More precisely, we analyze the quantitative and qualitative changes to the classical Hall effect, for electrons propagating hydrodynamically in a lead. Apart from the Lorentz force, a parity-breaking fluid's motion is also impacted by the Hall-viscous force; the shear-stress force induced by the Hall-viscosity. We show that the interplay of these two forces leads to a hydrodynamic Hall voltage with non-linear dependence on the magnetic field. More importantly, the Lorentz and Hall-viscous forces become equal at a non-vanishing magnetic field, leading to a trivial hydrodynamic Hall voltage. Moreover, for small magnetic fields we provide analytic results for the dependence of the hydrodynamic Hall voltage on all experimentally-tuned parameters of our simulations, such as temperature and density. These dependences, along with the zero of the hydrodynamic Hall voltage, are distinct features of hydrodynamic transport and can be used to verify our predictions in experiments. Last but not least, we consider how a distinctly electronic property, spin, can be included into the hydrodynamic framework. In particular, we construct an effective action for non-dissipative spin hydrodynamics up to first order in a suitably defined derivative expansion. We also show that interesting spin-transport effects appear at second order in the derivative expansion. Namely, we show that the fluid's rotation polarizes its spin. This is the hydrodynamic manifestation of the Barnett effect and provides us with an example of hydrodynamic spintronics. To conclude this thesis, we discuss several possible extensions of our research, as well as proposals for research in related directions.}, subject = {Hydrodynamics}, language = {en} } @phdthesis{Bendias2018, author = {Bendias, Michel Kalle}, title = {Quantum Spin Hall Effect - A new generation of microstructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-168214}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {The presented thesis summarizes the results from four and a half years of intense lithography development on (Cd,Hg)Te/HgTe/(Cd,Hg)Te quantum well structures. The effort was motivated by the unique properties of this topological insulator. Previous work from Molenkamp at al.\ has proven that the transport through such a 2D TI is carried by electrons with opposite spin, counter-propagating in 1D channels along the sample edge. However, up to this thesis, the length of quantized spin Hall channels has never been reported to exceed 4 µm. Therefore, the main focus was put on a reproducible and easy-to-handle fabrication process that reveals the intrinsic material parameters. Every single lithography step in macro as well as microscopic sample fabrication has been re-evaluated. In the Development, the process changes have been presented along SEM pictures, microgaphs and, whenever possible, measurement responses. We have proven the conventional ion milling etch method to damage the remaining mesa and result in drastically lower electron mobilities in samples of microscopic size. The novel KI:I2:HBr wet etch method for macro and microstructure mesa fabrication has been shown to leave the crystalline structure intact and result in unprecedented mobilities, as high as in macroscopic characterization Hall bars. Difficulties, such as an irregular etch start and slower etching of the conductive QW have been overcome by concentration, design and etch flow adaptations. In consideration of the diffusive regime, a frame around the EBL write field electrically decouples the structure mesa from the outside wafer. As the smallest structure, the frame is etched first and guarantees a non-different etching of the conductive layer during the redox reaction. A tube-pump method assures reproducible etch results with mesa heights below 300 nm. The PMMA etch mask is easy to strip and leaves a clean mesa with no redeposition. From the very first attempts, to the final etch process, the reader has been provided with the characteristics and design requirements necessary to enable the fabrication of nearly any mesa shape within an EBL write field of 200 µm. Magneto resistance measurement of feed-back samples have been presented along the development chronology of wet etch method and subsequent lithography steps. With increasing feature quality, more and more physics has been revealed enabling detailed evaluation of smallest disturbances. The following lithography improvements have been implemented. They represent a tool-box for high quality macro and microstructure fabrication on (CdHg)Te/HgTe of almost any kind. The optical positive resist ECI 3027 can be used as wet and as dry etch mask for structure sizes larger than 1 µm. It serves to etch mesa structures larger than the EBL write field. The double layer PMMA is used for ohmic contact fabrication within the EBL write field. Its thickness allows to first dry etch the (Cd,Hg)Te cap layer and then evaporate the AuGe contact, in situ and self-aligned. Because of an undercut, up to 300 nm can be metalized without any sidewalls after the lift-off. An edge channel mismatch within the contact leads can be avoided, if the ohmic contacts are designed to reach close to the sample and beneath the later gate electrode. The MIBK cleaning step prior to the gate application removes PMMA residuals and thereby improves gate and potential homogeneity. The novel low HfO2-ALD process enables insulator growth into optical and EBL lift-off masks of any resolvable shape. Directly metalized after the insulator growth, the self-aligned method results in thin and homogeneous gate electrode reproducibly withholding gate voltages to +-10 V. The optical negative resist ARN 4340 exhibits an undercut when developed. Usable as dry etch mask and lift-off resist, it enables an in-situ application of ohmic contacts first etching close to the QW, then metalizing AuGe. Up to 500 nm thickness, the undercut guarantees an a clean lift-off with no sidewalls. The undertaken efforts have led to micro Hall bar measurements with Hall plateaus and SdH-oszillations in up to now unseen levels of detail. The gap resistance of several micro Hall bars with a clear QSH signal have been presented in Quantum Spin Hall. The first to exhibit longitudinal resistances close to the expected h/2e2 since years, they reveal unprecedented details in features and characteristics. It has been shown that their protection against backscattering through time reversal symmetry is not as rigid as previously claimed. Values below and above 12.9 kΩ been explained, introducing backscattering within the Landauer-B{\"u}ttiker formalism of edge channel transport. Possible reasons have been discussed. Kondo, interaction and Rashba-backscattering arising from density inhomogeneities close to the edge are most plausible to explain features on and deviations from a quantized value. Interaction, tunneling and dephasing mechanisms as well as puddle size, density of states and Rashba Fields are gate voltage dependent. Therefore, features in the QSH signal are fingerprints of the characteristic potential landscape. Stable up to 11 K, two distinct but clear power laws have been found in the higher temperature dependence of the QSH in two samples. However, with ΔR = Tα, α = ¼ in one (QC0285) and α = 2 in the other (Q2745), none of the predicted dependencies could be confirmed. Whereas, the gap resistances of QC0285 remains QSH channel dominated up to 3.9 T and thereby confirmed the calculated lifting of the band inversion in magnetic field. The gate-dependent oscillating features in the QSH signal of Q2745 immediately increase in magnetic field. The distinct field dependencies allowed the assumption of two different dominant backscattering mechanisms. Resulting in undisturbed magneto transport and unprecedented QSH measurements The Novel Micro Hall Bar Process has proven to enable the fabrication of a new generation of microstructures.}, subject = {Quecksilbertellurid}, 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{Gottscholl2022, author = {Gottscholl, Andreas Paul}, title = {Optical Accessible Spin Defects in Hexagonal Boron Nitride: Identification, Control and Application of the Negatively Charged Boron Vacancy VB-}, doi = {10.25972/OPUS-27432}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-274326}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {In this work, a bridge was built between the so-far separate fields of spin defects and 2D systems: for the first time, an optically addressable spin defect (VB-) in a van der Waals material (hexagonal boron nitride) was identified and exploited. The results of this thesis are divided into three topics as follows: 1.) Identification of VB-: In the scope of this chapter, the defect ,the negatively charged boron vacancy VB-, is identified and characterized. An initialization and readout of the spin state can be demonstrated optically at room temperature and its spin Hamiltonian contributions can be quantified. 2.) Coherent Control of VB-: A coherent control is required for the defect to be utilized for quantum applications, which}, subject = {Bornitrid}, language = {en} } @phdthesis{Betzold2022, author = {Betzold, Simon}, title = {Starke Licht-Materie-Wechselwirkung und Polaritonkondensation in hemisph{\"a}rischen Mikrokavit{\"a}ten mit eingebetteten organischen Halbleitern}, doi = {10.25972/OPUS-26665}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-266654}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {Kavit{\"a}ts-Exziton-Polaritonen (Polaritonen) sind hybride Quasiteilchen, die sich aufgrund starker Kopplung von Halbleiter-Exzitonen mit Kavit{\"a}tsphotonen ausbilden. Diese Quasiteilchen weisen eine Reihe interessanter Eigenschaften auf, was sie einerseits f{\"u}r die Grundlagenforschung, andererseits auch f{\"u}r die Entwicklung neuartiger Bauteile sehr vielversprechend macht. Bei Erreichen einer ausreichend großen Teilchendichte geht das System in den Exziton-Polariton-Kondensationszustand {\"u}ber, was zur Emission von laserartigem Licht f{\"u}hrt. Organische Halbleiter als aktives Emittermaterial zeigen in diesem Kontext großes Potential, da deren Exzitonen neben großen Oszillatorst{\"a}rken auch hohe Bindungsenergien aufweisen. Deshalb ist es m{\"o}glich, unter Verwendung organischer Halbleiter selbst bei Umgebungsbedingungen {\"a}ußerst stabile Polaritonen zu erzeugen. Eine wichtige Voraussetzung zur Umsetzung von integrierten opto-elektronischen Bauteilen basierend auf Polaritonen ist der kontrollierte r{\"a}umliche Einschluss sowie die Realisierung von frei konfigurierbaren Potentiallandschaften. Diese Arbeit besch{\"a}ftigt sich mit der Entwicklung und der Untersuchung geeigneter Plattformen zur Erzeugung von Exziton-Polaritonen und Polaritonkondensaten in hemisph{\"a}rischen Mikrokavit{\"a}ten, in die organische Halbleiter eingebettet sind.}, subject = {Exziton-Polariton}, language = {de} } @phdthesis{Fischer2023, author = {Fischer, Mathias}, title = {Transient Phenomena and Ionic Kinetics in Hybrid Metal Halide Perovskite Solar Cells}, doi = {10.25972/OPUS-32220}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322204}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {The fact that photovoltaics is a key technology for climate-neutral energy production can be taken as a given. The question to what extent perovskite will be used for photovoltaic technologies has not yet been fully answered. From a photophysical point of view, however, it has the potential to make a useful contribution to the energy sector. However, it remains to be seen whether perovskite-based modules will be able to compete with established technologies in terms of durability and cost efficiency. The additional aspect of ionic migration poses an additional challenge. In the present work, primarily the interaction between ionic redistribution, capacitive properties and recombination dynamics was investigated. This was done using impedance spectroscopy, OCVD and IV characteristics as well as extensive numerical drift-diffusion simulations. The combination of experimental and numerical methods proved to be very fruitful. A suitable model for the description of solar cells with respect to mobile ions was introduced in chapter 4.4. The formal mathematical description of the model was transferred by a non-dimensionalization and suitable numerically solvable form. The implementation took place in the Julia language. By intelligent use of structural properties of the sparse systems of equations, automatic differentiation and the use of efficient integration methods, the simulation tool is not only remarkably fast in finding the solution, but also scales quasi-linearly with the grid resolution. The software package was released under an open source license. In conventional semiconductor diodes, capacitance measurements are often used to determine the space charge density. In the first experimental chapter 5, it is shown that although this is also possible for the ionic migration present in perovskites, it cannot be directly understood as doping related, since the space charge distribution strongly depends on the preconditions and can be manipulated by an externally applied voltage. The exact form of this behavior depends on the perovskite composition. This shows, among other things, that experimental results can only be interpreted within the framework of conventional semiconductors to a very limited extent. Nevertheless, the built-in 99 potential of the solar cell can be determined if the experiments are carried out properly. A statement concerning the type and charge of the mobile ions is not possible without further effort, while their number can be determined. The simulations were applied to experimental data in chapter 6. Thus, it could be shown that mobile ions make a significant contribution to the OCVD of perovskite solar cells. j-V characteristics and OCVD transients measured as a function of temperature and illumination intensities could be quantitatively modeled simultaneously using a single global set of parameters. By the simulations it was further possible to derive a simple experimental procedure to determine the concentration and the diffusivity of the mobile ions. The possibility of describing different experiments in a uniform temperaturedependent manner strongly supports the model of mobile ions in perovskites. In summary, this work has made an important contribution to the elucidation of ionic contributions to the (photo)electrical properties of perovskite solar cells. Established experimental techniques for conventional semiconductors have been reinterpreted with respect to ionic mass transport and new methods have been proposed to draw conclusions on the properties for ionic transport. As a result, the published simulation tools can be used for a number of further studies.}, subject = {Simulation}, language = {en} } @phdthesis{Schwemmer2023, author = {Schwemmer, Tilman}, title = {Relativistic corrections of Fermi surface instabilities}, doi = {10.25972/OPUS-31964}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-319648}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {Relativistic effects crucially influence the fundamental properties of many quantum materials. In the accelerated reference frame of an electron, the electric field of the nuclei is transformed into a magnetic field that couples to the electron spin. The resulting interaction between an electron spin and its orbital angular momentum, known as spin-orbit coupling (SOC), is hence fundamental to the physics of many condensed matter phenomena. It is particularly important quantitatively in low-dimensional quantum systems, where its coexistence with inversion symmetry breaking can lead to a splitting of spin degeneracy and spin momentum locking. Using the paradigm of Landau Fermi liquid theory, the physics of SOC can be adequately incorporated in an effective single particle picture. In a weak coupling approach, electronic correlation effects beyond single particle propagator renormalization can trigger Fermi surface instabilities such as itinerant magnetism, electron nematic phases, superconductivity, or other symmetry broken states of matter. In this thesis, we use a weak coupling-based approach to study the effect of SOC on Fermi surface instabilities and, in particular, superconductivity. This encompasses a weak coupling renormalization group formulation of unconventional superconductivity as well as the random phase approximation. We propose a unified formulation for both of these two-particle Green's function approaches based on the notion of a generalized susceptibility. In the half-Heusler semimetal and superconductor LuPtBi, both SOC and electronic correlation effects are prominent, and thus indispensable for any concise theoretical description. The metallic and weakly dispersive surface states of this material feature spin momentum locked Fermi surfaces, which we propose as a possible domain for the onset of unconventional surface superconductivity. Using our framework for the analysis of Fermi surface instability and combining it with ab-initio density functional theory calculations, we analyse the surface band structure of LuPtBi, and particularly its propensity towards Cooper pair formation. We study how the presence of strong SOC modifies the classification of two-electron wave functions as well as the screening of electron-electron interactions. Assuming an electronic mechanism, we identify a chiral superconducting condensate featuring Majorana edge modes to be energetically favoured over a wide range of model parameters.}, subject = {Supraleitung}, language = {en} } @phdthesis{Scheffler2023, author = {Scheffler, Lukas}, title = {Molecular beam epitaxy of the half-Heusler antiferromagnet CuMnSb}, doi = {10.25972/OPUS-32283}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-322839}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {This work presents a newly developed method for the epitaxial growth of the half-Heusler antiferromagnet CuMnSb. All necessary process steps, from buffer growth to the deposition of a protective layer, are presented in detail. Using structural, electrical, and magnetic characterization, the material parameters of the epitaxial CuMnSb layers are investigated. The successful growth of CuMnSb by molecular beam epitaxy is demonstrated on InAs (001), GaSb (001), and InP (001) substrates. While CuMnSb can be grown pseudomorphically on InAs and GaSb, the significant lattice mismatch for growth on InP leads to relaxation already at low film thicknesses. Due to the lower conductivity of GaSb compared to InAs, GaSb substrates are particularly suitable for the fabrication of CuMnSb layers for lateral electrical transport experiments. However, by growing a high-resistive ZnTe interlayer below the CuMnSb layer, lateral transport experiments on CuMnSb layers grown on InAs can also be realized. Protective layers of Ru and Al2O3 have proven to be suitable for protecting the CuMnSb layers from the environment. Structural characterization by high resolution X-ray diffraction (full width at half maximum of 7.7 ′′ of the rocking curve) and atomic force microscopy (root mean square surface roughness of 0.14 nm) reveals an outstanding crystal quality of the epitaxial CuMnSb layers. The half-Heusler crystal structure is confirmed by scanning transmission electron microscopy and the stoichiometric material composition by Rutherford backscattering spectrometry. In line with the high crystal quality, a new minimum value of the residual resistance of CuMnSb (𝜌0 = 35 μΩ ⋅ cm) could be measured utilizing basic electrical transport experiments. An elaborate study of epitaxial CuMnSb grown on GaSb reveals a dependence of the vertical lattice parameter on the Mn/Sb flux ratio. This characteristic enables the growth of tensile, unstrained, and compressive strained CuMnSb layers on a single substrate material. Additionally, it is shown that the N{\´e}el temperature has a maximum of 62 K at stoichiometric material composition and thus can be utilized as a selection tool for stoichiometric CuMnSb samples. Mn-related defects are believed to be the driving force for these observations. The magnetic characterization of the epitaxial CuMnSb films is performed by superconducting quantum interference device magnetometry. Magnetic behavior comparable to the bulk material is found, however, an additional complex magnetic phase appears in thin CuMnSb films and/or at low magnetic fields, which has not been previously reported for CuMnSb. This magnetic phase is believed to be localized at the CuMnSb surface and exhibits both superparamagnetic and spin-glass-like behavior. The exchange bias effect of CuMnSb is investigated in combination with different in- and out-of-plane ferromagnets. It is shown that the exchange bias effect can only be observed in combination with in-plane ferromagnets. Finally, the first attempts at the growth of fully epitaxial CuMnSb/NiMnSb heterostructures are presented. Both magnetic and structural studies by secondary-ion mass spectrometry indicate the interdiffusion of Cu and Ni atoms between the two half-Heusler layers, however, an exchange bias effect can be observed for the CuMnSb/NiMnSb heterostructures. Whether this exchange bias effect originates from exchange interaction between the CuMnSb and NiMnSb layers, or from ferromagnetic inclusions in the antiferromagnetic layer can not be conclusively identified.}, subject = {Molekularstrahlepitaxie}, language = {en} } @phdthesis{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} }