Universität Stuttgart
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Item Open Access Excitonic Fano resonances in Ta2NiSe5 and Ta2NiS5(2016) Larkin, Timofei I.; Keimer, Bernhard (Prof. Dr.)Item Open Access Proximity effects and Josephson currents in ferromagnet : spin-triplet superconductors junctions(2015) Terrade, Damien; Metzner, Walter (Prof. Dr.)Spin-triplet superconductivity, first attached to the description of He3, is now generally considered to also occur in heavy-fermions compounds and in perovskite ruthenium oxide Sr2RuO4. The latter material is especially interesting since many experiments show strong evidences for a unitary chiral spin-triplet state. Moreover, the recent fabrication of thin heterostructures made of ferromagnetic SrRuO3 on the top of Sr2RuO4 strongly encourages new theoretical studies on the interplay between spin-triplet superconductor and ferromagnet in similar fashion to spin-singlet superconductors. Using an extended tight-binding Hamiltonian to model the superconductor, we discuss in this thesis the specific proximity effects of such interface by solving self-consistently the Bogoliubov-De Gennes equations on two- and three-dimensional lattices in the ballistic limit. We obtain the spatial profile of the superconducting order parameters at the interface as well as the spin-polarisation and the current across the Josephson junctions. In contrast to heterostructures made of spin-singlet superconductor, we show that the physical properties at the interface are not only controlled by the strength of the magnetization inside the ferromagnet but also by its orientation due to the existence of a finite pair spin projection of the spin-triplet Cooper pairs. We analyse in the first part the spin-polarisation and the Gibbs free energy at the three-dimensional ferromagnet-chiral spin-triplet superconductor interface. Then, the second part of the thesis is dedicated to the study of the Josephson junctions made of a chiral spin-triplet superconductor and a ferromagnetic barrier. More precisely, we analyse the existence of 0-Pi state transitions in two- and three-dimensional junctions with respect to the strength and the orientation of the magnetization. Finally, we study the proximity effects at the interface of helical spin-triplet superconductors. They differ from the chiral superconductor by the direction of the pair spin polarisation of the Cooper pairs and by the properties of the edge states, present at the boundaries, which can sustain dissipationless spin-current.Item Open Access Role of disorder and interactions on the surface of topological superconductors(2015) Queiroz, Raquel; Metzner, Walter (Prof. Dr.)In this work we study the surface properties of topological systems, with a special focus on topological superconductors without inversion symmetry. These materials provide a rich playground for multiple topological phenomena, showing boundary modes with linear and (or) flat dispersion arising from complex nodal structures. A remarkable characteristic of topological phases is their robustness to local perturbations. In the present work, we explore the extent to which this robustness can be generalized to gapless topological phases. We numerically test the robustness of topological boundary modes against local disorder and explore the contrast between different disorder strengths and distributions. Additionally, we study the interplay between topology and electron-electron interactions at the surface of nodal superconductors, where the infinitely degenerate flat bands are susceptible to spontaneous symmetry breaking. Finally, we briefly look into possible symmetry preserving interactions that can lead to the destruction of the boundary modes.Item Open Access Structural and electronic properties of nickelate heterostructures(2016) Wrobel, Friederike; Keimer, Bernhard (Prof. Dr.)The fabrication of thin films and multilayers has led to the discovery of novel functional properties which are widely used in electronic devices nowadays. The limit of such a material design is atomic layer-by-layer deposition which was made possible through shuttered molecular beam epitaxy (MBE) growth. In the course of this thesis project a newly developed oxide MBE system was used to grow two different types of nickelate heterostructures, namely superlattices (SLs) consisting of metallic and paramagnetic LaNiO3 sandwiched between a large band-gap insulator and a combination of lanthanum nickelate and cuprate layers into a single hybrid structure. The former type was intensively studied in the last years and a transition to a weakly insulating, antiferromagnetically ordered state was observed in samples where the LaNiO3 thickness had been reduced to only two unit cells. So far little was known about the influence of the growth method on the defects in the samples and consequently on their physical properties. The use of oxide MBE enabled us to improve the overall sample quality of nickelate SLs and to design a novel material. We first optimized the growth of LaNiO3 and thoroughly analyzed the heterostructures by synchrotron-based x-ray diffraction, transmission electron microscopy, and temperature-dependent electrical resistivity. Furthermore we conducted in-depth studies, including x-ray absorption and magneto-transport measurements. The knowledge gained thereby was used grow new, layered nickelate-cuprate hybrid structures with novel electronic and magnetic properties.Item Open Access Ordering phenomena in nickelate heterostructures studied by elastic and inelastic photon scattering(2016) Hepting, Matthias; Keimer, Bernhard (Prof. Dr.)Item Open Access Magnetic and transport properties of YBa2Cu3O7 - La0.7Ca0.3MnO3 heterostructures(2016) Mustafa, Luqman; Keimer, Bernhard (Prof. Dr.)The exploration of interface properties in complex oxide heterostructures and superlattices is one of the new exciting fields in condensed matter sciences. This is particularly originating from the technological advances in synthesizing heterostructures with atomic scale precision by advanced thin film deposition techniques. There is a plethora of novel achievements culminating in unexpected results, such as generating artificial multifunctional materials with the prominent example of the appearance of interface electrical conductivity and even superconductivity in between insulating films (SrTiO3 - LaAlO3). In this thesis a special case of heterostructures is treated. Here, heterostructures composed of superconducting YBa2Cu3O7 and half-metallic ferromagnetic La2/3Ca1/3MnO3 are investigated and the interplay of the two long-range antagonistic ordering principles - superconductivity and ferromagnetism - is intended to be studied. Whereas the physics of such structures with the CuO2 planes of the superconducting YBa2Cu3O7 oriented parallel to the substrate plane ( i.e. the short coherence length of the superconductor, ξc ~ 0.1 nm is facing the interface perpendicular) has been explored in great detail, little is known in the case of the CuO2 planes oriented perpendicular to the substrate plane and thus ab ~ 1.6 nm is pointing perpendicular to the interface. In the former case, the properties of the heterostructures and superlattices are determined by an interplay of charge transfer and orbital reconstruction, but the mechanisms occurring in the latter case are unknown so far. Prior to elaborated experiments to study the interface properties at an atomistic scale, the technology of fabricating such structures has to be accomplished and their macroscopic properties (structure, transport and magnetic properties) have to be investigated. It is the goal of this thesis to prepare the ground for the atomistic studies by developing the technological prerequisites for the growth of (110)-oriented YBCO-LCMO heterostructures and characterize their structural, electric and magnetic macroscopic properties. Due to the sensitivity of the macroscopic properties of such structures to the crystallographic perfection of the interfaces a substantial part of this thesis is devoted to the corresponding enabling technology. Advanced PLD techniques are used to fabricate single layer (110)-oriented YBCO and LCMO films, bilayers as well as twin-free (103)-oriented LCMO-YBCO-LCMO trilayers and (110)-oriented YBCO-LCMO-YBCO structures with ultrathin LCMO films (nominally 1-2 nm ) in between 50 nm YBCO. These (110)-oriented trilayers serve as a precursor for a prototype planar Josephson junction technology. A comparison of the experimental results for (001) and (110) - oriented heterostructures reveals distinct changes in the ordering temperatures Tc and TCurie giving a fingerprint of different microscopic mechanisms taking place at the interfaces. Furthermore, in the twin-free (103)-oriented trilayer samples a novel positive Meissner effect has been observed which is ascribed to the magnetic domain arrangement of the LCMO.Item Open Access In situ characterization of phase evolution in LiFePO4(2015) Ohmer, Nils; Maier, Joachim (Prof. Dr.)Among the candidates for electrodes in future Li-based batteries, LiFePO4 (LFP) is one of the most important and most frequently studied materials, undergoing a phase transformation upon delithiation to FePO4 (FP). In spite of the great scientific and practical interest in this material, there is still an extensive debate on the mechanism of this phase transformation and the underlying factors of influence. Within the framework of this thesis, first studies are carried out ex situ on multi-particle, full electrode LFP materials, being electrochemically cycled and analyzed at various states of charge by a combination of highly spatially resolved methods (high-resolution transmission electron microscopy and electron energy loss spectroscopy (HRTEM, EELS)) and integral measurement techniques (analyzing the X-ray diffraction and X-ray absorption near edge structure (XRD, XANES)). This combination of characterization techniques allows one to distinguish between the cycling behaviour of differently sized crystallites within the same electrode. It is found that for electrodes with hydrothermally grown LFP as active material, a particle size dependent cycling behaviour exists, with nanosized particles apparently not participating in the charging process at all. A turbostratic stacking of layers in these nanosized particles is found and identified to be responsible for sluggish lithium insertion and extraction. These higher dimensional defects prevent the small particles from participating in the charging process, most likely by disturbing the lithium diffusion along the 1-dimensional channels, as well as impair the transport along the other directions in the LFP host structure and thus blocking the lithium transport, resulting in a comparibly lower practical capacity during electrochemical cycling. To study the lithium exchange mechanism upon charging a LFP thin film cathode, an all-solid-state thin film battery cell with a lateral design concept is developed and realized by pulsed laser deposition (PLD) and thermal evaporation techniques. Using PLD and shadow masks LFP cathode, Li2O-V2O5-SiO2 (LVSO) electrolyte and LiAl anode thin films are deposited sequentially in a way that the Li transport pathway in the resulting battery is along the X-ray transparent commercial Si3N4 membrane substrate. This enables the usability of synchrotron-based energy resolved scanning transmission X-ray microscopy (STXM) with its high chemical and spatial resolution to perform in situ absorption measurements at the Fe L3 edge. Upon delithiation, a shift in the main absorption feature from 708 to 710 eV is used to fingerprint the change in the local state of charge, identifying areas containing Fe2+ (lithiated) and Fe3+ (delithiated), respectively. The initial lithiation process of a LFP thin film cathode material has been followed by in situ STXM, with a lateral resolution of 30 nm, during electrochemical charging of the thin film battery. The observed initial lithiation process does not follow the classical particle by particle mechanism, typical for multi-particle LFP cathodes, but instead a rather simultaneous, although inhomogeneous, lithiation is observed. The reason for this change in mechanism, compared to multi-particle powder electrodes, is found in mechanical interactions within the thin film upon lithiation, i.e. in the corresponding volume expansion and formation of high energy surfaces, changing the shape of the single-particle chemical potential to a monotone form upon lithiation. This has far-reaching consequences: not only the many-particle mechanism is changed to a concurrent lithiation, but also the single-particle mechanism is changed from a two-phase to a single-phase mechanism upon lithiation. Furthermore, a vanishing hysteresis loop and the disappearing of the memory effect is predicted. These findings are rather general and applicable to all kind of thin films of phase separating intercalation materials, undergoing a volume change upon lithium exchange. To fill the gap in literature on in situ observations of the (L)FP phase evolution on a single-particle level with appreciable space and time resolution, a micrometer-sized all-solid-state thin film battery is built with a defect-chemically well characterized LFP single crystal as cathode material with dimensions of 16x1x0.2 micrometer. Using STXM, the phase evolution along the fast (010) orientation is followed during in situ electrochemical (de)lithiation on a micro-meter scale with a lateral resolution of 30 nm and with minutes of time resolution. Furthermore, the STXM measurements performed on this sample are one of the few experiments ever taken on LFP materials with a well defined defect chemistry, even though fundamentally necessary for an overall understanding of the materials behaviour. This combination discloses not only the mechanism of LFP transformation on a single-particle level, but also the significance of elastic effects on the (de)lithiation process. Using a defect chemical analysis, the position of phase formation is found to be determined by the defect chemical situation, while the growth pattern of both LFP and FP is found to be dominated by elastic effects.Item Open Access Oxidische FETs mit sub-100 nm Gatelänge(2016) Woltmann, Carsten; Mannhart, Jochen (Prof. Dr.)Item Open Access Transport properties of electrolyte gated graphene devices(2015) Paolucci, Federico; von Klitzing, Klaus (Prof. Dr.)Graphene is a single layer of carbon atoms arranged in a honeycomb lattice. It is the building block of graphite. The latter is made out of weakly coupled (van der Waals force) graphene layers stacked one on the other. Graphene was isolated in 2004 through micro-mechanical cleavage of graphite. The interaction between lattice and charge carriers produces a linear electronic dispersion relation. Therefore, the charge carriers in graphene mimic chiral particles with zero mass. Many interesting physical properties were shown in graphene including room temperature integer quantum Hall effect, fractional quantum Hall effect, high temperature ballistic transport, and Hofstadter's butterfly. Superconductivity is predicted in graphene at extremely high carrier concentrations, but it has never been experimentally proven. Electrolyte gating allows inducing high charge carrier concentration in a wide range of materials. These achievable densities are one order of magnitude lower than chemical doping, but two orders of magnitude higher than classic solid gating. Contrary to chemical doping, electric field induced charges do not affect the crystal structure of the studied material. In multilayer graphene also intercalation of ions in between the graphene planes is conceivable in electrolyte gated devices. It causes changes in the physical properties of graphene.Item Open Access Experimental investigation of VO2-based field-effect devices(2016) Lanzano, Marco; Mannhart, Jochen (Prof. Dr.)Field-effect transistors based on Mott insulators could overcome the limits encountered with modern electronic devices. In this work I investigate the construction of an all-oxide device amiming at characterizing the electric-field-induced metal-insulator transition in vanadium oxide (VO2) in order to define the potential of this material for next-generation electronic devices.