Universität Stuttgart
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Item Open Access High quality graphene for magnetic sensing(2022) Herlinger, Patrick; Smet, Jurgen (Dr.)In this thesis, we investigated the reliable fabrication of high quality graphene and its use as Hall transducer material. Charged impurities and random strain fluctuations were identified as main culprits that deteriorate the electrical properties of graphene devices. It was shown that these extrinsic sources of disorder can be reduced through optimized device processing steps as well as the use of a proper substrate material for graphene such as hexagonal boron nitride (hBN). This insulating material is atomically flat and possesses a very low intrinsic density of charged impurities. By performing Raman spectroscopy and electrical transport measurements, both without and with applied magnetic field, on a large number of different types of graphene devices, it was demonstrated that the encapsulation of graphene between hexagonal boron nitride thin films is the best way to obtain high quality graphene devices. However, even for these hBN-encapsulated devices, we still observed a notable sample-to-sample variation of the electrical properties. Therefore, we developed a post-processing technique that allows us to improve the electrical properties of such devices both significantly and reliably. Since our technique is applied after device fabrication, we could also demonstrate its beneficial effect by comparing one and the same device before and after treatment. We then assessed the application of such high quality graphene as Hall transducer material. The dependencies on and between all relevant operating parameters were explored. This allowed us to develop a deep understanding and empirical model for graphene Hall elements, including the interplay between thermal and 1/f noise in these devices. All key performance indicators for Hall sensors were measured and their typical values reported. For comparable device dimensions, hBN-encapsulated graphene Hall elements were found to have the potential to become a strong competitor to existing materials that are used in today's commercial Hall sensors. Unfortunately, the large-scale fabrication of hBN thin films still remains an unresolved challenge for the industrialization of large area, high quality graphene Hall elements. Also, the Si CMOS integration demands further development. Even though the application of graphene in Hall devices is promising, as shown in this work, this use case alone does likely not justify the significant efforts and investments we expect to be necessary to industrialize the fabrication of high quality graphene devices. Instead, these efforts and costs must be shared by developing a common technology platform for 2D materials that can address several commercially attractive applications where graphene or another 2D material offers superior performance as well. We hope that the insights provided in this work can help to accelerate this process.Item Open Access Ultrafast spectroscopy of single quantum dots(2012) Wolpert, Christian; Lippitz, Markus (Juniorprofessor Dr.)In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.Item Open Access Polarized neutron reflectometry study of complex magnetism and hydrogen incorporation in thin-film structures(2022) Guasco, Laura; Keimer, Bernhard (Prof. Dr.)In this thesis, we present the study of the structural and magnetic properties of simple metals and complex oxide thin films by means of polarized neutron reflectometry. The nuclear and electronic properties of thin films were modified via two routes, namely via hydrogen incorporation, in the case of niobium systems and complex oxide layers, and via depth modulated hole doping, in the case of manganite heterostructures.Item Open Access Structure and electronic properties of epitaxial monolayer WSe2(2019) Mohammed, Avaise; Takagi, Hidenori (Prof. Dr.)Item Open Access From Hermitian to non-Hermitian topological phases of matter(2019) Rui, Wenbin; Metzner, Walter (Prof. Dr.)The focus of this thesis lies on extending the theory of topological phases of matter from the Hermitian to the non-Hermitian regime. This includes not only the extension of conventional concepts such as topological invariants and topological boundary states in the theory of Hermitian topological phases, but also the exploration and characterization of entirely new topological phases unique to non-Hermitian systems.Item Open Access Investigating superconductivity by tunneling spectroscopy using oxide heterostructures(2017) Fillis-Tsirakis, Evangelos; Mannhart, Jochen (Prof. Dr.)Item Open Access Real-space spectroscopy of interacting quasiparticles in exotic semimetals(2022) He, Qingyu; Takagi, Hidenori (Prof. Dr.)Item Open Access The superconductor-insulator-transition in the LaAlO3-SrTiO3 electron system : tunneling and field effect investigations(2017) Kürten, Lukas; Mannhart, Jochen (Prof. Dr.)The work described in this thesis investigated the superconductor-insulator- and BKT-transitions in a two-dimensional superconductor based on transition-metal oxides: the electron system at the LaAlO3-SrTiO3 interface. Thin films of insulating LaAlO3 grown onto insulating SrTiO3 substrates exhibit a two-dimensional electron system (2DES) with a superconducting ground state at the interface of the two materials. The ground state can be switched from superconducting to insulating behavior by application of a gate voltage. Therefore it is a useful tool for the investigation of these phase transitions. Additionally, LaAlO3-SrTiO3 exhibits a phase similar to the cuprate pseudogap phase in which electron transport is resistive, but a gap can be observed in the density of states in tunneling experimentswhich is not explicable by the standard BCS theory of superconductivity. The phase transitions were investigated by two complimentary pathways: by tunneling experiments on backgated samples and through the construction of superconducting transistors in which the electron system is influenced by the application of topgate voltages.Item Open Access Realistic calculations for correlated materials(2019) Toloui-Mantadakis, Daniil; Hansmann, Philipp (Dr.)Strongly correlated fermionic systems nowadays stand in the forefront of condensed matter physics. A plethora of phenomena, ranging from unconventional superconductivity, gigantic and colossal magneto-resistance and metal-to-insulator transitions, are attributed to the effects of electron correlation. Given the spectacular progress on the experimental side, today - more than ever - the understanding of the underlying microscopic mechanisms, and the explanation or even prediction of experimental observations becomes a necessity. The advancements of theoretical and computational methodologies together with a concurrent increase of computational power, allows for both the ab initio study of realistic materials and the investigation of low-energy effective Hamiltonians inspired and designed to resemble whole classes of compounds. This work is conceptually divided into two major parts. In Chapter 3 and Chapter 4, we present our results, obtained by the state-of-the-art merger of density functional and dynamical mean-field theory, for two realistic systems: the layered LaNiO2/LaGaO3 superstructure, where we focus on the orbital resolved single-particle spectral functions and study the effect of electron and hole doping; and the ruthenate system Ca2RuO4, for which we provide a clear understanding and theoretical support of the experimentally observed semi-metallic state under the application of DC current. The second conceptual part of this work deals with the study of low-energy effective Hamiltonians. In Chapter 4, we investigate a generic t2g model Hamiltonian in the presence of non-spherical crystal-field potentials and/or spin-orbit coupling in order to shed more light on the distinct features that arise on the single-particle level and, most importantly, on the two-particle observables, such as the uniform and static magnetic susceptibilities. In Chapter 5, we investigate the multi-orbital extension of the periodic Anderson model, as inspired by the family of cerium-based heavy-fermion compounds, with a clear focus on the evolution of the dynamic behavior of the systems' moments.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.