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 Electronic and structural instabilities in the putative excitonic insulator Ta2NiSe5(2025) Zhang, Yuanshan; Takagi, Hidenori (Prof. Dr.)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 Exploring the growth of refractory metal and sapphire films by thermal laser epitaxy(2024) Majer, Lena N.; 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 Atomic scale electron spin resonance at high Zeeman energies(2022) Kot, Piotr; Ast, Christian (Dr. habil.)Item Open Access Long-range order, bosonic fluctuations, and pseudogap in strongly correlated electron systems(2022) Bonetti, Pietro Maria; Metzner, Walter (Prof. Dr.)Item Open Access Electronic properties of rare earth vanadate heterostructures(2022) Radhakrishnan, Padma; Keimer, Bernhard (Prof. Dr.)RVO3 compounds are a class of strongly correlated Mott insulators that harbour a complex interplay of spin, orbital and lattice degrees of freedom, which gives rise to an equally intricate phase diagram. They display two distinct types of spin ordering and orbital ordering (C-OO/G-SO and G-OO/C-SO) phases as a function of the rare-earth cation size and temperature. Due to the competing nature of its two ground states, these compounds present an ideal playground for exploring the prospects of heterostructuring to manipulate their properties. In this thesis we study different aspects of heteroepitaxy on RVO3 (R=Y, La), focusing mainly on YVO3, which sits at the edge of the phase boundary of the two spin-orbital phases, and exhibits both phases as a function of temperature. We synthesized films and superlattices of RVO3 using pulsed laser deposition. In the first part, we used resonant reflectometry to investigate YVO3-LaAlO3 superlattices and found an inversion of orbital polarization of the xz and yz orbitals between the interface and central layers of the YVO3 slab, which is stable down to 30 K. We explain these results based on epitaxial strain and spatial confinement by LaAlO3, at the interface. Further, we delve deeper into the relationship between the structural and electronic properties using a combination of DFT+U calculations and extensive structural analysis. The results reveal that the substrate facet can be used to obtain the desired orientation for YVO3 heterostructures, which, together with the thickness of the YVO3 layers and the presence of spacer layers (such as LaAlO3) in a superlattice, govern the resulting orbital polarization. In the second part, we used polarized Raman spectroscopy and ellipsometry to explore the effect of strain on the orbital ordering patterns of YVO3 and LaVO3 films. We found that the c-axis compression for (110)-oriented films stabilizes the G-OO phase, whereas c-axis elongation for (001)-oriented films favours the C-OO phase. By isolating the influences of the sign of strain, degree of strain and orientation of the unit cell, we attribute these results to strain-induced bond-length changes which in turn alter the superexchange interactions and lattice effects that are known to compete in bulk RVO3. Our results reveal that strain and interface engineering are a promising route to substantially modify the properties of these compounds and thus illustrate the diverse prospects of heteroepitaxy to tailor the properties of strongly-correlated transition metal oxides.Item Open Access Crystalline topological semimetals : classification and properties(2022) Leonhardt, Andreas; Metzner, Walter (Prof. Dr.)Item Open Access Scanning tunneling microscopy study on topological material candidates with orbital and spin degrees of freedom(2024) Que, Xinglu; Takagi, Hidenori (Prof. Dr.)