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%0 Thesis
%A Behner, Gerrit
%T Quantum transport, interference and multi-terminal effects in topological insulator nano-devices : towards topological superconductivity
%I RWTH Aachen University
%V Dissertation
%C Aachen
%M RWTH-2025-08393
%P 1 Online-Ressource : Illustrationen
%D 2025
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, RWTH Aachen University, 2025
%X As classical transistors approach atomic dimensions, quantum mechanical effects become increasingly significant, imposing fundamental limits on further miniaturization and performance enhancement. In response to these constraints, quantum computing has emerged as a transformative paradigm, harnessing quantum superposition and entanglement to enable computational capabilities that surpass those of classical systems. Despite substantial progress in recent years, significant challenges persist on the path towards universal quantum computing, particularly with respect to scalability and error mitigation. Topological quantum computing, an approach to realizing qubits, the fundamental building blocks of quantum computers, using exotic quasiparticles known as Majorana zero modes, addresses these challenges by encoding quantum information in a manner that is inherently protected from local errors. This intrinsic robustness significantly reduces decoherence effects and minimizes the need for complex error correction. One approach to creating a topological qubit involves combining a one-dimensional topological insulator, a material class characterized by conducting surface states and an insulating bulk, realized in a nanoribbon—with an s-wave superconductor. Unlike other types of qubits, the topological qubit has not yet been experimentally realized, as the existence of localized Majorana zero modes (MZMs) remains unproven and is still a subject of ongoing research. The focus of this thesis is the search for topological superconductivity, referring to superconducting properties in the material's surface states as a step towards demonstrating the existence of Majorana zero modes. For this, experiments in topological insulator and hybrid topological insulator/superconductor nanostructures are performed. In a first step, standard material characterization is performed using selectively grown Hall bars. It can be shown that this rather young ( ≈  10 years) material class still needs to undergo rigorous growth optimization, as growth defects lead to a manifold of undesired effects that destroy the proposed properties of topological insulators. As a consequence nanoscale devices are used to investigate the existence of surface states and their electronic transport properties. Scaling down the devices to nanometer sizes significantly increases the surface to bulk ration and should lead to an enhancement of surface state effect. For later hybrid devices including superconductors it is crucial to understand the transport dynamics regarding the phase coherence of charge carriers and the influence on in-plane magnetic fields on carriers in multi-terminal structures as both are important components of topological quantum computing architectures. Therefore, Aharonov-Bohm rings are probed to investigate the transport properties of the surface state charge carriers with a focus on phase-coherence effects. It could be shown that two different transport regimes coexist in topological insulator materials: a diffusive one, arising from bulk channels due to intrinsic doping as a result of growth defects, and ballistic channels that can be attributed to the surface states of the material. The surface states are inherently decoupled from the rest of the system and show ballistic behaviour even under large defect concentrations. Subsequently, multi-terminal and kinked nanoribbons are investigated to gain insight into the influence of in-plane magnetic fields on transport in these systems. Electron in the surface states experience a Lorentz force due to the unaligned component of in-plane magnetic fields when traversing the nanoribbon leading to a trapping of carriers on the bottom or top side of the ribbon. This in turn, depending on the orientation of the in-plane magnetic field result in a coupling or decoupling of in- and output states into the system. As a result π-periodic conductance oscillations arise, which can only be explained by phase-coherent surface states on the circumference of the ribbon. Since the existence of phase-coherent and robust surface states has been proven, these materials can be combined with superconductors. This allows to study the influence of superconducting correlations on transport in these hybrid systems as a result of the proximity effect. A novel fabrication technique is used to create in-situ Josephson junctions and multi-terminal Josephson junctions. In the Josephson junctions, it was possible to show that the induced superconductivity in the surface states combined with their ballistic nature results in a Josephson diode effect. The Josephson Diode effect describes the presence of a non-reciprocal supercurrent in the system which is the result of three-symmetry breaking mechanism which can only be explained by superconducting correlations in the topological surface states of the Josephson junctions weak link. In a next step, the single terminal Josephson junctions are extended to multi-terminal structures. These multi-terminal Josephson junctions fulfill all conditions of the multi-terminal Josephson effect defined in experiments with semiconductor-superconductor hybrid structures. Analogous to the simple Josephson junctions some of the devices even show effects that can only be explained by the proximization of the surface states with the parent superconductor. Finally, preliminary experiments are presented where the manipulation of the phase in the terminals of a multi-terminal junction influences transport, demonstrating the vast possibilities of these systems.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2025-08393
%U https://publications.rwth-aachen.de/record/1019417