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@PHDTHESIS{Behner:1019417,
author = {Behner, Gerrit},
othercontributors = {Schäpers, Thomas and Morgenstern, Markus},
title = {{Q}uantum transport, interference and multi-terminal
effects in topological insulator nano-devices : towards
topological superconductivity},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2025-08393},
pages = {1 Online-Ressource : Illustrationen},
year = {2025},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2025},
abstract = {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
($\approx$ 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 $\pi$-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.},
cin = {134610 / 130000},
ddc = {530},
cid = {$I:(DE-82)134610_20140620$ / $I:(DE-82)130000_20140620$},
pnm = {EXC 2004: Matter and Light for Quantum Computing (ML4Q)
(390534769)},
pid = {G:(BMBF)390534769},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2025-08393},
url = {https://publications.rwth-aachen.de/record/1019417},
}
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