% IMPORTANT: The following is UTF-8 encoded. This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.
@PHDTHESIS{Schmitt:991063,
author = {Schmitt, Tobias Werner},
othercontributors = {Grützmacher, Detlev and Morgenstern, Markus},
title = {{E}xploring proximity induced superconductivity in
topological insulator based hybrid devices},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-07661},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2024},
abstract = {As a proposed platform to realize elusive Majorana zero
modes (MZMs), the superconducting proximity effect of an
s-wave superconductor (S) in the surface states of a 3D
topological insulator (TI) forms an attractive field of
research driven by both, the fundamental interest in the
mere existence of these non-Abelian modes and their
envisioned application for fault-tolerant quantum computing.
While prior research on several S-TI hybrid devices has
already found experimental signatures which are assumed to
be related to the unconventional induced superconductivity
and MZMs, extensive experimental work is required to
reinforce this interpretation and enhance the technological
readiness of the S-TI platform. This thesis contributes to
this field of research by exploring the realization and
characterization of S-TI hybrid devices based on molecular
beam epitaxy (MBE) grown tetradymite TIs. For a pristine
fabrication of the respective S-TI hybrid devices, this work
relies on a scalable ultra-high vacuum (UHV) process. This
fabrication technique, which combines selective area growth
of TIs and stencil lithography metallization, is further
developed in the scope of this thesis to meet the
experimental requirements of the respective devices. At
first, the integration of topological insulator nanoribbons
in superconducting qubit circuits is showcased which is
motivated by recent proposal for the detection of MZMs via
circuit quantum electrodynamic (cQED) techniques. In order
to mitigate dielectric microwave losses in these qubits a
local stencil lithography approach is developed. Using this
process, a first generation of superconducting transmon
qubits based on (Bi,Sb)$_2$Te$_3$ nanoribbons is fabricated.
The characterization of these devices finds qubit
frequencies tuned by fabrication to the desired transmon
regime and yields coherent qubit control and temporal
quantum coherence. Thus, important capabilities for future
cQED experiments based on S-TI hybrid qubit devices are
demonstrated. In these TI nanoribbons, the proximity-induced
superconductivity is moreover investigated in transport
spectroscopy experiment. The respective devices are
fabricated via an extensive multi-step stencil lithography
process, which allows for the additional UHV fabrication of
the required barriers and normal probes. In electrical
transport experiments, signatures of the induced
superconductivity in the (Bi,Sb)$_2$Te$_3$ nanoribbons are
found, including low energy conductance features assigned to
the induced superconductivity in spurious bulk states. These
features hamper the analysis of the proximity effect in the
surface states. For a mitigation in future experiments,
device optimizations are presented to enable junctions in an
effectively shorter limit. This thesis further studies the
general use of Al as parent superconductor for UHV
fabricated tetradymite TIs via the characterization of
Josephson junction based on the binary TI Bi$_2$Te$_3$. As a
potential substitute for Nb, Al offers some advantageous
characteristics and could thus help to improve S-TI hybrid
devices. While the application of pure Al on Bi$_2$Te$_3$ is
found to inhibit induced superconductivity, thin interlayers
of 3 nm Nb, Pd, Pt, or Ti are applied for an optimization of
the interface. With the finding of supercurrents, excess
currents, and subharmonic gap structures on respective
Josephson junctions, all interlayers demonstrate to improve
the interface quality. These junction characteristics are
analyzed for a qualitative comparison of the interlayers.
This discussion also covers an observed anomalous
temperature dependence of the multiple Andreev reflections,
which is assigned to distinct transport contributions from
bulk and surface states.},
cin = {134610 / 130000 / 080044},
ddc = {530},
cid = {$I:(DE-82)134610_20140620$ / $I:(DE-82)130000_20140620$ /
$I:(DE-82)080044_20160218$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-07661},
url = {https://publications.rwth-aachen.de/record/991063},
}
guest ::