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@PHDTHESIS{Schleenvoigt:999456,
author = {Schleenvoigt, Michael},
othercontributors = {Grützmacher, Detlev and Morgenstern, Markus},
title = {{M}olecular beam epitaxy of magnetic topological insulators
and their integration into superconducting hybrid devices},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-12113},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2025; Dissertation, RWTH Aachen University, 2024},
abstract = {Among the few approaches towards quantum computing (QC) as
the future of computing in a post-Moore’s-law time, the
recent push for topological QC is one of the most
auspicious. It promises fault tolerant QC by employing
exotic Majorana zero mode (MZM) quasiparticle states, which
build the basis of the topological QC qubit. Intrinsic
protection against relaxation and dephasing distinguishes
those elusive qubits from other platforms, like
superconducting and spin qubits. Nanowires of 3D topological
insulator (TI) – superconductor (SC) hybrid structures are
predicted to exhibit MZM at each end of the wire. The
one-dimensional confinement however leads to a loss of the
topological phase needed to realize the MZMs. An alternative
capable of avoiding this problem has gotten much traction
lately, which are magnetic topological insulators (MTI),
which incorporate magnetic dopants into the TI itself. This
thesis focuses on the growth of magnetic TIs via molecular
beam epitaxy (MBE) from established recipes for non-magnetic
TIs. It will show how complex MTI-SC hybrid devices can be
fabricated in situ and will illustrate the difficulties that
arise when trying to combine MTIs with SC. First, the growth
of 3D TI (BiSb)2Te3 via MBE is optimized to reach a low
level of charge carrier density and a high mobility,
signifying an appropriate host material for the magnetic
dopants. By supplying Cr during thin film growth, high
quality magnetically doped films are deposited. First films
exhibit an anomalous Hall effect, indicating homogeneous and
strong magnetism. In similar Mn-doped films, investigated in
parallel, the magnetization is found to less homogeneous,
ultimately leading to an focusing onto the more reliable Cr
doped MTI films. By creating a trilayer film of CBST and
thinning it down to <8 nm a quantization in the Hall
resistance is observed, signaling a transition into a
quantum anomalous Hall insulator. To investigate the
interaction of the MTI and SC, Josephson junction (JJ)
devices are created via a stencil lithography process
established in our group. The JJ devices show no
supercurrent over the junctions, but rather indications for
a barrier at the MTI-SC interface due to bad interface
transparency between MTI and SC. To rule out ex situ
contaminations and to relate the barrier height to the
magnetization, a new device layout is developed. It allows
for full in situ deposition and enables Hall and JJ
measurements in one device. While the low transparency issue
persists, indications of induced superconductivity are found
when decreasing the magnetic doping in these devices. To
further investigate the interplay of topology, magnetism and
superconductivity on atomic scales, a novel process is
developed in the last chapter: a fully in situ process
utilizing removable large-scale ultra-high vacuum
lithography (LUL). With LUL, (M)TI and SC films can be grown
selectively in situ and aligned to each other with nm
precision. Soft cappings of Tellurium or Selenium are
investigated to protect the functional surfaces. By
exfoliating the stencil mask layer, samples suitable for
investigations in scanning tunneling microscopes (STM) are
created. In STM, atomic resolution is achieved after capping
removal. Tunneling spectra of the SC gap, the TI surface and
the magnetic gap are obtained after successful mask and
capping removal, showing a short-ranged SC gap profile on
the TI. In conclusion, this thesis establishes multiple new
material systems in our institute, showcases novel UHV
lithography methods for the combination of (M)TIs and SCs,
paving the way for the creation of MZMs in hybrid devices,
and lastly, by utilizing LUL, provides an innovative process
for creating structures and devices of combinations of
arbitrary quantum materials in situ that enables
measurements in ways unprecedented in STM.},
cin = {134610 / 130000},
ddc = {530},
cid = {$I:(DE-82)134610_20140620$ / $I:(DE-82)130000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-12113},
url = {https://publications.rwth-aachen.de/record/999456},
}
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