Universität zu Köln

Rößler, Matthias (2023). Development and Electronic Phenomena of Etched Bulk-Insulating Topological Insulator Nanowires. PhD thesis, Universität zu Köln.

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Abstract

Topological quantum matter serves as a fertile ground to study novel transport phenomena as well as fundamental aspects of modern condensed matter physics since about two decades. More recently it was realized that topological insulator nanowires (TINWs) that exhibit quantum-confined topological surface states (TSS) with peculiar spin-physics can, when proximitized by a superconductor, harbour exotic quasi-particles with non-abelian exchange statistics called Majorana bound states (MBS), which are of great fundamental and technological interest. In this work, a TINW platform was developed and experimentally investigated that provides the necessary ingredients to realize next-generation proposals to study and manipulate such quasi-particles. The prepared TINWs were etched from the bulk-insulating 3D topological insulator material (Bi_{1-x}Sb_{x})_2Te_3 (BST) and have been electrically probed in low temperature transport measurements. In such experiments, after proving the existence of the TSS, manipulation of the TSS enabled the discovery of a non-reciprocal transport effect that occurs due to a large magnetochiral anisotropy (MCA). Furthermore, after proximitizing the TSS with a superconductor, S-TI-S Josephson Junctions on TINWs of varying width have been studied. Key ingredients for advanced detection and control techniques for MBS are a well-controlled and high material quality, confirmation of the existence and reliable control of the TSS as well as sufficiently robust induced superconductivity in a nanowire platform that is compatible with scalable device designs. While the potential for a larger size of the relevant energy scale in TINWs compared to other proposals makes 3D topological insulators (TI) an attractive material class for studying MBS, preparation of bulk-insulating narrow TINWs with fabrication techniques that are compatible with advanced proposals, which require several deterministically branched and interconnected NWs, has not been reported before. Additionally, inducing robust superconductivity in a reproducible manner is not easily achieved in such bulk-insulating wires. For these reasons, not all the requirements for experiments for an unambiguous detection of MBS in 3D TINWs had been met before. To tackle these challenges, here a versatile approach to prepare TINWs etched from bulk-insulating Molecular-beam epitaxy (MBE) grown thin films of the 3D TI BST has been developed which fulfills the aforementioned requirements. From low temperature transport measurements it is shown that narrow NWs prepared in this way maintain the high quality of the as-grown material, are gate-tuneable as well as bulk-insulating and show magnetic-field-dependent electrical resistance oscillations that are consistent with theoretical expectations, a manifestation and proof of the existence of the TSS. Furthermore, a well-controllable rectification effect stemming from magnetochiral anisotropy in the TSS has been discovered that can be quantified by a coefficient |γ| ~ 100000 A^{-1} T^{-1}, an extraordinarily large value for a normal conductor. Additionally, S-TI-S Josephson junctions have been studied to investigate and improve induced superconductivity in such TINWs. Here, in experiments with microwave irradiation a fractional Josephson effect has been observed that reveals transport signatures previously associated with an unconventional 4π-periodic supercurrent contribution. These results demonstrate that TINWs can be realized by top-down patterning from planar 3D TI BST thin films in a quality at least on par with previously existing preparation techniques, that they can be well used to study novel electronic transport phenomena that arise from discrete TSS and that they can further be successfully proximitized with a superconductor to study unconventional superconducting transport, all of which is interesting from both a fundamental point of view as well as for potential technical applications such as novel diode effects and topological quantum computation.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Rößler, Matthias matthias.roessler@rwth-aachen.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-726838
Date:	2023
Place of Publication:	Köln
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Topological Insulator, Nanowire, Aharonov−Bohm oscillations, Josephson Junction, Superconductivity, Shapiro steps, Low temperature transport, Magnetochiral anisotropy, Majorana, Majorana Bound States, Quantum Computing, ML4Q, Matter and Light for Quantum Computation UNSPECIFIED
Date of oral exam:	29 November 2023
Referee:	Name Academic Title Ando, Yoichi Prof. Dr. Bocquillon, Erwann Prof. Dr. Altland, Alexander Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/72683