Angle-resolved photoemission and first-principles studies of topological thin-film heterostructures
Li, Yao
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https://hdl.handle.net/2142/120382
Description
Title
Angle-resolved photoemission and first-principles studies of topological thin-film heterostructures
Author(s)
Li, Yao
Issue Date
2023-04-24
Director of Research (if dissertation) or Advisor (if thesis)
Chiang, Tai-Chang
Doctoral Committee Chair(s)
Eckstein, James N
Committee Member(s)
Stone, Michael
El-khadra, Aida X
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
angle-resolved photoemission spectroscopy (ARPES)
topological insulator
molecular beam epitaxy (MBE)
topological superconductor
thin-film heterostructure
topological proximity coupling
Abstract
Since their first theoretical prediction and experimental realization a decade ago, topological materials have been an integral part of condensed matter research owning to their combination of topologically nontrivial bulk states and symmetry-protected conducting boundary states. Recently, increasing effort has been made to study topological thin-film heterostructures, in which a plethora of fascinating emergent phenomena such as proximity-induced quantum anomalous Hall effect and topological superconductivity have been reported. This thesis research focuses on studying the electronic band structures of a complementary pair of topological insulator (TI)-based heterostructures Sb/Bi2Te3 and Bi2Te3/Sb, and metallic Sb-based heterostructures Bi/Sb and Pb/Sb. All heterostructures are prepared by molecular beam epitaxy (MBE) and are characterized by reflection high-energy electron diffraction (RHEED) and angle-resolved photoemission spectroscopy (ARPES). First-principles theoretical calculations are also conducted in parallel to support experimental findings.
A major part of this thesis is dedicated to studying emergent and topologically nontrivial band structure modifications in Sb/Bi2Te3 and Bi2Te3/Sb heterostructures. In bulk form, Sb (a semimetal) and Bi2Te3 (an insulator) both host topological surface states (TSSs) with the same topological order but substantially different decay lengths and dispersions, whereas ultrathin Sb and Bi2Te3 films by themselves are fully-gapped trivial insulators. ARPES band mappings, aided by theoretical calculations, confirm the formation of emergent TSSs in both heterostructures. Furthermore, energy of the topological Dirac points is found to vary as a function of overlayer thickness. The variation is non-monotonic, indicating nontrivial effects in the formation of topological heterostructure systems.
In a related project, ARPES studies of Bi/Sb and Pb/Sb heterostructures are conducted to investigate the coupling between metallic and topological thin films. Proximity-induced topological band structure modifications are observed in Bi/Sb with sharp heterostructure interface. By controlling MBE growth parameters, we establish the tunability of TSSs in Bi/Sb through the formation of an alloy-terminated interface with varying Bi concentration. In Pb/Sb heterostructure, we observe coexistence of Pb and Sb bulk quantum well states, which indicates coherent coupling between the two materials. We also find evidence of propagation of Sb TSSs to the Pb/vacuum interface, making Pb/Sb heterostructure a very promising candidate as a topological superconductor.
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