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Title:Angle-resolved photoemission studies of thin-film topological materials
Author(s):Xu, Caizhi
Director of Research:Chiang, Tai-Chang
Doctoral Committee Chair(s):Cooper, S. Lance
Doctoral Committee Member(s):Stone, Michael; Selen, Mats
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Angle-resolved photoemission spectroscopy (ARPES)
Topological insulators
Topological semimetals
Electronic structure
Thin film
Molecuar-beam epitaxy (MBE)
Abstract:This dissertation examines the electronic properties of topological materials in their thin film forms, including a prototypical topological insulator (TI), Bi2Te3, and a newly discovered topological Dirac semimetal (TDS), α-Sn under suitable strain. TIs and TDSs have nontrivial surface states and unique bulk electronic structures. Because of that, they possess many unusual physical properties and are promising materials for realizing novel device applications, such as low-power electronics, spintronics and quantum computation. In this thesis research, high-quality thin-film topological materials are prepared in situ by molecular beam epitaxy (MBE) and characterized by experimental tools, including reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD). The electronic properties of the thin-film topological materials are studied by a combined method of angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. We have studied the electronic structure of Bi2Te3 films using ARPES with circularly-polarized lights. Our ARPES results show a thickness-dependent electronic structure of Bi2Te3 films. In addition, we have studied the circular dichroism (CD) from Bi2Te3 films in a wide range of photon energies and film thicknesses. Our comprehensive measurements show that it has a complicated behavior with photon energy and film thickness, which is explained by our theoretical model of CD. Our results establish the nontrivial connection between the spin-orbit texture and CD from TIs. Finally, based on first-principles calculations, we have proposed that α-Sn, an ordinary zero-gap semimetal, becomes a TDS under suitable strain. High-quality α-Sn films have been successfully grown on InSb(111) substrate. XRD characterizations demonstrate that the α-Sn films are strained in the desired way discussed in our theoretical calculations. Using ARPES, we have observed the evidences for TDS phase in strained α-Sn films, which are in excellent agreement with our theoretical predictions. Our results establish the first known case of TDS based on a simple elemental material.
Issue Date:2017-12-01
Rights Information:Copyright 2017 Caizhi Xu
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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