Angle-resolved photoemission and first-principles studies of topological semimetals and heterostructures

Wong, Man-Hong

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Description

Title

Angle-resolved photoemission and first-principles studies of topological semimetals and heterostructures

Author(s)

Wong, Man-Hong

Issue Date

2017-11-29

Director of Research (if dissertation) or Advisor (if thesis)

Chiang, Tai-Chang

Doctoral Committee Chair(s)

Cooper, S. Lance

Committee Member(s)

• Stone, Michael
• Yang, Liang

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Topological
• Photoemission

Abstract

Topologically nontrivial systems are electronic materials that have protected conducting states on their edge or surface. These widely studied materials are of great interest due to their exotic physical properties and potential for applications in spin-related electronic devices and quantum computation. In this study, angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations are used to study the electronic properties of topological semimetals and topological heterostructures. The topological heterostructures are prepared using molecular beam epitaxy (MBE) and characterized by experimental techniques such as reflective-high energy electron diffraction (RHEED) and low-energy electron diffraction (LEED). This thesis focuses on the topological semimetal antimony (Sb) and the topological heterostructures Bi2Se3/Nb and Ag/Bi/Bi2Te3. First-principles calculations are used to determine the electronic band structure of the topological semimetal Sb for various spin-orbit coupling (SOC) strengths. Transitioning to a topological state typically involves a gap reversal caused by a strong SOC. By theoretically adjusting the SOC strength, we can observe the changes in the electronic band structure and make conclusions about the topological phase of the system. Using this method, we constructed a topological phase diagram as a function of SOC strength for antimony that shows various quantum phase transitions. Particularly interesting is that for Sb, the topological phase transition does not involve the usual gap reversal and the surface states survive the transition; instead, the system changes from a metal to a topological semimetal when the SOC is tuned from 0% to its natural value. The topological heterostructure Bi2Se3/Nb is studied using ARPES to determine the effects of coupling a topological insulator (TI) with a superconductor (SC). Topological superconductors, such as Bi2Se3/Nb, are predicted to host exotic states such as Majorana fermions. By using a novel “flip-chip” preparation method, we cleaved these samples in situ to expose a clean surface for temperature and thickness dependent ARPES measurements. Utilizing the proximity effect, superconductive features were observed on the surface of the topological insulator. Both a leading-edge shift in the ARPES spectra and a coherence peak were observed when the heterostructure was cooled below TCNb = 9.4 K. These features were observed in all Bi2Se3 thicknesses measured in this study (4-10 quintuple layers (QL)), but with decreasing intensity as the thickness increased. By fitting symmetrized data with the Dynes function, we deduced a superconducting gap of 1.1 meV for a 4 QL Bi2Se3/Nb sample, which is the largest measured superconducting gap for a topological heterostructure. Finally, we studied the effects of coupling a topological insulator with a metal. Theoretical predictions state that coupling these two types of materials may allow for the topological surface states of the TI to propagate through the metal film. To realize this, we grew the topological heterostructure Ag/Bi/Bi2Te3. The use of a bismuth (Bi) bilayer is to prevent the silver (Ag) atoms from intercalating into the Bi2Te3 film while preserving the topological classification of the substrate for the Ag film. Using ARPES, the band structure of this heterostructure shows linearly dispersing surface states that cross to form a Dirac cone, which differs from the bulk surface state of Ag that has a parabolic dispersion. We conclude that the surface states originate from the underlying topological insulator. Studying both the metal/TI and TI/SC heterostructures provides a new direction to the ongoing research into these unique materials.

Graduation Semester

2017-12

Type of Resource

text

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Copyright 2017 Man-Hong Wong

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