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Title:Angle-resolved photoemission studies of topological thin films and superconducting heterostructures
Author(s):Hlevyack, Joseph Andrew
Director of Research:Chiang, Tai-Chang
Doctoral Committee Chair(s):Madhavan, Vidya
Doctoral Committee Member(s):Stone, Michael; Stack, John
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Angle-resolved photoemission spectroscopy (ARPES), Topological insulators, Topological superconductivity, Proximity pairing, Topological Dirac semimetals, Molecular beam epitaxy (MBE), Thin films
Abstract:In recent years, topological phases of matter have remained at the forefront of condensed-matter physics research due to the presence of unusually robust, conducting boundary states in these systems. Their unique bulk electronic band structures in tandem with these topologically protected states allows these materials to potentially display many fascinating physical properties, including the quantum anomalous Hall effect, emergent phenomena such as Majorana fermions, supersymmetry, and skyrmions, and unconventional superconductivity. Many of these materials are thus highly suitable for applications in spintronics and fault-tolerant quantum computing. During this thesis research, high-quality, single-crystalline ultrathin topological films and heterostructures are prepared using a combination of molecular beam epitaxy (MBE), DC magnetron sputtering, and ex situ sample-cleavage preparation techniques. Films grown using MBE are characterized in situ during the growths using electron diffraction techniques, such as reflection high-energy electron diffraction (RHEED). Angle-resolved photoemission spectroscopy (ARPES) is employed to quantify the electronic band structure of all topological films and associated heterostructures. Systems fabricated during this dissertation research include the topological insulators (TIs) Bi2Te3 and Bi2Se3, the TI/superconductor (SC) heterostructure (Bi{1-x}Sbx)2Te3/Nb, and the Type-II Dirac semimetal candidate NiTe2. A major component of this thesis research was the engineering of clean, nearly intrinsic TI/SC systems to probe the mechanism of proximity-induced pairing in these systems. Interfacing a TI with a simple isotropic s-wave superconductor may initiate exotic p-wave-like pairing in the topological surface states (TSSs). Realizing this unconventional superconductivity is dependent on the quantum-mechanical coupling between the bulk and topological surface states; however, the underlying physics is still under debate. By using a cleavage-based flip-chip approach, we have fabricated single-crystalline, bulk insulating (Bi{1-x}Sbx)2Te3 films (of thicknesses N = 2 – 10 layers) each of a predetermined layer thickness and with a strategically chosen composition ratio x = 0.62 on superconducting Nb films. Our ARPES characterizations demonstrate that each film prepared on Nb is slightly n-doped and bulk insulating by design, in both the bulk and ultrathin-film limits. Using ultrahigh-resolution laser-ARPES, proximity-induced superconducting gaps in the (Bi{1-x}Sbx)2Te3/Nb system are measured as a function of TI film thickness and temperature; these results are then compared with corresponding results from our prior studies on heavily n-doped TI films (with bulk carriers). We discover that superconductivity is greatly suppressed in slightly n-doped, bulk insulating (Bi{1-x}Sbx)2Te3/Nb, suggesting that bulk carriers are required for transiting superconductivity from the superconducting substrate to the surface of a TI film. Lastly, the other component of this thesis research focused on the fabrication and characterization of newly proposed topological systems. To that end, high-quality, single-crystalline thin films of the Type-II topological Dirac semimetal (TDS) candidate NiTe2 are grown on bilayer-graphene-terminated 6H-SiC(0001) (BLG/SiC). Our ARPES results demonstrate that this Dirac semimetal candidate is a semimetal with a complex Fermi surface in the thin-film limit. Thickness-dependent evolution of the electronic band structure is also evident in the ARPES spectra presented herein, though the band structure of the thinnest films is complicated due to their tendency to form multilayer films in the ultrathin-film regime. Consistent with prior studies, we confirm that the bulk Type-II Dirac point of NiTe2 apparently lies very near and above the Fermi level, quite unlike other Type-II Dirac semimetal systems such as PtTe2 and PdTe2; this suggests that creating alloy systems such as Pt{1-x}NixTe2 may likely be an efficient means for tuning the binding energy position of the bulk Type-II Dirac point relative to the Fermi level and thus the underlying transport properties.
Issue Date:2020-03-02
Rights Information:Copyright 2020 J. A. Hlevyack
Date Available in IDEALS:2020-08-27
Date Deposited:2020-05

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