UBC Theses and Dissertations
Angle-resolved photoemission and density functional theory studies of topological materials Zhu, Zhi-Huai
Topological insulators (TIs), with a gapless surface state located in a large bulk band gap, define a new class of materials with strong application potential in quantum electronic devices. However, real TI materials have many critical problems, such as bulk conductivity and surface instability, which hinder us from utilizing their exotic topological states. Another fundamental question in the TI field is what the realistic spin texture of the topological surface states (TSSs) is; no conclusive answer has yet been reached, despite extensive studies. We present two studies of doping the prototypical TI materials via in situ potassium deposition at the surface of Bi₂Se₃ and by adding magnetic impurities into the bulk Bi₂Te₃ during crystal growth. We show that potassium deposition can overcome the instability of the surface electronic properties. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with tunable, reversible, and highly stable spin splitting. Our density functional theory (DFT) calculations reveal that these Rashba states are derived from quantum well states associated with a K-induced 5 nm confinement potential. The Mn impurities in Bi₂Te₃ have a dramatic effect on tailoring the spin-orbit coupling of the system, manifested by decreasing the size of the bulk band gap even at low concentrations (2%--5%). This result suggests an efficient way to induce a quantum phase transition from TI to trivial insulators. We also explicitly unveil the TSS spin texture in TI materials. By a combination of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and DFT slab calculations, we find that the surface states are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. We predict a way to probe the intrinsic spin texture of TSS, and to continuously manipulate the spin polarization of photoelectrons all the way from 0 to +/-100% by an appropriate choice of photon energy and linear polarization. Our spin-resolved ARPES experiment confirms these predictions and establishes a generic rule for the manipulation of photoelectron spin polarization. This work paves a new pathway towards the long-term goal of utilizing TIs for opto-spintronics.
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