Photoemission studies of a new topological insulator class: Experimental discovery of the bismuth-X3 topological insulator class.

摘要

Topological insulators are materials with a bulk band gap, which carry conducting surface states that are protected against disorder. In three dimensions, the insulators carry 2D Dirac fermions on their surfaces. The opening of a magnetic surface gap can exhibit a topological magnetoelectric effect, and support Majorana fermions which can be manipulated for quantum computation. Previous spin and angle-resolved photoemission studies have shown that Bi 1-xSbx alloy belongs to this class of materials, with a characteristic number nu 0 = 1. Some materials challenges with Bi1-x Sbx alloy however are the significant degree of bulk disorder and a small band gap. Both problems make gating difficult for the manipulation and control of the charge carriers.;While ordinary materials such as superconductors and liquid crystals can be described by an order parameter, topological insulators are not associated with a local order parameter resulting from a spontaneous broken symmetry. Rather, they manifest a topological order which requires a direct probe of how their energy bands are connected. Measurement techniques designed to detect a particular order parameter are therefore insufficient to identify the topological character of a material. Alternatively, one can look for properties analogous to the quantum Hall effect as a signature of a topologically ordered system. However, using transport probes to isolate the surface states of the topological insulator requires a pristine bulk with minimal charge carrier density. While advances have been made recently in this direction, a good candidate for such measurements has been elusive.;In this thesis, we describe a systematic study of a new topological insulator class with a large band gap and a single surface state Fermi surface. Using synchrochon-based angle-resolved photoemission spectroscopy (ARPES), we measured the topological character of these materials by observing the dispersion of their metallic electronic states confined to the surface. Additionally, we confirmed the unusual spin texture of these surface states using spin-sensitive ARPES. In Chapter 1, we first give a brief summary of the theoretical developments leading to the proposal of the topological insulator. In Chapter 2, a description of the experimental techniques of spin and angle-resolved PES is provided. Chapter 3 presents experimental data for three members of the topological class: Bi2Se3, Bi2Te3 and Sb2Te3. In each of the discussions, a comparison with the respective theoretical surface state calculations is presented. Finally, in Chapter 4, we present several techniques for manipulating the metallic surface states of the topological insulators.