Search for a Topological Phase Transition in BiTeClxBr1-x: A Photoemission Spectroscopy

Abstract

Topological insulators represent a new phase of matter and an emerging field for condensed matter research. Topological materials can host surface states which have been proposed as sources for Majorana fermions and for applications to quantum computing or quantum information theory[1][2]. This thesis primarily seeks to analyze a proposed topological phase transition in the bismuth tellurohalide (BT) family between BiTeCl and BiTeBr. A topological phase transition was sought through the examination of samples of BiTeClxBr1-x via photoemission spectroscopy (PES). The bismuth tellurohalide (BT) BiTeCl has been controversially labeled a strong inversion asymmetric topological insulator (SIATI) candidate[3] while BiTeBr is a centrosymmetric trivial insulator[4][5]. The transition from BiTeCl to BiTeBr thus could consist of both a topological and a crystal symmetry transition. Such a symmetry-breaking topological phase transition can occur with a preserved bulk valence-conduction band gap or as a gapless Weyl semimetal[6][7]. Evidence of such a phase transition was analyzed through data fitting of the bulk bands and surface states of BiTeClxBr1-x. A topological phase transition in BiTeClxBr1-x was not found in this study. As expected, BiTeBr displayed the band structure of a trivial insulator while BiTeCl displayed surface Dirac cones. However, there were no systematic changes in the bulk or surface fitting parameters of BiTeClxBr1-x with x. Such findings suggest that the surface states of BiTeCl are derived only from the surface-adjacent layers of the crystal and that BiTeCl and BiTeBr combine non-adiabatically. The surface states appeared to be largely unaffected by bulk concentrations of Cl and Br, and the presence of Cl-termination regardless of Cl-concentration suggests that microdomains of BiTeCl and BiTeBr formed in the crystal. Sections of BiTeCl may then be found in PES scans of the most intensely reactive sections of the exposed crystal. This thesis thus provides a counterargument to the interpretation of BiTeCl as a SIATI and demonstrates techniques for seeking a topological phase transition.