Surface States of the Topological Insulator Bi_{1-x}Sb_x

TL;DR

This paper investigates the surface states of the topological insulator BiSb, demonstrating its strong topological nature through theoretical models, and explores how symmetries influence surface band structures and their experimental implications.

Contribution

It provides a detailed analysis of surface states in BiSb, linking bulk parity invariants to surface features, and introduces the concept of mirror Chern numbers and mirror chirality in topological insulators.

Findings

BiSb surface states enclose an odd number of TRIM, confirming strong topological insulator behavior.

Surface fermion parity is directly linked to bulk parity eigenvalues.

Discrepancies in mirror chirality predictions suggest reevaluation of tight binding parameters.

Abstract

We study the electronic surface states of the semiconducting alloy BiSb. Using a phenomenological tight binding model we show that the Fermi surface of the 111 surface states encloses an odd number of time reversal invariant momenta (TRIM) in the surface Brillouin zone confirming that the alloy is a strong topological insulator. We then develop general arguments which show that spatial symmetries lead to additional topological structure, and further constrain the surface band structure. Inversion symmetric crystals have 8 Z_2 "parity invariants", which include the 4 Z_2 invariants due to time reversal. The extra invariants determine the "surface fermion parity", which specifies which surface TRIM are enclosed by an odd number of electron or hole pockets. We provide a simple proof of this result, which provides a direct link between the surface states and the bulk parity eigenvalues. We then make specific predictions for the surface state structure for several faces of BiSb. We next show that mirror invariant band structures are characterized by an integer "mirror Chern number", n_M. The sign of n_M in the topological insulator phase of BiSb is related to a previously unexplored Z_2 parameter in the L point k.p theory of pure Bi, which we refer to as the "mirror chirality", \eta. The value of \eta predicted by the tight binding model for Bi disagrees with the value predicted by a more fundamental pseudopotential calculation. This explains a subtle disagreement between our tight binding surface state calculation and previous first principles calculations on Bi. This suggests that the tight binding parameters in the Liu Allen model of Bi need to be reconsidered. Implications for existing and future ARPES experiments and spin polarized ARPES experiments will be discussed.