$\mbox{Bi}_{1}\mbox{Te}_{1}$: a dual topological insulator

TL;DR

This study combines theory and experiments to demonstrate that Bi1Te1 is a dual topological insulator exhibiting both weak topological and topological crystalline insulator properties, with distinct surface states protected by different symmetries.

Contribution

It provides the first combined theoretical and experimental evidence that Bi1Te1 is a dual topological insulator with coexisting weak and crystalline topological phases.

Findings

Bi1Te1 exhibits a non-trivial $ ext{Z}_2$ topological invariant.

Bi1Te1 has a mirror Chern number of -2.

Surface states are protected by time-reversal and mirror symmetries.

Abstract

A combined theoretical and experimental study reveals evidence for the dual topological insulating character of the stoichiometric natural superlattice phase $\mathrm{Bi_{1}Te_{1}}=\mathrm{[Bi_{2}]_{1}[Bi_{2}Te_{3}]_{2}}$, being a stack of alternating Bi bilayers and two quintuple layers of $\mathrm{Bi_{2}Te_{3}}$. We identify $\mathrm{Bi_{1}Te_{1}}$ by density functional theory to exhibit a non trivial time-reversal symmetry-driven character of $\mathbb{Z}_{2}=(0;001)$ and additionally a mirror-symmetry induced mirror Chern number of $n_{{\cal M}}=-2$, which indicates that $\mathrm{Bi_{1}Te_{1}}$ is both a weak topological insulator and a topological crystalline insulator. The coexistence of the two phenomena preordains distinct crystal planes to host topological surface states that are protected by the respective symmetries. The surface perpendicular to the stacking direction is the 'dark' surface of the weak topological insulator, while hosting mirror-symmetry protected surface states along the $\bar{\Gamma\mathrm{M}}$ direction at non-time-reversal invariant momenta points. We confirm the stacking sequence of our MBE-grown $\mathrm{Bi_{1}Te_{1}}$ thin films by X-ray diffraction and transmission electron microscopy, and find indications of the topological crystalline and weak topological character in the surface electronic spin structure by spin- and angle-resolved photoemission spectroscopy, which nicely match the results from density functional theory.