Nontrivial gapless electronic states at the stacking faults of weak topological insulators

TL;DR

Stacking faults in weak topological insulators can host their own nontrivial gapless states, potentially serving as sources of symmetry-protected conducting channels within the bulk material.

Contribution

This paper demonstrates that stacking faults in WTIs can support topologically nontrivial gapless states, revealing a new way defects can induce nontrivial topology.

Findings

Stacking faults can host topologically nontrivial gapless states.

Stacking faults can form 2D topological insulators within WTIs.

Stacking faults can create topologically protected 2D semimetals.

Abstract

Lattice defects such as stacking faults may obscure electronic topological features of real materials. In fact, defects are a source of disorder that can enhance the density of states and conductivity of the bulk of the system and they break crystal symmetries that can protect the topological states. On the other hand, in recent years it has been shown that lattice defects can act as a source of nontrivial topology. Motivated by recent experiments on three-dimensional (3D) topological systems such as Bi$_2$TeI and Bi$_{14}$Rh$_3$I$_9$, we examine the effect of stacking faults on the electronic properties of weak topological insulators (WTIs). Working with a simple model consisting of a 3D WTI formed by weakly-coupled two-dimensional (2D) topological layers separated by trivial spacers, we find that 2D stacking faults can carry their own, topologically nontrivial gapless states. Depending on the WTI properties, as well as the way in which the stacking fault is realized, the latter can form a topologically protected 2D semimetal, but also a 2D topological insulator which is embedded in the higher-dimensional WTI bulk. This suggests the possibility of using stacking faults in real materials as a source of topologically nontrivial, symmetry-protected conducting states.