Shear-resistant topology in quasi one-dimensional van der Waals material Bi$_4$Br$_4$

TL;DR

This study discovers a new shear-strain-induced in-plane shifted structure of Bi4Br4, revealing its topological insulator properties and expanding understanding of its electronic phases.

Contribution

The paper reports a new Bi4Br4 structure with a b/3 chain shift caused by shear strain, and confirms its higher-order topological insulator nature through experiments and DFT calculations.

Findings

Identified a new Bi4Br4 structure with b/3 chain shift

Observed bulk insulating gap and metallic edge states

Confirmed topological insulator properties via DFT

Abstract

Bi$_4$Br$_4$ is a prototypical quasi one-dimensional (1D) material in which covalently bonded bismuth bromide chains are arranged in parallel, side-by-side and layer-by-layer, with van der Waals (vdW) gaps in between. So far, two different structures have been reported for this compound, $\alpha$-Bi$_4$Br$_4$ and $\beta$-Bi$_4$Br$_4$ , in both of which neighboring chains are shifted by $\mathbf{b}/2$, i.e., half a unit cell vector in the plane, but which differ in their vertical stacking. While the different layer arrangements are known to result in distinct electronic properties, the effect of possible in-plane shifts between the atomic chains remains an open question. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we report a new Bi$_4$Br$_4$(001) structure, with a shift of $\mathbf{b}/3$ between neighboring chains in the plane and AB layer stacking. We determine shear strain to be the origin of this new structure, which can readily result in shifts of neighboring atomic chains because of the weak inter-chain bonding. For the observed $b/3$ structure, the (residual) atomic chain shift corresponds to an in-plane shear strain of $\gamma\approx7.5\%$. STS reveals a bulk insulating gap and metallic edge states at surface steps, indicating that the new structure is also a higher-order topological insulator, just like $\alpha$-Bi$_4$Br$_4$, in agreement with density functional theory (DFT) calculations.