Topological edge states in single- and multi-layer Bi$_{4}$Br$_{4}$

TL;DR

This study demonstrates that multilayer Bi$_{4}$Br$_{4}$ maintains robust topological edge states suitable for dissipationless transport, with minimal interlayer effects and potential for device applications.

Contribution

It reveals that multilayer Bi$_{4}$Br$_{4}$ preserves topological edge states with weak interlayer coupling, expanding the material's applicability for quantum spin Hall devices.

Findings

Multilayer Bi$_{4}$Br$_{4}$ has similar band gaps to single-layer.

Topological edge states are weakly coupled across layers.

Decoupled edge states enable multi-channel dissipationless transport.

Abstract

Topological edge states at the boundary of quantum spin Hall (QSH) insulators hold great promise for dissipationless electron transport. The device application of topological edge states has several critical requirements for the QSH insulator materials, e.g., large band gap, appropriate insulating substrates, and multiple conducting channels. In this paper, based on first-principle calculations, we show that Bi$_{4}$Br$_{4}$ is a suitable candidate. Single-layer Bi$_{4}$Br$_{4}$ was demonstrated to be QSH insulator with sizable gap recently. Here, we find that, in multilayer systems, both the band gaps and low-energy electronic structures are only slightly affected by the interlayer coupling. On the intrinsic insulating substrate of Bi$_{4}$Br$_{4}$, the single-layer Bi$_{4}$Br$_{4}$ well preserves its topological edge states. Moreover, at the boundary of multilayer Bi$_{4}$Br$_{4}$, the topological edge states stemming from different single-layers are weakly coupled, and can be fully decoupled via constructing a stair-stepped edge. The decoupled topological edge states are well suitable for multi-channel dissipationless transport.