Topological edge states in single- and multi-layer Bi₄Br₄

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 QSH insulator materials, e.g. a large band gap, appropriate insulating substrates, and multiple conducting channels. In this paper, based on first-principles calculations, we show that Bi₄Br₄ is a suitable candidate. Single-layer Bi₄Br₄ was recently demonstrated to be a QSH insulator with sizable gap. 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 bulk Bi₄Br₄, the single-layer Bi₄Br₄ preserves its topological edge states well. Moreover, at the boundary of multilayer Bi₄Br₄, the topological edge states stemming from different single-layers are weakly coupled, and can be fully decoupled by constructing a stair-stepped edge. The decoupled topological edge states are very suitable for multi-channel dissipationless transport. Our work indicates that an ideal QSH insulator can be prepared by nano-fabricaton on the cleaved surface of layered Bi₄Br₄ single crystal. (paper)