Tetragonal Bismuth Bilayer: A Stable and Robust Quantum Spin Hall Insulator

TL;DR

This paper predicts a stable tetragonal bismuth bilayer as a high-performance quantum spin Hall insulator with a large, tunable band gap and robust edge states, promising for spintronic applications.

Contribution

It introduces a new 2D bismuth structure with confirmed stability and topological properties, expanding the family of quantum spin Hall insulators.

Findings

Stable tetragonal bismuth bilayer predicted by first-principles calculations.

Large nontrivial band gap tunable by strain.

Robust helical edge states with high Fermi velocity.

Abstract

Topological insulators (TIs) exhibit novel physics with great promise for new devices, but considerable challenges remain to identify TIs with high structural stability and large nontrivial band gap suitable for practical applications. Here we predict by first-principles calculations a two-dimensional (2D) TI, also known as a quantum spin Hall (QSH) insulator, in a tetragonal bismuth bilayer (TB-Bi) structure that is dynamically and thermally stable based on phonon calculations and finite-temperature molecular dynamics simulations. Density functional theory and tight-binding calculations reveal a band inversion among the Bi-p orbits driven by the strong intrinsic spin-orbit coupling, producing a large nontrivial band gap, which can be effectively tuned by moderate strains. The helical gapless edge states exhibit a linear dispersion with a high Fermi velocity comparable to that of graphene, and the QSH phase remains robust on a NaCl substrate. These remarkable properties place TB-Bi among the most promising 2D TIs for high-speed spintronic devices, and the present results provide insights into the intriguing QSH phenomenon in this new Bi structure and offer guidance for its implementation in potential applications.