Stable nontrivial Z2 topology in ultrathin Bi (111) films: a first-principles study

TL;DR

This study uses first-principles calculations to demonstrate that ultrathin Bi (111) films possess a stable nontrivial Z2 topological phase regardless of thickness, challenging previous beliefs about triviality oscillations.

Contribution

It reveals that ultrathin Bi (111) films maintain a nontrivial Z2 topology independent of thickness, driven by band inversions and inter-bilayer coupling, with implications for topological insulator design.

Findings

All ultrathin Bi (111) films have nontrivial Z2 topology.

The nontrivial topology is stable across different film thicknesses.

Surface adsorption can eliminate metallic surface states in thick films.

Abstract

Recently, there have been intense efforts in searching for new topological insulator (TI) materials. Based on first-principles calculations, we find that all the ultrathin Bi (111) films are characterized by a nontrivial Z2 number independent of the film thickness, without the odd-even oscillation of topological triviality as commonly perceived. The stable nontrivial Z2 topology is retained by the concurrent band gap inversions at multiple time-reversal-invariant k-points and associated with the intermediate inter-bilayer coupling of the multi-bilayer Bi film. Our calculations further indicate that the presence of metallic surface states in thick Bi(111) films can be effectively removed by surface adsorption.