Stable nontrivial Z2 topology in ultrathin Bi (111) films: a   first-principles study
  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. 
Zheng Liu, Chao-Xing Liu, Yong-Shi Wu, Wen-Hui Duan, Feng Liu and Jian   Wu
cond-mat.mtrl-sci cond-mat.mes-hall