The emergence of topologically protected surface states in epitaxial   Bi(111) thin films
  Quantum transport measurements including the Altshuler-Aronov-Spivak (AAS) and Aharonov-Bohm (AB) effects, universal conductance fluctuations (UCF), and weak anti-localization (WAL) have been carried out on epitaxial Bi thin films ($10-70$ bilayers) on Si(111). The results show that while the film interior is insulating all six surfaces of the Bi thin films are robustly metallic. We propose that these properties are the manifestation of a novel phenomenon, namely, a topologically trivial bulk system can become topologically non-trivial when it is made into a thin film. We stress that what's observed here is entirely different from the predicted 2D topological insulating state in a single bilayer Bi where only the four side surfaces should possess topologically protected gapless states. 
Kai Zhu, Lin Wu, Xinxin Gong, Shunhao Xiao, Shiyan Li, Xiaofeng Jin,   Mengyu Yao, Dong Qian, Meng Wu, Ji Feng, Qian Niu, Fe...
cond-mat.mes-hall