  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.
