Topological metal at the surface of an ultrathin Bi1-xSbx alloy film

We have studied the growth and electronic/transport properties of ultrathin Bi1-xSbx alloy films, which the bulk is reported to be a topological insulator for 0.07 < x < 0.22. We found that single crystal epitaxial films as thin as similar to 30 angstrom could be grown on silicon for 0 <= x < 0.32. The Z(2) topological number of the films was shown to be nontrivial from the surface Fermi surface. Moreover a crossover from an insulating to a metallic behavior was observed upon reducing the film thickness, revealing the clear detection of the topological-metal conductivity. Our results settle the controversial issues concerning the metallicity of Bi1-xSbx at low temperature and verify that the surface/volume ratio must be extensively enhanced to properly understand the nature of these surface states.