Quantum oscillations in a topological insulator Bi1-xSbx

We have studied transport and magnetic properties of Bi1-xSbx, which is believed to be a topological insulator-a new state of matter where an insulating bulk supports an intrinsically metallic surface. In nominally insulating Bi0.91Sb0.09 crystals, we observed strong quantum oscillations of the magnetization and the resistivity originating from a Fermi surface which has a clear two-dimensional character. In addition, a three-dimensional Fermi surface is found to coexist, which is possibly due to an unusual coupling of the bulk to the surface. This finding demonstrates that quantum oscillations can be a powerful tool to directly probe the novel electronic states in topological insulators.