Quantum oscillations in a topological insulatorBi1−xSbx

We have studied transport and magnetic properties of ${\text{Bi}}_{1-x}{\text{Sb}}_x$, 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 ${\text{Bi}}_{0.91}{\text{Sb}}_{0.09}$ 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.