Topological quantum phase transition from mirror to time reversal symmetry protected topological insulator

Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z(2) class, which possess an odd number of metallic surface states with dispersion of a Dirac cone. Topological crystalline insulators are merely protected by individual crystal symmetries and exist for an even number of Dirac cones. Here, we demonstrate that Bi-doping of Pb(1−x)Sn(x)Se (111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z(2) topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy and induces a gap at [Formula: see text] , while the three Dirac cones at the [Formula: see text] points of the surface Brillouin zone remain intact. We interpret this new phase transition as caused by a lattice distortion. Our findings extend the topological phase diagram enormously and make strong topological insulators switchable by distortions or electric fields.