Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z2 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 Pb1-x Sn x Se (111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 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.Transitions between topological phases of matter protected by different symmetries remain rare. Here, Mandal et al. report a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator by doping Bi into Pb1-x Sn x Se (111) thin films.

Department of Condensed Matter Physics Masaryk University Kotlářská 267 2 61137 Brno Czech Republic

Helmholtz Zentrum Berlin für Materialien und Energie Albert Einstein Strasse 15 12489 Berlin Germany

Institut für Physik und Astronomie Universität Potsdam Karl Liebknecht Street 24 25 14476 Potsdam Germany

Institute for Semiconductor and Solid State Physics Johannes Kepler Universität Altenberger Strasse 69 4040 Linz Austria

National Technical University Kharkiv Polytechnic Institute Frunze Street 21 61002 Kharkiv Ukraine

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