Observation of a topological crystalline insulator phase and topological phase transition in Pb _1-x Sn _x Te

A topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion. The topological phase in the Bi _1-x Sb _x system is due to an odd number of band inversions. A related spin-orbit system, the Pb _1-x Sn _x Te, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb _1-x Sn _x Te class of materials that has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi _1-x Sb _x . Our observation of the spin-polarized Dirac surface states in the inverted Pb _1-x Sn _x Te and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.