Observation of a topological crystalline insulator phase and topological phase transition in Pb1−xSnxTe

A topological insulator protected by time-reversal symmetry is realized via spin–orbit interaction-driven band inversion. The topological phase in the Bi1−xSbx system is due to an odd number of band inversions. A related spin–orbit system, the Pb1−xSnxTe, 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 Pb1−xSnxTe 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 Bi1−xSbx. Our observation of the spin-polarized Dirac surface states in the inverted Pb1−xSnxTe 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.