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. In the recently proposed topological crystalline insulators, the topological states result from crystalline symmetries rather than time-reversal symmetry. Xu et al. report the experimental observation of a topological crystalline insulator phase in Pb1-xSnxTe by spin-resolved photoemission spectroscopy.