A Z2 topological insulator protected by time-reversal symmetry is realizedvia spin-orbit interaction driven band inversion. For example, the topologicalphase in the Bi-Sb system is due to an odd number of band inversions. A relatedspin-orbit system, the (Pb/Sn)Te class, has been known to contain an evennumber of inversions based on band theory. Here we experimentally investigatethe possibility of a mirror symmetry protected topological crystallineinsulator phase in the (Pb/Sn)Te class of materials which has beentheoretically predicted to exist in its non-alloyed version. Our experimentalresults show that at a finite-Pb composition above the topological inversionphase transition, the surface exhibits even number of spin-polarized Dirac conestates (as opposed to odd as observed in Bi-Sb alloy or Bi2Se3) revealingmirror protected topological order distinct from that observed in Bi-Sb orBi2Se3. Our observation of the spin-polarized Dirac surface states in theinverted (Pb/Sn)Te and their absence in the non-inverted compounds related viaa topological phase transition (spin-orbit induced band inversion type) providethe experimental groundwork for opening the research on novel topological orderin future quantum devices.
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