A topological crystalline insulator (TCI) phase via topological phase transition and crystalline mirror symmetry

TL;DR

This paper experimentally demonstrates a mirror symmetry protected topological crystalline insulator phase in (Pb/Sn)Te, revealing even number of Dirac cones and distinct topological order from known Z2 topological insulators, opening new avenues for quantum devices.

Contribution

It provides the first experimental evidence of a topological crystalline insulator phase in (Pb/Sn)Te with mirror symmetry protection, differing from traditional Z2 topological insulators.

Findings

Observation of even number of spin-polarized Dirac cones in (Pb/Sn)Te

Surface states appear above the topological phase transition

Experimental validation of mirror symmetry protected topological order

Abstract

A Z2 topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction driven band inversion. For example, the topological phase in the Bi-Sb system is due to an odd number of band inversions. A related spin-orbit system, the (Pb/Sn)Te class, has 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/Sn)Te class of materials which has been theoretically predicted to exist in its non-alloyed version. 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 (as opposed to odd as observed in Bi-Sb alloy or Bi2Se3) revealing mirror protected topological order distinct from that observed in Bi-Sb or Bi2Se3. Our observation of the spin-polarized Dirac surface states in the inverted (Pb/Sn)Te and their absence in the non-inverted compounds related via a topological phase transition (spin-orbit induced band inversion type) provide the experimental groundwork for opening the research on novel topological order in future quantum devices.