Haim Beidenkopf1 Nurit Avraham1 Claudia Felser2 Binghai Yan1 Yan Sun2

1, Weizmann Institute of Science, Rehovot, , Israel
2, Max Planck Institute for Chemical Physics of Solids, Dresden, , Germany

The established diversity of electronic topology classes lends the opportunity to pair them into dual topological complexes. Bulk-surface correspondence then ensures the coexistence of a combination of boundary states that cannot be realized but only at the various surfaces of such a dual topological material. We show that the layered compound Bi2TeI realizes a dual topological insulator. It exhibits band inversions at two time reversal symmetry points of the bulk band which classify it as a weak topological insulator with metallic states on its ‘side’ surfaces. Additional mirror symmetry of the crystal structure concurrently classifies it as a topological crystalline insulator. Bi2TeI is therefore predicted to host a pair of Dirac cones protected by time reversal symmetry on its ’side’ surfaces and three pairs of Dirac cones protected by mirror symmetry on its top and bottom surfaces. We spectroscopically map the top cleaved surface of Bi2TeI, and crystallographic step edges therein. We show the existence of both two dimensional surface states which are susceptible to mirror symmetry breaking, as well as one dimensional channels that resides along the step edges. Their mutual coexistence on the step edge where both facets join is facilitated by momentum and energy segregation. Our observations of a dual topological insulator make way to additional pairing of other dual topology classes with distinct surface manifestations coexisting at their boundaries.