Experimental demonstration of the robustness of topological surface states on a PbTaSe₂ superconductor

Conventional semiconductor electronic surface states are determined by the details of its surface geometry, and the chemical nature of the surface atoms. Conversely, topological surface states arise entirely from the non-trivial bulk band topology. In topological insulators, the bulk electronic structure is always gapped while the surface is metallic everywhere independent of surface details. This bulk-boundary correspondence is the key manifestation of topology in a condensed matter system.
Unfortunately, there has yet to be solid experimental demonstrated in a real material. Here, on superconducting Z₂ topological insulator PbTaSe₂(001), we uncover two types of surfaces with distinct surface geometry and chemistry, namely 1 × 1-Se and 2 × 2-Pb surfaces. By means of scanning tunneling microscopy, we find metallic surface states on both types of surfaces. Moreover, we discern the superconducting gap to be uniform on both surfaces. Our research serves as the first experimental proof that the topological surface state is robust against surface geometry reconstruction, variation in chemical termination of the surface, and superconducting interactions, possibly opening new pathways for research in topological materials.