Observation of topologically protected surface states in a Bi-Pd superconductor

A layered Bi-Pd superconductor is investigated by spin- and angle-resolved photoemission spectroscopy. Beside the spin-degenerate bulk bands, several spin-polarized surface bands, some of which crossing the Fermi level, are clearly observed. These surface states are evaluated to be topologically protected, based on the Z$_{2}$ invariant analysis in analogy to 3-dimensional strong topological insulators. It indicates that this material is likely to be a topological superconductor realized without any carrier doping or applying pressure.