Topologically protected surface states in a centrosymmetric superconductor beta-PdBi2

The topological aspects of electrons in solids emerge in realistic matters as represented by topological insulators. They are expected to show a variety of new magneto-electric phenomena, and especially the ones hosting superconductivity are strongly desired as the candidate for topological superconductors (TSC). Possible TSC materials have been mostly developed by introducing carriers into topological insulators, nevertheless, those exhibiting indisputable superconductivity free from inhomogeneity are very few. Here we report on the observation of topologically-protected surface states in a centrosymmetric layered superconductor, beta-PdBi2, by utilizing spin- and angle-resolved photoemission spectroscopy. Besides the bulk bands, several surface bands, some of which crossing the Fermi level, are clearly observed with symmetrically allowed in-plane spin-polarizations. These surface states are precisely evaluated to be topological, based on the Z2 invariant analysis in analogy to 3-dimensional strong topological insulators. beta-PdBi2 may offer a TSC realized without any carrier-doping or applying pressure, i.e. a solid stage to investigate the topological aspect in the superconducting condensate.