  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.
