  Spontaneous symmetry breaking has been a paradigm to describe the phase
transitions in condensed matter physics. In addition to the continuous
electromagnetic gauge symmetry, an unconventional superconductor can break
discrete symmetries simultaneously, such as time reversal and lattice
rotational symmetry. In this work we report a characteristic in-plane 2-fold
behaviour of the resistive upper critical field and point-contact spectra on
the superconducting semimetal PbTaSe2 with topological nodal-rings, despite its
hexagonal lattice symmetry (or D_3h in bulk while C_3v on surface, to be
precise). However, we do not observe any lattice rotational symmetry breaking
signal from field-angle-dependent specific heat. It is worth noting that such
surface-only electronic nematicity is in sharp contrast to the observation in
the topological superconductor candidate, CuxBi2Se3, where the nematicity
occurs in various bulk measurements. In combination with theory,
superconducting nematicity is likely to emerge from the topological surface
states of PbTaSe2, rather than the proximity effect. The issue of time reversal
symmetry breaking is also addressed. Thus, our results on PbTaSe2 shed new
light on possible routes to realize nematic superconductivity with nontrivial
topology.
