Evidence for nematic superconductivity of topological surface states in PbTaSe.

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 PbTaSe with topological nodal-rings, despite its hexagonal lattice symmetry (or D in bulk while C on surface, to be precise). The 2-fold behaviour persists up to its surface upper critical field H even though bulk superconductivity has been suppressed at its bulk upper critical field H≪H, signaling its probable surface-only electronic nematicity. In addition, we do not observe any lattice rotational symmetry breaking signal from field-angle-dependent specific heat within the resolution. It is worth noting that such surface-only electronic nematicity is in sharp contrast to the observation in the topological superconductor candidate, CuBiSe, where the nematicity occurs in various bulk measurements. In combination with theory, superconducting nematicity is likely to emerge from the topological surface states of PbTaSe, rather than the proximity effect. The issue of time reversal symmetry breaking is also addressed. Thus, our results on PbTaSe shed new light on possible routes to realize nematic superconductivity with nontrivial topology.