Evidence for nematic superconductivity of topological surface states in PbTaSe2

PbTaSe2拓扑表面态诱导的向列超导电性

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 D3 h in bulk while C3 v on surface,to be precise).The 2-fold behaviour persists up to its surface upper critical field Hc2R even though bulk superconductivity has been suppressed at its bulk upper critical field Hc2HC<<Hc2R,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,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.

通过平面内磁场转角的电阻和点接触谱的测量,作者发现拥有Dirac节点线的拓扑材料PbTaSe2中超导能隙存在明显的二重对称行为,低于PbTaSe2六角晶格结构的对称性,表明向列超导电性的存在.这种二重对称性会一直持续到表面超导的上临界场Hc2R,尽管体超导在体上临界场c2RH<<Hc2R以上已经被抑制.该结果表明二重对称行为起源于表面的向列超导电性.同时,在测量的精度范围内,磁场转角比热的数据也并没有观测到体超导的各向异性.值得注意的是, PbTaSe2的向列超导电性与拓扑超导候选材料CuxBi2Se3有所不同,后者的二重对称行为来自于体态超导.结合理论分析,本文认为PbTaSe2的向列超导电性可能起源于其拓扑表面态形成的拓扑超导态而不是体态超导的近邻效应,该理论预言PbTaSe2的表面超导会破坏时间反演对称.PbTaSe2的系统研究有助于进一步探索向列超导电性与拓扑超导之间的关系.