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). The 2-fold behaviour persists up to its surface upper critical field $H_{\mathrm{c}2}^{\mathrm{R}}$ even though bulk superconductivity has been suppressed at its bulk upper critical field $H_{\mathrm{c}2}^{\mathrm{HC}}\ll H_{\mathrm{c}2}^{\mathrm{R}}$, 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, ${\text{Cu}}_x$Bi₂Se₃, 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.

自发对称性破缺一直是描述凝聚态物理学中相变的范例。除了连续的电磁规范对称性之外，非常规超导体还可以同时打破离散对称性，例如时间反演和晶格旋转对称性。在这项工作中，我们报告了具有拓扑节环的超导半金属 PbTaSe ₂的电阻上临界场和点接触光谱的特征面内 2 倍行为，尽管其具有六方晶格对称性（或${丁}_{\mathrm{3个}H}$批量同时$C_{\mathrm{3个}v}$准确地说，在表面上）。2 倍行为一直持续到其表面上临界场$H_{\mathrm{C}\mathrm{2个}}^{\mathrm{R}}$尽管体超导性在其体上临界场已被抑制$H_{\mathrm{C}\mathrm{2个}}^{\mathrm{碳氢化合物}}\ll H_{\mathrm{C}\mathrm{2个}}^{\mathrm{R}}$，表明其可能仅表面的电子向列性。此外，我们没有在分辨率内观察到来自视场角相关比热的任何晶格旋转对称破坏信号。值得注意的是，这种仅表面的电子向列性与拓扑超导体候选者中的观察结果形成鲜明对比，${\text{铜}}_X$Bi ₂ Se ₃，其中向列性出现在各种整体测量中。结合理论，超导向列性很可能来自PbTaSe ₂的拓扑表面态，而不是邻近效应。还解决了时间反转对称性破缺的问题。因此，我们在 PbTaSe ₂上的结果为实现具有非平凡拓扑结构的向列超导性的可能途径提供了新的思路。