Nematic phases, in which electrons in a solid spontaneously break rotational symmetry while preserving translational symmetry, exist in several families of unconventional superconductors. Superconductivity mediated by nematic fluctuations is well established theoretically, but it has yet to be unambiguously identified experimentally. One major challenge is that nematicity is often intertwined with other degrees of freedom, such as magnetism and charge order. The FeSe_1−xS_x family of superconductors provides an opportunity to explore this concept, as it features an isolated nematic phase that can be suppressed by sulfur substitution at a quantum critical point where the nematic fluctuations are the largest. Here we determine the momentum structure of the superconducting gap near the centre of the Brillouin zone in FeSe_0.81S_0.19—close to the quantum critical point—and find that it is anisotropic and nearly nodal. The gap minima occur in a direction that is rotated 45° with respect to the Fe–Fe direction, unlike the usual isotropic gaps due to spin-mediated pairing in other tetragonal Fe-based superconductors. Instead, we find that the gap structure agrees with theoretical predictions for superconductivity mediated by nematic fluctuations, indicating a change in the pairing mechanism across the phase diagram of FeSe_1−xS_x.

向列相，其中固体中的电子自发打破旋转对称性，同时保持平移对称性，存在于几个非常规超导体族中。由向列脉动介导的超导性在理论上已经确立，但尚未在实验中明确确定。一个主要挑战是向向性通常与其他自由度交织在一起，例如磁力和电荷顺序。FeSe_1−xS_x 系列超导体为探索这一概念提供了机会，因为它具有孤立的向列相，可以在向列波动最大的量子临界点通过硫取代来抑制。在这里，我们确定了 FeSe_0.81S_0.19 附近布里渊区中心附近（靠近量子临界点）的超导间隙的动量结构，发现它是各向异性的，几乎是节点。间隙最小值出现在相对于 Fe-Fe 方向旋转 45° 的方向上，这与其他四方 Fe 基超导体中由于自旋介导的配对而导致的通常各向同性间隙不同。相反，我们发现间隙结构与向列涨落介导的超导性理论预测一致，表明 FeSe _1-xS_x 相图上的配对机制发生了变化。