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Electronic nematic exist in several families of unconventional superconductors. Although superconductivity mediated by nematic fluctuations is well established theoretically [1-4], it has yet to be unambiguously identified experimentally [5, 6]. A 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 iron-based superconductors provides a unique opportunity to explore this concept, as it features an isolated nematic phase that can be suppressed by sulfur substitution at a quantum critical point (QCP) near x_c = 0.17, where nematic fluctuations are the largest [7, 8]. We performed scanning tunneling spectroscopy measurements to visualize Boguliubov quasiparticle interference patterns, from which we determined the momentum structure of the superconducting gap near the Brillouin zone Γ point of FeSe_0.81S_0.19. The results reveal an anisotropic, near nodal gap with minima that are 45° rotated with respect to the Fe-Fe direction, characteristic of a nematic pairing interaction. In contrast, the gap maxima in pristine FeSe are aligned with the Fe-Fe direction. Thus, the measurements reveal a gap structure that not only is at odds with general expectations from the traditional spin-fluctuation scenario, but which also displays the form expected for superconductivity mediated by nematic fluctuations. While the qualitative analysis of the data already supports the scenario of superconductivity due to nematic fluctuations, we also made a quantitative comparison with a theoretical model, further substantiating our conclusions.

 

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