Visualizing superconductivity mediated by nematic fluctuations in the Fe-based superconductor FeSe_1-xS_x: Part 2

The FeSe_1-xS_x system presents an ideal platform for studies of intertwined nematicity and superconductivity (SC), a topic of ongoing interest in Fe-based superconductors, due to its lack of magnetic ground state at ambient pressure. In the parent compound FeSe, the SC, mediated by spin fluctuations, coexists with a nematic phase. S-substitution (x) in FeSe_1-xS_x suppresses the nematic phase at the quantum critical point (QCP), x_c~0.17, where nematic fluctuations become largest. The pairing mechanism in the x>x_c regime is undetermined, presenting an ideal test for theoretically-predicted SC mediated by nematic fluctuations.



In this two-part talk, we present the first direct experimental evidence of SC mediated by nematic fluctuations through our sub-Kelvin scanning tunneling microscopy/spectroscopy (STM/S) studies on FeSe_0.81S_0.19. Here, in Part 2, we resolve Bogoliubov quasiparticle interference in tetragonal FeSe_1-xS_x for the first time. Invoking the octet model, famously used to describe superconducting gap structure in cuprates superconductors, we extract the gap symmetry, showing near-nodal minima that are rotated 45° compared to that of the parent nematic FeSe. We find that this anisotropic gap structure is remarkably consistent with theoretical predictions of SC mediated by nematic fluctuations, but qualitatively disagrees with scenarios where SC is mediated by spin fluctuations. Overall, our experiments reveal the existence of SC mediated by nematic fluctuations in FeSe_0.81S_0.19.