Part 1*

The FeSe_1-xS_x system presents an ideal platform for studies of intertwined nematicity and superconductivity, a topic of ongoing interest in Fe-based superconductors, due to the lack of magnetic ground state at ambient pressure. In the parent compound FeSe, the superconductivity, 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 superconductivity mediated by nematic fluctuations.

 

In this two-part talk, we present the first direct experimental evidence of superconductivity mediated by nematic fluctuations through our sub-Kelvin scanning tunneling microscopy/spectroscopy (STM/S) studies on FeSe_0.81S_0.19. Here, in Part 1, we visualize the superconducting gap momentum-structure near the Γ-point via quasiparticle interference (QPI) measurements. Constructing the angular dependence of the Fermi surface QPI signal, we find a striking change in the gap symmetry, with gap minima rotated 45° compared to that of xc samples, demonstrating a fundamental change of the pairing mechanism across the phase diagram of FeSe_1-xS_x.