Highly Anisotropic and Twofold Symmetric Superconducting Gap in Nematically Ordered FeSe0.93 S0.07

FeSe exhibits a novel ground state in which superconductivity coexists with a nematic order in the absence of any long-range magnetic order. Here, we report on an angle-resolved photoemission study on the superconducting gap structure in the nematic state of FeSe0.93S0.07, without the complications caused by Fermi surface reconstruction induced by magnetic order. We find that the superconducting gap shows a pronounced twofold anisotropy around the elliptical hole pocket near Z (0, 0, π), with gap minima at the end points of its major axis, while no detectable gap is observed around Γ (0, 0, 0) and the zone corner (π, π, kz). The large anisotropy and nodal gap distribution demonstrate the substantial effects of the nematicity on the superconductivity and thus put strong constraints on current theories.