Spectroscopic-imaging STM studies on lightly Te-doped FeSe

The iron-based superconductor FeSe exhibits electronic nematicity that coexists with superconductivity characterized by an anisotropic superconducting gap. Isovalent substitutions of S or Te for Se suppress the nematicity, offering opportunities to investigate the relationship between nematicity and superconductivity. It is also known that large amount of Te substitution (~ 50%) makes the band structure topologically nontrivial and creates a topological superconducting surface state, where the superconducting gap is isotropic. To investigate the roles of individual Te dopants on such electronic properties, we performed spectroscopic-imaging STM on lightly Te doped FeSe. We found that the isolated Te dopant hardly affects the anisotropic superconducting gap, whereas it alters the asymmetry in the density of states between filled and empty states. This may suggest that the change in the superconducting gap from anisotropic to isotropic upon Te substitution is related to the nematic and/or topological transitions.