Strong-coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS

Abstract

We investigate the electronic properties of the tetragonal FeS superconductor by using scanning tunneling microscopy/spectroscopy. It is found that the typical tunneling spectrum on the top layer of sulfur can be nicely fitted with an anisotropic $s$ wave or a combination of two superconducting components in which one may have a highly anisotropic or nodal-like superconducting gap. The fittings lead to the superconducting gap of about ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{max}}\ensuremath{\approx}\phantom{\rule{4pt}{0ex}}0.90\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$, which yields a ratio of $2{\mathrm{\ensuremath{\Delta}}}_{\mathrm{max}}/{k}_{B}{T}_{c}\ensuremath{\approx}$ 4.65. This value is larger than that of the predicted value 3.53 by the BCS theory in the weak-coupling limit, indicating a strong-coupling superconductivity. Two kinds of defects are observed on the surface, which can be assigned to the defects on the S sites (fourfold image) and Fe sites (dumbbell shape). Impurity-induced resonance states are found only for the defects on the S sites and stay at zero-bias energy.