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 microscope/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 maximum superconducting gap ${\Delta }_{max}\approx$ 0.90meV, which yields a ratio of 2${\Delta }_{max}/k_B{T}_c\approx$ 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 (four-fold image) and Fe sites (dumbbell shape). Impurity induced resonance states are found only for the former defects and stay at zero-bias energy.

arXiv comment6 pages, 5 figures

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