Strong coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS

TL;DR

This study uses scanning tunneling microscopy to reveal strong coupling superconductivity in tetragonal FeS, characterized by an anisotropic or nodal gap and impurity effects, advancing understanding of its electronic properties.

Contribution

It provides direct spectroscopic evidence of strong coupling superconductivity and detailed gap structure in tetragonal FeS, with impurity effects analyzed at the atomic level.

Findings

Superconducting gap $ ightarrow$ approximately 0.90 meV

Gap ratio $2\Delta_{max}/k_BT_c$ $ ightarrow$ about 4.65, indicating strong coupling

Impurity states at S sites show zero-bias resonance, affecting local electronic properties

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 $\Delta_{max}\approx$ 0.90$\;$meV, which yields a ratio of 2$\Delta_{max}/k_BT_c\approx$ 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.