Evidence for two-gap superconductivity in (Ba,K)Fe_2As_2 by directional point contact Andreev reflection spectroscopy

TL;DR

This study uses directional point-contact Andreev reflection spectroscopy on Ba$_{0.55}$K$_{0.45}$Fe$_2$As$_2$ single crystals to provide evidence for two-gap superconductivity, revealing anisotropic gap features and differing coupling strengths.

Contribution

It demonstrates the presence of two distinct superconducting gaps in (Ba,K)Fe$_2$As$_2$ and highlights the anisotropic nature of the superconducting state through directional spectroscopy.

Findings

Two superconducting gaps detected in the $ab$ plane

Large gap shows strong coupling, small gap below BCS value

Reduced conductance persists above $T_c$ in the $c$ direction

Abstract

Directional point-contact Andreev-reflection spectroscopy measurements on the Ba$_{0.55}$K$_{0.45}$Fe$_2$As$_2$ single crystals are presented. The spectra show significant differences when measured in the $ab$ plane in comparison with those measured in the $c$ direction of the crystal. In the latter case only a reduced point-contact conductance around zero bias has been revealed persisting well above $T_c$ and probably related to the structural and magnetic transitions in the system. Within the $ab$ plane two superconducting energy gaps are detected below $T_c$. Here a reduced conductance above $T_c$ could also be found. The fits of the $ab$-plane data to the superconducting s-wave two-gap model indicate that the smaller gap has a size below the BCS value while the large gap reveals much higher coupling strength.