Evidence for two-gap superconductivity inBa0.55K0.45Fe2As2from directional point-contact Andreev-reflection spectroscopy

Abstract

Directional point-contact Andreev-reflection spectroscopy measurements on the ${\text{Ba}}_{0.55}{\text{K}}_{0.45}{\text{Fe}}_{2}{\text{As}}_{2}$ 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.