Evidence for two-gap superconductivity in (Ba,K)Fe₂As₂ by directional point contact Andreev reflection spectroscopy

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.