Evidence for two-gap superconductivity in Ba0.55K0.45Fe2As2 from directional point-contact Andreev-reflection spectroscopy

Directional point-contact Andreev-reflection spectroscopy measurements on the Ba0.55K0.45Fe2As2 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.