Evidence for two-gap superconductivity in Ba_0.55K_0.45Fe₂As₂ from directional point-contact Andreev-reflection spectroscopy

Directional point-contact Andreev-reflection spectroscopy measurements on the Ba_0.55K_0.45Fe₂As₂ 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.