Study of the Two-Gap Superconductivity in GdO(F)FeAs by ScS-Andreev Spectroscopy

Current-voltage characteristics and dynamic conductance of the superconductor -constriction - superconductor junctions in GdO(F)FeAs polycrystalline samples with critical temperatures TlocalC = 46 ÷ 53 K were investigated. Two superconducting gaps, the large ΔL = 10.5±2 meV, and the small one Δs = 2.3 ± 0.4 meV were clearly observed at T = 4.2 K. The 2ΔL/kBTlocalC = 5.5 ± 1 ratio gives support to the strong coupling mechanism which is responsible for the high Tc value. Temperature dependence of the large gap ΔL(T) indicates the presence of intrinsic proximity effect (in κ-space) between two superconducting condensates.