Structure and Anisotropy of the Superconducting Order Parameter in Ba0.65K0.35Fe2As2 Probed by Andreev Spectroscopy

The superconducting order parameters in optimally doped Ba 0.65 K 0.35 Fe 2 As 2 single crystals have been directly measured using multiple Andreev reflection effect spectroscopy of superconductor–normal metal–superconductor break-junctions. We determine two superconducting gaps, which are nodeless in the k x k y -plane of the momentum space, and resolve a substantial in-plane anisotropy of the large gap. The temperature dependences of the gaps indicate a strong coupling within the bands where Δ L develops, a weak coupling in the condensate with the small gap Δ S , and a moderate interband interaction between the two condensates. The own critical temperatures of both condensates have been estimated (under the hypotherical assumption of zero interband interaction).