Structure and anisotropy of the superconducting order parameter in Ba$_{0.65}$K$_{0.35}$Fe$_2$As$_2$ 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 $k_xk_y$-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 $\Delta_{\mathrm{L}}$ develops, a weak coupling in the condensate with the small gap $\Delta_S$, 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).