Full Gap Superconductivity in Ba0.6K0.4Fe2As2 Probed by Muon Spin Rotation

Superfluid density (ns) in the mixed state of an iron pnictide superconductor Ba1-xKxFe2As2 is determined by muon spin rotation for a sample with optimal doping (x = 0.4). The temperature dependence of it, is perfectly reproduced using the conventional BCS model for s-wave paring, where the order parameter can be either it single-gap structure with Delta = 8.35(6) meV [2 Delta/k(B)T(c) = 5.09(4)] or double-gap structure with Delta(1) - 12 meV (fixed) [2 Delta(1)/k(B)T(c) - 7.3] and Delta(2) = 6.8(3) meV [2 Delta(2)/k(B)T(c) = 4.1(2)]. The latter is consistent with the recent result obtained by the angle-resolved photoemission spectroscopy. The large gap parameters (2 Delta/k(B)T(c)) indicate the extremely Strong coupling of carriers to bosons that mediate the Cooper pairing.