Full gap superconductivity in Ba _0:6K _0:4Fe ₂As ₂ probed by muon spin rotation

Superfluid density (n _s) in the mixed state of an iron pnictide superconductor Ba _1-xK _xFe ₂As ₂ is determined by muon spin rotation for a sample with optimal doping (x = 0:4). The temperature dependence of n _s is perfectly reproduced using the conventional BCS model for s-wave paring, where the order parameter can be either a single-gap structure with Δ = 8:35(6) meV [2Δ/k _BT _c = 5:09(4)] or double-gap structure with Δ ₁ = 12 meV (fixed) [2Δ ₁/k _BT _c = 7:3] and Δ ₂ = 6:8(3) meV [2Δ ₂/ k _BT _c = 4:1(2)]. The latter is consistent with the recent result obtained by the angle-resolved photoemission spectroscopy. The large gap parameters (2Δ/k _BT _c) indicate the extremely strong coupling of carriers to bosons that mediate the Cooper pairing.