Optical conductivity of Ba0.6K0.4Fe2As2: The effect of in-plane and out-of-plane doping in the superconducting gap

We measured the in-plane optical conductivity of a nearly optimally doped (T-c = 39.1K) single crystal of Ba0.6K0.4Fe2As2. Upon entering the superconducting state the optical conductivity vanishes below similar to 20meV, indicating a fully gapped system. A quantitative modeling of the optical response of this material requires two different isotropic gaps, the larger of which dominates the London penetration depth. The temperature dependence of these gaps indicate a strong interband interaction, but no impurity-scattering-induced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped BaFe1.84Co0.08As2 and strongly supports an s +/- gap symmetry for both compounds. Copyright (C) EPLA, 2013