Eliashberg approach to infrared anomalies induced by the superconducting state of Ba0.68K0.32Fe2As2 single crystals

We report the full complex dielectric function of high-purity Ba0.68K0.32Fe2As2 single crystals with T-c = 38.5 K determined by wideband spectroscopic ellipsometry at temperatures 10 <= T <= 300 K. We discuss the microscopic origin of superconductivity-induced infrared optical anomalies in the framework of a multiband Eliashberg theory with two distinct superconducting gap energies, 2 Delta(A) approximate to 6 k(B)T(c) and 2 Delta(B) approximate to 2.2 k(B)T(c). The observed unusual suppression of the optical conductivity in the superconducting state at energies up to 14 k(B)T(c) can be ascribed to spin-fluctuation-assisted processes in the clean limit of the strong-coupling regime.