We report the full complex dielectric function of high-purity $\textrm{ Ba}_{ 0.68}\textrm{ K}_{ 0.32}\textrm{ Fe}_2\textrm{ As}_2$ single crystals with $T_{ \mathrm{ c}}=38.5\ \textrm{ K}$ determined by wide-band spectroscopic ellipsometry at temperatures $10\leq T\leq300\ \textrm{ K}$. 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 $2\Delta_{ \mathrm{ A}}\approx6\ k_{ \mathrm{ B}}T_{ \mathrm{ c}}$ and $2\Delta_{ \mathrm{ B}}\approx2.2\ k_{ \mathrm{ B}}T_{ \mathrm{ c}}$. The observed unusual suppression of the optical conductivity in the superconducting state at energies up to $14\ k_{ \mathrm{ B}}T_{ \mathrm{ c}}$ can be ascribed to spin-fluctuation--assisted processes in the clean limit of the strong-coupling regime.
Categories: cond-mat.supr-con cond-mat.str-el