Multiband Description of Optical Conductivity in Ferropnictide Superconductors

We study optical properties of the multiband superconductors with an s ± order parameter symmetry. By comparing results of our theory with experimental data on optical conductivity for Ba0.68K0.32Fe2As2 single crystals, we show that satisfactory description of the novel superconductors can be obtained only considering a strong electron-boson coupling. We reexamine the effect of disorder and demonstrate that multiband superconductors are more robust with respect to it than naively expected by simple analogy with paramagnetic impurities in single-band superconductors. Moreover, disorder may give rise to new effects, in particular to a phase transition s ±→s ++. We discuss how the systematic study of disorder impact on the density of states and the optical conductivity may provide information on the underlying order parameter structure.