A multiband Eliashberg-approach to iron-based superconductors

Since the discovery of the Fe-based superconductors (FeSC) a lot of efforts has been applied to elucidate the microscopic mechanism behind cooper pairing and the symmetry of the order parameter. Examination of effective interactions of electrons with spin-fluctuations in these systems yield that couplings are not weak and the materials have to be considered in a strong coupling scheme. In the present article, we review some aspects of the description of iron-based superconductors in the frame of the Eliashberg approach for multiband systems. The application of the Eliashberg scheme is demonstrated with two compounds Ba0.6K0.4Fe2As2 and Ca0.32Na0.68Fe2As2. In both cases, it was found that these materials are in the intermediate coupling regime. Further, we consider effects of impurities on physical properties of multiband superconductors. It is demonstrated that the impurities affect nontrivially the superconducting properties. In particular, they may trigger a symmetry change of superconducting order parameters. This effect can be used for determination of the superconducting order parameter. Finally, we consider KFe2As2 and show that its thermodynamic properties are consistent with a nodal superconducting order parameter