Perturbation theory of iron-pnictide superconductivity

To study the high-Tc superconductivity in iron pnictides, we solve the Eliashberg equation for a five-orbital Hubbard model by expanding the pairing interaction perturbatively to third order in the on-site Coulomb integrals. The most favorable pairing symmetry is an s±-wave with sign change of the order parameter between the hole and electron pockets. It is suggested that the superconductivity disappears in high-doping regions where the electron pockets or the hole pockets disappear.