Perturbation theory of high- superconductivity in iron pnictides

Nomura, Takuji

High- superconductivity discovered recently in iron pnictides is analyzed within a perturbation theory. Specifically, the probable pairing symmetry, the doping dependence of and a pairing mechanism etc. are studied by solving the Eliashberg equation for multi-band (2- and 5-band) Hubbard models. The effective pairing interaction is expanded perturbatively in the on-site Coulomb integrals up to third order. Our weak-coupling approach suggests: sufficiently large eigenvalues of the Eliashberg equation are obtained to explain the actual high for realistic on-site Coulomb integrals in the 5-band model. Thus unconventional (non-phonon-mediated) superconductivity is much likely to be realized. The superconducting order parameter does not change its sign on the Fermi surfaces, but change between the electron and hole Fermi surfaces. Consequently the probable pairing symmetry is always "nodeless extended -wave (more specifically, -wave)". It is suggested that the 2-band model is insufficient to explain actual high 's.