Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor

For the iron-based high Tc superconductor LaFeAsO1−xFx, we construct a minimal model, where all of the five Fe d bands turn out to be involved. We then investigate the origin of superconductivity with a five-band random-phase approximation by solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realise, basically, an extended s-wave pairing, where the gap changes sign across the nesting vector.