Unconventional superconductivity originating from disconnected Fermi surfaces in correlated superconductors : A case study for iron pnictides

We first generally summarize the effect of disconnected Fermi surfaces in spin fluctuation mediated superconductivity. We argue that disconnected Fermi surfaces are favorable in that the sign of the superconducting gap can be changed without nodal lines intersecting the Fermi surface. Then, as an example of actual materials that have disconnected Fermi surfaces, we focus on the iron-based high T_c superconductors. We construct a model that contains all of the five Fe d bands, and apply random-phase approximation. We find that multiple spin fluctuation modes develop due to the nesting between disconnected Fermi surfaces, and the superconductivity originating from the cooperation or competition between these multiple spin fluctuation modes depends on the lattice structure. In particular, the appearance of the Fermi surface around (π, π) in the unfolded Brillouin zone is sensitive to the pnictogen height h_Pn measured from the Fe plane, and the height can act as a switch between high T_c nodeless and low T_c nodal pairings. In the high T_c case, the superconducting gap is fully open on all of the five Fermi surfaces, but changes sign across the nesting vectors that bridge the disconnected Fermi surfaces.