Abstract

The iron-based oxypnictide compound recently discovered by Hosono is theoretically challenging as a new class of superconductors. Here we study the pairing mechanism in two steps: We first construct an effective model for the material from first principles in terms of the maximally localized Wannier functions, where the model turns out to involve all the five Fe 3d bands. We then examine the Eliashberg equation with the five-band RPA . A disconnected Fermi surface, comprising two sets of Fermi pockets, has multiple nesting vectors, which in turn give rise to an unconventional pairing, where the pairing symmetry is roughly an extended s with a sign change between the pockets, but the gap function is in fact a 5 by 5 matrix. We shall discuss the implications of the matrix gap function and the material/pressure dependence. This work is a collaboration with K. Kuroki, S. Onari, R. Arita, H. Usui, Y. Tanaka and H. Kontani.