Model construction and pairing symmetry for the iron-based oxypnictides

PC-19-INV: In order to clarify the mechanism of superconductivity in the iron-based compound recently discovered by Hosono's group, we have first constructed a tight-binding model in terms of the maximally localized Wannier orbitals from a first-principles electronic structure calculation. The model has turned out to involve all the five Fe 3d bands. This is used to calculate the spin and charge susceptibilities with the five-band random-phase approximation, which are then plugged into the linearised Eliashberg equation. For a doped system we obtain an unconventional s-wave pairing with sign-reversing gap functions. To be more precise, the gap function is a 5×5 matrix, for which the diagonal elements mainly comprise dx2-y2 and dyz,dxz orbital components. The strong dependence of the gap between different orbitals may be observed experimentally.