Superconductivity in a Two-Orbital Hubbard Model with Electron and Hole Fermi Pockets : Application to Iron Oxypnictide Superconductors

We investigate electronic states of the one-dimensional two-orbital Hubbard model with band splitting by using the exact diagonalization method. The Luttinger liquid parameter Kp is calculated to obtain superconducting (SC) phase diagram as a function of on-site interactions: the intra- and inter-orbital Coulomb U and U ', the Hund coupling J and the pair transfer J '. In this model, electron and hole Fermi pockets are originated when the Fermi level crosses both upper and lower orbital bands. We find that the system shows two types of SC phases, the SC I for U > U ' and the SC II for U < U ', in the wide parameter region including both weak and strong correlation regimes. Pairing correlation functions indicate that the most dominant pairing for the SC I (SC II) is the intersite (on-site) intra-orbital spin-singlet with (without) sign reversal of the order parameters between the two Fermi pockets. The result of the SC I is consistent with the sign-reversing s-wave pairing recently proposed for iron oxypnictide superconductors