Abstract
We investigate interplay between magnetic fluctuations and superconductivity in the effective five-band Hubbard model for iron-pnictide superconductors on the basis of the fluctuation-exchange approximation. As for the normal-state properties, we find the pseudogap behavior in the NMR relaxation rate 1/T1T in the electron-doped region, while we find strong enhancement of 1/T1T in the hole-doped region. Such pseudogap behavior originates from the band structure effect, that is, existence of high density of states just below the Fermi level and its broadening by the correlation effect. Solving the superconducting Eliashberg equation, we find that the most probable candidate for the pairing symmetry is the sign-changing s-wave spin-singlet state. For small exchange coupling J, the eigenvalue is not so sensitive to carrier doping, and seems to be irrelevant with antiferromagnetic (AF) spin fluctuation. We suggest that the orbital degrees of freedom is important as the pairing mechanism as well as the AF spin fluctuation.
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