Magnetic fluctuation and Gap symmetry in Iron-Pnictide Superconductors

Since the recent discovery of high-$T_c$ superconductivity in LaFeAsO$_{1-x}$F$_x$, the Fe-based superconductors have been intensively investigated. In order to understand the overall feature of the phase diagram, we investigate the unfolded 5-band Hubbard model, which can describe the band structure near the Fermi level in LaFeAsO, within the fluctuation-exchange approximation. We find the enhancement of the antiferromagnetic (AF) spin fluctuation in the hole-doped region and the pseudogap behavior of NMR-$1/T_1$ in the electron-doped region. Evaluating the Eliashberg equation, we find that the sign-reversing $s_\pm$-wave pairing state appears in close proximity to the AF phase. Roughly speaking, the gap function is fully-gapped in the hole-doped region, and remarkably anisotropic in the electron-doped region. The eigenvalue $\lambda$ is relatively small, and insensitive to carrier doping for small Hund coupling $J$. On the contrary, for large $J$, it is relatively large, and sensitive to carrier doping and the presence of the AF phase.