Non-Fermi-liquid transport phenomena and superconductivity driven by orbital fluctuations in iron pnictides: Analysis by fluctuation-exchange approximation

We study the five-orbital Hubbard model including the charge quadrupole interaction for iron pnictides. Using the fluctuation-exchange approximation, orbital fluctuations evolve inversely proportional to the temperature, and therefore the resistivity shows linear or convex T dependence for a wide range of temperatures. We also analyze the Eliashberg gap equation, and show that an s-wave superconducting state without sign reversal (s ₊₊-wave state) emerges when the orbital fluctuations dominate over the spin fluctuations. When both fluctuations are comparable, their competition gives rise to a nodal s-wave state. The present study offers us a unified explanation for both the normal and superconducting states.