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Non-Fermi-Liquid-Like Behaviors and Superconductivity Driven by Orbital Fluctuations in Iron Pnictides: Analysis by Fluctuation-Exchange Approximation

We study the five-orbital Hubbard-Holstein model for iron pnictides with small electron-phonon interaction due to Fe-ion Einstein oscillators. Using the fluctuation-exchange (FLEX) approximation, orbital fluctuations evolve inversely proportional to the temperature, and therefore the resistivity shows linear or convex T-dependence for wide range of temperatures. We also analyze the Eliashberg gap equation, and show that s-wave superconducting state without sign reversal (s_{++}-wave state) emerges when the orbital fluctuations dominate 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.