Magnetic and superconducting instabilities in a hybrid model of itinerant/localized electrons for iron pnictides

We study a unified mechanism for spin-density-wave (SDW) and superconductivity in a minimal model in which itinerant electrons and local moments coexist as previously proposed for the iron pnictides [Kou, Li, Weng, EPL 88, 17010 (2009)]. The phase diagram obtained at the mean-field level is in qualitative agreement with the experiment, which shows how the magnetic and superconducting (SC) instabilities are driven by the critical coupling between the itinerant/localized electrons. The spin and charge response functions at the random-phase-approximation level further characterize the dynamical evolution of the system. In particular, the dynamic spin susceptibility displays a Goldstone mode in the SDW phase, which evolves into a gapped resonance-like mode in the SC phase. The latter persists all the way into the normal state above T(c) where a strong scattering between the itinerant electrons and local moments is restored, as an essential feature of the model.