A High-Tc Mechanism of Iron Pnictide Superconductivity due to Cooperation of Ferro-orbital and Antiferromagnetic Fluctuations

The electronic states and superconductivity in iron pnictides are studied on the basis of the 16 band $d$-$p$ model which includes both the onsite Coulomb interaction between Fe $d$ electrons and the intersite one between Fe $d$ and pnictogen $p$ electrons. The model well accounts for experimentally observed two fluctuations: the $d$-$d$ interaction-enhanced antiferromagnetic (AFM) fluctuation and the $d$-$p$ interaction-enhanced ferro-orbital (FO) fluctuation responsible for the $C_{66}$ elastic softening. The AFM fluctuation induces the repulsive pairing interaction for $\bm{q}\sim \bm{Q}_{\rm AF}$ while the FO does the attractive one for $\bm{q}\sim \bm{0}$ resulting in the $s_{\pm}$-wave superconductivity where the two fluctuations cooperatively enhance the superconducting transition temperature $T_{c}$ without any competition by virtue of the $\bm{q}$-space segregation.