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 $p$ ele
ctrons. 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.
We investigate a two-orbital model for iron-based superconductors to elucidate the effect of interplay between electron correlation and Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory combined with the exact diagonalizat
ion method. When the intra- and inter-orbital Coulomb interactions, $U$ and $U$, increase with $U=U$, both the local spin and orbital susceptibilities, $chi_{s}$ and $chi_{o}$, increase with $chi_{s}=chi_{o}$ in the absence of the Hunds rule coupling $J$ and the electron-phonon coupling $g$. In the presence of $J$ and $g$, there are distinct two regimes: for $J stackrel{>}{_sim} 2g^2/omega_0$ with the phonon frequency $omega_0$, $chi_{s}$ is enhanced relative to $chi_{o}$ and shows a divergence at $J=J_c$ above which the system becomes Mott insulator, while for $J stackrel{<}{_sim} 2g^2/omega_0$, $chi_{o}$ is enhanced relative to $chi_{s}$ and shows a divergence at $g=g_c$ above which the system becomes bipolaronic insulator. In the former regime, the superconductivity is mediated by antiferromagnetic fluctuations enhanced due to Fermi-surface nesting and is found to be largely dependent on carrier doping. On the other hand, in the latter regime, the superconductivity is mediated by ferro-orbital fluctuations and is observed for wide doping region including heavily doped case without the Fermi-surface nesting.
We investigate the electronic state and the superconductivity in the 5-orbital Hubbard model for iron pnictides by using the dynamical mean-field theory in conjunction with the Eliashberg equation. The renormalization factor exhibits significant orbi
tal dependence resulting in the large change in the band dispersion as observed in recent ARPES experiments. The critical interactions towards the magnetic, orbital and superconducting instabilities are suppressed as compared with those from the random phase approximation (RPA) due to local correlation effects. Remarkably, the s++-pairing phase due to the orbital fluctuation is largely expanded relative to the RPA result, while the s+--pairing phase due to the magnetic fluctuation is reduced.
We investigate the periodic Anderson model with $bm{k}$-dependent $c$-$f$ mixing reproducing the point nodes of the hybridization gap by using the dynamical mean-field theory combined with the exact diagonalization method. At low temperature below a
coherence temperature $T_0$, the imaginary part of the self-energy is found to be proportional to $T^2$ and the pseudogap with two characteristic energies $tilde{it Delta}_1$ and $tilde{it Delta}_2$ is clearly observed for $Tll T_0$, while the pseudogap is smeared with increasing $T$ and then disappears at high temperature $T simg T_0$ due to the evolution of the imaginary self-energy. When the Coulomb interaction between $f$ electrons $U$ increases, $tilde{it Delta}_1$, $tilde{it Delta}_2$, and $T_0$ together with $T_{rm max}$ at which the magnetic susceptibility is maximum decrease in proportion to the renormalization factor $Z$ resulting in a heavy-fermion semiconductor with a large mass enhancement $m^*/m=Z^{-1}$ for large $U$. We also examine the effect of the external magnetic field $H$ and find that the magnetization $M$ shows two metamagnetic anomalies $H_1$ and $H_2$ corresponding to $tilde{it Delta}_1$ and $tilde{it Delta}_2$ which are reduced due to the effect of $H$ together with $Z$. Remarkably, $Z^{-1}$ is found to be largely enhanced due to $H$ especially for $H_1 siml H siml H_2$, where the field induced heavy-fermion state is realized. The obtained results seem to be consistent with the experimental results observed in the anisotropic Kondo semiconductors such as CeNiSn.
