Metal-insulator transition and superconductivity in the two-orbital Hubbard-Holstein model for iron-based superconductors

Abstract

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 diagonalization method. When the intra- and inter-orbital Coulomb interactions, $U$ and $U^{\prime }$, increase with $U=U^{\prime }$, both the local spin and orbital susceptibilities, ${\chi }_s$ and ${\chi }_o$, increase with ${\chi }_s={\chi }_o$ in the absence of the Hund's 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.

9 pages, 8 figures. arXiv admin note: text overlap with arXiv:1209.4954

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