Dynamical Mean-Field Study of Local Pairing Interaction Mediated by Spin and Orbital Fluctuations in Iron Pnictide Superconductors

Abstract

We investigate the two-orbital Hubbard model, which reproduces the electron and hole Fermi surfaces in the iron pnictide superconductors, in the presence of the Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory. 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 ${\chi }_s={\chi }_o$ because of the spin-orbital symmetry. Due to the Hund's rule coupling $J$, ${\chi }_s$ is enhanced and dominates over ${\chi }_o$ resulting in the repulsive local pairing interaction $V_{\mathrm{loc}}>0$, while due to the electron-phonon coupling $g$, ${\chi }_o$ is enhanced and dominates over ${\chi }_s$ resulting in the attractive one $V_{\mathrm{loc}}<0$ which induces the intra-orbital s-wave pairing. Remarkably, $V_{\mathrm{loc}}$ is weakly dependent on doping and can be attractive for heavily electron-doped regime where the superconductivity is observed without Fermi surface nesting.

4 pages, 4 figures