Effect of Electron-Phonon Interactions on Orbital Fluctuations in Iron-Based Superconductors

To investigate the possibility of whether electron-phonon coupling can enhance orbital fluctuations in iron-based superconductors, we develop an ab initio method to construct the effective low-energy models including the phonon-related terms. With the derived effective electron-phonon interactions and phonon frequencies, we estimate the static part (omega = 0) of the phonon-mediated effective on site intra-or interorbital electron-electron attractions as similar to - 0.4 eV and exchange or pair-hopping terms as similar to - 0.02 eV. We analyze the model with the derived interactions together with the Coulomb repulsions within the random phase approximation. We find that the enhancement of the orbital fluctuations due to the electron-phonon interactions is small, and that the spin fluctuations enhanced by the Coulomb repulsions dominate. It leads to the superconducting state with the sign reversal in the gap functions (s(+/-) wave).