Correlation-enhanced electron-phonon interaction in a strongly correlated electron system

In order to investigate effects of the Coulomb correlation on the electron-phonon interaction in a strongly correlated system, the electron-phonon coupling constant (λph) and the electron-spin-fluctuation coupling constant (λsf) are evaluated for the Hubbard Hamiltonian system. It is found that the Coulomb correlation in the Hubbard model gives rise to a phonon hardening and an enhancement of λph. Further, both λph and λsf are found to be reduced significantly in overall magnitudes, if an effect of the finite bandwidth is taken into account. In particular, we have found that λph becomes larger than λsf for a system with a nearly filled band, which indicates that a phonon-mediated BCS superconductivity may be possible in strongly correlated systems.