Superconductivity in the Half-Filled Hubbard-Holstein Model in the Antiadiabatic Region

We study an electron-phonon model with on-site Coulomb repulsions U e e at half filling by exact diagonalization combined with a mean-field approximation which gives accurate results in the narrow-bandwidth case. Superconductivity with off-site pairing is found to exist with an inverse isotope effect if and only if the phonon-mediated attraction is almost offset by U e e .