Effect of pair hopping and magnitude of intra-atomic interaction on exchange-mediated superconductivity

It is shown that the kinetic exchange Hamiltonian for a partially filled narrow band contains a term responsible for hopping of neighboring pairs of electrons in the singlet spin state. In this way we introduce explicitly the dynamics of the resonant valence bond into the formalism describing the exchange-mediated high-Tc superconductivity. The role of the magnitude of the intraatomic Coulomb interaction on the onset of superconducting phase is also discussed.