2-BAND MECHANISM OF SUPERCONDUCTIVITY IN DIMERIC HUBBARD MODELS

In the two-chain Hubbard model the on-site Coulomb repulsion generates a remarkably large pair-transfer coupling constant between bonding an d antibonding orbitals of a dimer made of the two sites lying on the o pposite chains and linked by a transfer energy. It leads to a pair-tra nsfer interaction between two bands composed of these orbitals, promot ing superconductivity (SC). Since it is found to be as large as intrab and Coulomb coupling constants, our previous work suggests that SC can occur in this model. In fact we have obtained a clear indication of S C in restricted-size systems carrying out the exact-diagonalization wi th 6 electrons and a variational diagonalization with up to 14. The SC occurs in suitable band situations for a wide range of on-site Coulom b energies. We also obtained such an SC indication in a two-dimensiona l Hubbard model. We mention some candidate materials substantiating th e present mechanism and suggest it to be quite general.