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 and antibonding orbitals of a dimer made of the two sites lying on the opposite chains and linked by a transfer energy. It leads to a pair-transfer interaction between two bands composed of these orbitals, promoting superconductivity (SC). Since it is found to be as large as intraband Coulomb coupling constants, our previous work suggests that SC can occur in this model. In fact we have obtained a clear indication of SC in restricted-size systems carrying out the exact-diagonalization with 6 electrons and a variational diagonalization with up to 14. The SC occurs in suitable band situations for a wide range of on-site Coulomb energies. We also obtained such an SC indication in a two-dimensional Hubbard model. We mention some candidate materials substantiating the present mechanism and suggest it to be quite general.