Abstract
We study on the two-band model with the interband pair-transfer interaction between oxygen 2p orbitals at the same atomic site. The Cu d x2−y2 orbital is included in the model in order to reflect a realistic electronic structure in the oxide high-Tc superconductors. The exact-diagonalization technique is applied to a cluster cut out of the CuCO2 plane, i.e., Cu4O8. When the on-site Coulomb energy Up = 0 for 2p orbitals, as the second band consisting of O pπ orbitals comes near the Fermi level, the superconducting (SC) pair correlation function enhances stepwise, indicating a SC state occurring. However, the SC region was found to be restricted to a modest value of Up. Analysis of the wave function in the case of one hole doping indicates that a doped hole in O pσ orbital forms a localized singlet with the intrinsic hole at Cu site, competing with the SC state.
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