Superconducting ground state of the two-band model indicated by the superconducting correlation function

The superconducting (SC) ground state of the finite-size two-band model is looked for by calculating the SC, SDW and CDW correlation functions obtained by using the Lanczos method. When the second band approaches to or crosses the Fermi level lying in the first band, the increase of the SC correlation function clearly indicates the occurrence of SC. The diagram of the SC phase is obtained in the space of relevant parameters. It is close to that expected from the mean-field theory. The SC region tends to expand with increase of the system size but the calculation is limited up to 7 sites and 6 electrons. It is already wide enough in the size of on-site Coulomb energies to justify serious application of the model to the oxides. (orig.)