SUPERCONDUCTING PHASE-DIAGRAM OF THE 2-BAND MODEL INVESTIGATED BY THE EXACT-DIAGONALIZATION METHOD

By means of the exact-diagonalization method we have studied the finite-size two-band model with the interband pair-transfer interaction by calculating the superconducting (SC) correlation function P(s) etc. in the ground state. Occurrence of SC was clearly indicated by the increase of P(s). The diagram of the observed SC region was constructed in the plane of the relative position of the second band versus on-site Coulomb energy U. In the low-density limit it proved close to the region where a negative-U situation is realized by the pair-transfer interaction. On the high-density side, the SC region was found to be more extended than the latter in accord with the previous interband-ladder-diagram results. Inclusion of the exchange interaction was found to reduce the SC region in a restricted region where two bands substantially share electrons. The two-band situations in cuprates are argued to be relevant to the high T(c), through the present mechanism.