Superconducting Phase Diagram of the Two-Band Model Investigated by the Exact Diagonalization Method

Abstract

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 by means of the Lanczos diagonalization method. When the second band approaches the Fermi level lying in the first band, the increase of P_s clearly indicates occurrence of SC. The diagram of the SC phase is obtained in a parameter space. Although calculation is limited up to 7 sites and 6 electrons, the SC phase is wide enough to justify serious application of the model to the cuprates. Assuming that SC occurs by the present mechanism in a conducting channel made of the O 2p_σ orbitals, we qualitatively explain the observed systematics on T_c with the Madelung potential differences and with the doped hole density.