Superconductivity in a two-band Hubbard model

As a model for high-${\mathit{T}}_{\mathit{c}}$ superconductors, a two-band Hubbard model is proposed. When the Coulomb interactions are introduced between the α and β orbitals, an effective attraction of carriers in the β (doped) band is shown to arise in an appropriate condition. The quantum Monte Carlo method is used to confirm the occurrence of Cooper pairing in finite two-band Hubbard systems. The attraction, which also appears in the density-density correlation, enhances the on-site singlet Cooper pair-pair correlation in the β band and suppresses the antiferromagnetic spin-spin correlation in the α band. The criterion for the pairing and the nature of the pairing are discussed. Thus we provide a mechanism of superconductivity specific to strongly correlated two-band systems.