Two-Band Model for High Tc Superconductivity

A two-band model is dealt with by the canonical perturbation theory of projection operators. An effective Hamiltonian with Cu-Cu, Cu-O and O-O couplings is obtained. A unified treatment for such a coupled system gives the variations of transition temperatures for Cu-Cu, Cu-O and O-O parings with the concentration of O-holes. A window of δ for superconductivity is obtaihed and compared with experimental data qualitatively and quantitatively.