MANY-BODY DERIVATION OF QUASI-PARTICLE BANDS AND EFFECTIVE INTERACTIONS OF FERMIONS IN CUO2 LAYERS

We discuss normal-state properties and the phase diagram of high-Tc copper oxide superconductors on the basis of a five-states model (one d state and two bonding and two nonbonding p states per unit cell) for interacting electrons in a single CuO2 layer. In particular, quasiparticle bands and densities of states have been derived by applying a generalized Hubbard-I scheme to the five-states model and to the reduced three-states model. We find that in both cases and for realistic values of model parameters the insulating gap opens between an antibonding band of almost pure p character and the antibonding band of mainly d character. The insulating state is thus of the charge-transfer type and introducing holes into the p-like antibonding band metallizes the system. This would mean that in the presence of an attractive interaction at least a part of the Cooper pairs are formed out of only weakly correlated p-type quasiparticles. Insight of the normal-state behavior can be gained from a discussion of the spectral distribution of strongly correlated and more localized d electrons among mobile p electrons or holes. The work is supplemented by a brief discussion of the residual interactions among quasiparticles and by a comparison of our model with more strong-coupling models like the t-J model. Finally, we present formal solutions for the Greens functions by using a more refined decoupling scheme. The influence of the resulting lifetime effects and Kondo-type features on the quasiparticle spectra is briefly discussed. © 1990 The American Physical Society.