ELECTRONIC-PROPERTIES OF CUO2 PLANES

We study a 3-band model of CuO2 with ''bare bones'' interactions: hopp ing from copper ions to nearest-neighbor oxygens t(pd) only, a two-bod y interaction on the copper ions U-dd only, and an overlap copper-oxyg en parameter lambda(pd). In the limit t(pd) proportional to U-dd(1/2) --> infinity,t=t(pd)(2)/2U(dd) is the unit of energy with 8 lambda(pd )(2) as the only parameter of significance. If the two-body interactio n is invariant under particle-hole interchange, the low-lying states ( energy O(t)) can be described by conserved particles and can all be c lassified. They are quite distinct from the high-lying states (energie s O(U-dd)). The dynamics of the conserved fermion-like elementary part icles are well described by a modified t-J model with extended hopping and nearest-neighbor superexchange attraction. This is a scenario kno wn to be favorable to high-temperature superconductivity, but it must be noted that both the hopping range and the exchange are functions of 8 lambda(pd)(2). Moreover, if the Hamiltonian is not invariant under particle-hole interchange the dynamics becomes much more complex and p ossibly more inimical to high-temperature superconductivity. This may provide an explanation for the deleterious effect on superconductivity of very small concentrations of certain impurities.