Electronic properties of CuO2 planes

We study a 3-band model of CuO₂ with bare bones interactions: hopping from copper ions to nearest-neighbor oxygenst _pd only, a two-body interaction on the copper ionsU _dd only, and an overlap copper-oxygen parameter _pd. In the limitt _pd U _dd ^1/2, t^*=t _pd ²/2U _dd is the unit of energy with 8_pd ² as the only parameter of significance. If the two-body interaction is invariant under particle-hole interchange, the low-lying states (energy O(t^*)) can be described by conserved particles and can all be classified. They are quite distinct from the high-lying states (energies O(U _dd)). The dynamics of the conserved fermion-like elementary particles are well described by a modifiedt-J model with extended hopping and nearest-neighbor superexchange attraction. This is a scenario known to be favorable to high-temperature superconductivity, but it must be noted that both the hopping range and the exchange are functions of 8_pd ². Moreover, if the Hamiltonian isnot invariant under particle-hole interchange the dynamics becomes much more complex and possibly more inimical to high-temperature superconductivity. This may provide an explanation for the deleterious effect on superconductivity of very small concentrations of certain impurities.