CHARGE FLUCTUATIONS IN THE 4-BAND EXTENDED HUBBARD-MODEL

We investigate the extended four-band Hubbard model for the CuO2 planes of high-temperature superconductors. We consider the limit of U = infinity and we include the nearest-neighbor (n-n) Coulombic repulsion between the holes on 2p(x),2p(y) orbitals on oxygen and the holes on d(x2-y2) or d(3x2-r2) orbitals on copper (V-x and V-z respectively, with V-x > V-z). As in the three-band version of this model, a phase separation instability is induced by nearest-neighbor repulsions. The presence of holes of d(3z2-r2) character experiencing a smaller n-n repulsion than the d(x2-y2) holes leads to a reduction of the overall n-n repulsion. This reduces the intermediate-doping region of instability with respect to the three-band case. Nevertheless, the interplay between the d(x2-y2) and the d(3x2-r2) occupations introduces a new phase separation instability in the region of large differences between the copper and oxygen atomic levels. Both instabilities are driven by p-d charge fluctuations which provide an effective attractive interaction leading to superconductivity.