CHARGE FLUCTUATIONS IN THE 4-BAND EXTENDED HUBBARD-MODEL

We investigate the extended four-band Hubbard model for the CuO2 plane s of high-temperature superconductors. We consider the limit of U = in finity and we include the nearest-neighbor (n-n) Coulombic repulsion b etween 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 separ ation instability is induced by nearest-neighbor repulsions. The prese nce of holes of d(3z2-r2) character experiencing a smaller n-n repulsi on than the d(x2-y2) holes leads to a reduction of the overall n-n rep ulsion. This reduces the intermediate-doping region of instability wit h respect to the three-band case. Nevertheless, the interplay between the d(x2-y2) and the d(3x2-r2) occupations introduces a new phase sepa ration instability in the region of large differences between the copp er and oxygen atomic levels. Both instabilities are driven by p-d char ge fluctuations which provide an effective attractive interaction lead ing to superconductivity.