PHASE-SEPARATION AND SUPERCONDUCTIVITY IN THE U = INFINITY LIMIT OF THE EXTENDED MULTIBAND HUBBARD-MODEL

The three band Hubbard model with nearest neighbour repulsion is studied in the U = infinity limit using the large N expansion technique to order 1/N. A charge transfer (CT) instability is found like in weak coupling theory. However at small doping a major role is played by the Brinkman-Rice point. The CT instability is always associated with a diverging compressibility leading to a phase separation. The evaluation to order 1/N of the effective scattering amplitude in the Cooper channel shows the presence of superconducting instabilities in the s and d wave channel near the phase separation.