SUPERCONDUCTIVITY, PHASE-SEPARATION AND CHARGE-TRANSFER INSTABILITY IN THE U = INFINITY LIMIT OF THE 3-BAND MODEL OF THE CUO2 PLANES

The three band Hubbard model with nearest neighbour repulsion is studied in the U = infinity limit using the slave boson technique and the large N expansion. A charge transfer (CT) instability is found like in weak coupling theory. The CT instability is always associated with a diverging compressibility leading to a phase separation. The total density and CT susceptibilities are analyzed in both the static and dynamic limit to gain insight on the strong mixing between the density fluctuations and the excitonic modes. 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.