The U=∞ limit of the three-band Hubbard model with nearest-neighbor repulsion V is studied using the slave-boson approach and the large-N expansion technique to order 1/N. A charge-transfer instability is found as in weak-coupling theory. The charge-transfer instability is always associated with a diverging compressibility leading to a phase separation. Near the phase-separation, charge-transfer-instability region we find superconducting instabilities in the s- and d-wave channel. The requirement for superconductivity is that V be on the scale of the Cu-O hopping as suggested by Varma, Schmitt-Rink, and Abrahams.

• Received 19 July 1990

DOI:https://doi.org/10.1103/PhysRevLett.67.259