Standard weak-coupling methods are used to calculate the excitation spectrum and the instabilities of the metallic phase of a three-band model for the copper-oxide superconductors with both intra- and intersite repulsion. Besides the magnetic excitations, a strong charge-transfer resonance of $A_{1g}$ symmetry is predicted. For reasonable parameters, the metallic phase is found to be unstable to a charge-transfer instability at large deviations from half filling, and at small deviations to an antiferromagnet.

• Received 27 December 1988

DOI:https://doi.org/10.1103/PhysRevB.39.12371