Collective excitations in the normal state of Cu-O based superconductors.

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 inner-site repulsion. A strong charge transfer resonance of A sub (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.