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 intersite repulsion. Besides the magnetic excitations, a strong charge-transfer resonance of A1g 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.