Strong-Coupling Descriptions of High-Temperature Superconductors - Electronic Attraction From A Repulsive Potential

We consider the effect of the nearest-neighbour copper-oxygen repulsion, V, when coupled to the charge transfer resonances Cu2+ --> Cu3+ and Cu2+ --> Cu+ in the high-temperature cuprate superconductors. This is done by deriving effective low-energy Hamiltonians correct to second order in the copper-oxygen hybridization. Only hole doping is considered. When Cu2+ --> Cu3+ fluctuations dominate we derive an effective one-band model of 'Zhang-Rice' singlets with a nearest-neighbour repulsion between these singlets. When Cu2+ --> Cu+ fluctuations dominate we find rich and complex behaviour. If 0 much less than <V/DELTA less-than-or-equal-to 1/2 (where DELTA is the 'bare' copper-oxygen charge transfer pp) we show that clusters of charge are more stable than isolated charges. On the other hand, if 0 less-than-or-equal-to V/DELTA much less than 1/2 the Hamiltonian contains both weak attractive and repulsive two-body potentials. Calculations on clusters indicate that the attractive potentials have the same correlations as the more dominant 'single-particle' terms suggesting the possibility of 's'-wave pairing.