Strong-coupling descriptions of high-temperature superconductors: electronic attraction from a repulsive potential

Abstract

The authors consider the effect of the nearest-neighbour copper-oxygen repulsion, V, when coupled to the charge transfer resonances Cu2+ to Cu3+ and Cu2+ to 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+ to Cu3+ fluctuations dominate the authors derive an effective one-band model of 'Zhang-Rice' singlets with a nearest-neighbour repulsion between these singlets. When Cu2+ to Cu+ fluctuations dominate they find rich and complex behaviour. If 0<<V/ Delta <or=1/2 (where Delta is the 'bare' copper-oxygen charge transfer gap) the authors show that clusters of charge are more stable than isolated charges. On the other hand, if 0 <or= V/ Delta << 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 's'-wave pairing.