Effect of on-site Coulomb repulsion on superconductivity in the boson-fermion model

We study the influence of the repulsive Coulomb interactions on the thermodynamic properties of the boson-fermion model with an anisotropic (d-wave and extended s-wave) order parameter. Superconductivity is induced in this model from the anisotropic charge exchange interaction between the conduction-band fermions (electrons or holes) and the immobile hard-core bosons (the localized electron pairs). The on-site Coulomb repulsion competes with this pairing interaction and hence is expected to have a detrimental influence on superconductivity. We analyze this effect in some detail, considering the two opposite limits of the weak and strong repulsion. A possible crossover between both these regimes is also discussed.