EFFECTS OF FINITE PAIR BINDING-ENERGY IN A MODEL OF A SUPERCONDUCTOR WITH LOCAL ELECTRON PAIRING

A simple model of a superconductor with very short coherence length is studied for arbitrary electron density. The tight binding hamiltonian consists of the inter-site charge exchange term J, determining the ho pping of electron pairs, and the effective on-site interaction U. The evolution of phase diagrams, superconducting critical temperature T(c) and critical fields are determined as functions of interaction parame ters and electron density, and the temperature dependences of the elec tron pair concentration, magnetic susceptibility and specific heat are established. There are two well defined regimes of the model. In the local pair limit i.e. for large pair binding energies E(b) (E(b)/k(B) T(c) much greater than 1 ) the superconducting transition occurs throu gh Bose condensation of preexisting pairs, whereas in the opposite cas e, i.e. for E(b)/k(B)T(c) less than or similar to 1, the transition is determined by pair breaking excitations and there are essentially no preformed pain above T(c). It turns out that a crossover between the t wo regimes, occurring with increasing U, is smooth.