Superconductivity in a two-component model with local electron pairs

Superconductivity in the two component model of coexisting local electron pairs (hard-core charged bosons) and itinerant fermions coupled via charge exchange mechanism is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s-wave and d_{x^2-y^2} symmetries are analyzed for a 2D square lattice within the BCS-mean field approximation and the Kosterlitz-Thouless theory. The phase diagrams and superconducting characteristics of this induced pairing model as a function of the position of the local pair (LP) level and the total carrier concentration are determined. The model exhibits several types of interesting crossovers between the BCS like behavior and that of LP's. In addition, the Uemura plots are obtained for extended s and d_{x^2-y^2} pairing symmetries. Finally, we analyze the pairing fluctuation effects (in 3D) within a generalized T-matrix approach. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for high temperature superconductors.