GROUND-STATE AND THERMODYNAMIC PROPERTIES OF AN INDUCED-PAIRING MODEL

We study the phase diagrams and thermodynamic properties of a system o f coexisting wide-band current carriers (c-particles) and narrow-band electrons (d-electrons) which can form local pairs. There are two dist inct mechanisms of superconductivity in the model considered: (i) the intersubsystem charge exchange, which leads to the superconducting sta te involving both types of electrons and (ii) the pair hopping interac tion of d-electrons, leading to the superconducting state of d-particl es only. In contrast to previous works on the subject, we assume an ar bitrary value of on-site density interaction of d-electrons U, which a llows us to study the effects of reduced d-pair binding energy. Within the approach in which the U term is exactly treated we determine the evolution of superconducting properties as a function of interactions and relative position of the bands. In a definite range of parameters the system shows features which are intermediate between those of a lo cal pair superconductor and those of a classical BCS model. The increa sing on-site Coulomb repulsion U competing with the intersubsystem cha rge exchange and the pair hopping interaction reduces the superconduct ing critical temperature. Moreover, it can induce a change of the supe rconducting transition into the first order one. Above a critical valu e of U, dependent on concentration of electrons and other interactions , the superconducting state cannot be stable at any temperature.