Ground State and Thermodynamic Properties of an Induced-Pairing Model
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We study the phase diagrams and thermodynamic properties of a system of coexisting wide-band current carriers (c-particles) and narrow-band electrons (d-electrons) which can form local pairs. There are two distinct mechanisms of superconductivity in the model considered: (i) the intersubsystem charge exchange, which leads to the superconducting state involving both types of electrons and (ii) the pair hopping interaction of d-electrons, leading to the superconducting state of d-particles only. In contrast to previous works on the subject, we assume an arbitrary value of on-site density interaction of d-electrons U, which allows 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 local pair superconductor and those of a classical BCS model. The increasing on-site Coulomb repulsion U competing with the intersubsystem charge exchange and the pair hopping interaction reduces the superconducting critical temperature. Moreover, it can induce a change of the superconducting transition into the first order one. Above a critical value of U, dependent on concentration of electrons and other interactions, the superconducting state cannot be stable at any temperature.
- 74.25.Ha: Magnetic properties including vortex structures and related phenomena(for vortices, magnetic bubbles, and magnetic domain structure, see 75.70.Kw)
- 74.20.-z: Theories and models of superconducting state
- 71.28.+d: Narrow-band systems; intermediate-valence solids(for magnetic aspects, see 75.20.Hr and 75.30.Mb in magnetic properties and materials)
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