COMPOSITE BOSONS AND QUANTUM COHERENT EFFECTS IN THE NEGATIVE-U HUBBARD-MODEL

A real-time functional-integral approach at finite temperature is used for self-consistent formulation of the dynamics and superconducting phase transition of preformed composite bosons in the extended negative-U Hubbard model in the large-U limit. By employing the representation in terms of the amplitude and phase of the bound-pair field we derive the effective phase Hamiltonian and exploit the effects associated with coherent bound-pair tunneling at the microscopic level-responsible for the phase-coherent state, as well as the dissipative effects due to quasiparticle charge transfer. Striking formal similarity of the physical picture in the large-U limit to those modeled by Josephson-coupled arrays with charging effects is pointed out.