CROSSOVER FROM BCS TO COMPOSITE-BOSON (LOCAL-PAIR) SUPERCONDUCTIVITY IN QUASI-2D SYSTEMS

The crossover from cooperative Cooper pairing to independent bound-sta te (composite-bosons) formation and condensation in quasi-2D systems i s studied. It is shown that at low carrier density the critical superc onducting temperature is equal to the temperature of Bose condensation of ideal quasi-2D Bose-gas with heavy dynamical mass, meanwhile at hi gh densities the BCS result remains valid. The evident nonmonotonical behaviour of the critical temperature as a function of the coupling co nstant (the energy of the two-particle bound state) is a qualitative d ifference of quasi-2D crossover from the 3D one.