EVOLUTION FROM BCS SUPERCONDUCTIVITY TO BOSE-CONDENSATION - ROLE OF THE PARAMETER KF-XI

We argue that the natural variable to establish the crossover from Coo per-pair-based superconductivity to Bose-Einstein condensation of boun d-electron pairs is the product k(F)xi of Fermi electron wave vector t imes the coherence length for two-electron correlation, and that Coope r-pair-based superconductivity is stable against bosonization down to k(F)xi congruent-to 2pi. We also propose that the experimental plot by Uemura et al., relating the superconducting temperature T(c) to the F ermi temperature T(F) for a variety of ''exotic'' as well as conventio nal superconductors, can be rationalized by correlating the ratio T(c) /T(F) to k(F)xi. It appears in this way that the high-T(c) superconduc tors lie in the plot by Uemura et al. near the ''instability'' line k( F)xi = 2pi.