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

We argue that the natural variable to establish the crossover from Cooper-pair-based superconductivity to Bose-Einstein condensation of bound-electron pairs is the product k(F)xi of Fermi electron wave vector times the coherence length for two-electron correlation, and that Cooper-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 Fermi temperature T(F) for a variety of ''exotic'' as well as conventional 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) superconductors lie in the plot by Uemura et al. near the ''instability'' line k(F)xi = 2pi.