On the Nature of the Superconducting State in High-Tc Cuprates

We review the current evidence that cuprate superconductors and uncharged superfluids like 4He share the universal 3D xy properties in the fluctuation-dominated regimes T -> 0 and T -> Tc. The doping dependence of
superconducting properties, including the unconventional behavior of the magnetic penetration depth, is then traced back to the crossover from 3D xy to 4D xy critical point behavior. The remarkable consistency with experiment strongly suggests that the nature of superconductivity in the cuprates is essentially that of superfluidity in 4He. In both cases Tc signals the appearance or disappearance of phase ordering. Our analysis points uniquely to Bose condensation of tightly bound and interacting pairs as the mechanism that drives the transition.