Abstract

We provide strong evidence that cuprate superconductors and uncharged superfluids like 4He share the universal 3D x–y properties in the fluctuation dominated regimes. The universal relations between critical amplitudes and Tc, supplemented by the empirical phase diagram (doping-dependence of Tc) also imply – in agreement with recent kinetic-induction measurements on La2−xSrxCuO4 films and muon spin resonance (μSR) data – unconventional behavior of the penetration depth in the overdoped regime. The evidence of uncharged superfluid of 3D x–y behavior is completed in terms of the asymptotic low-temperature behavior of the penetration depth, because the sound-wave contribution of the uncharged superfluid accounts remarkably well for the experimental data. The dominant role of 3D x–y fluctuations, implying tightly bound and interacting pairs even above Tc, together with the doping-dependent specific heat singularity point uniquely to Bose condensation of hard pairs on a lattice as the mechanism that drives the transition from the normal to the superconducting state.

Considerable debate has arisen over the nature of superconductivity and the symmetry of the order parameter in high-Tc superconductors [1–9]. In view of the fact that thermal fluctuations reflect the structure of the order parameter and that these extreme type II materials exhibit pronounced fluctuation effects [10–17], we discuss in this paper the use of thermal fluctuations to elucidate the nature of the superconducting state.

The organization of this paper is as follows. First we provide strong evidence that cuprate superconductors and uncharged superfluids like 4He share the universal three-dimensional (3D) x–y properties in the fluctuation-dominated regimes.