On the consequences of the uncertainty principle on the superconducting fluctuations well inside the normal state

Abstract

We first argue that the collective behaviour of the Cooper pairs created by\nthermal fluctuations well above the superconducting transition temperature, Tc,\nis dominated by the uncertainty principle which, in particular, leads to a\nwell-defined temperature, T^C, above which the superconducting coherence\nvanishes. On the grounds of the BCS approach, the corresponding\nreduced-temperature, ln(T^C/Tc), is estimated to be around 0.55, i.e., above\nT^C \\approx 1.7Tc coherent Cooper pairs cannot exist. The implications of these\nproposals on the superfluid density are then examined using the\nGaussian-Ginzburg-Landau approximation. Then we present new measurements of the\nthermal fluctuation effects on the electrical conductivity and on the\nmagnetization in different low- and high-Tc superconductors with different\ndopings which are in excellent agreement with these proposals and that\ndemonstrate the universality of ln(T^C/Tc).\n