Fluctuation magnetoconductivity in (formula presented) Gaussian, three-dimensional (formula presented) beyond three-dimensional (formula presented) and lowest-Landau-level scaling

Systematic measurements of the in-plane fluctuation magnetoconductivity in a (formula presented) single crystal are presented. Fields between 0 and 14 T were applied either parallel or perpendicular to the Cu-O atomic planes. The data reveal the occurrence of a large Gaussian regime in the normal phase. Far above (formula presented) the mean-field fluctuation spectrum is effectively two dimensional. Decreasing the temperature towards (formula presented) a crossover to a three-dimensional (3D) Gaussian regime is seen at low applied fields. The analysis of these results allows the estimation of the coherence length perpendicular and parallel to the Cu-O planes, and reveals that superconductivity in (formula presented) is characterized by a double planar periodicity. The c lattice parameter is the relevant periodicity length of the 2D behavior, whereas the smallest distance between the double Cu-O layers plays an important role in the 3D fluctuation spectrum. In fields above 5 T applied parallel to the c axis, the fluctuation magnetoconductivity scales as predicted by the 3D lowest-Landau-level approximation of the Ginzburg-Landau theory. Very close to (formula presented) and for quite low values of the applied field, the results clearly show the occurrence of a genuine critical regime, where the exponent is consistent with the predictions of the full dynamic 3D (formula presented) universality class. Still closer to (formula presented) evidence is found for a fluctuation regime beyond 3D (formula presented) scaling. This new scaling raises the interesting possibility for the ultimate weakly first-order character of the superconducting transition in (formula presented) The whole set of data is condensed on (formula presented) diagrams, which display the regions of stability for the observed fluctuation regimes, in fields oriented parallel or perpendicular to the Cu-O layers.