Fluctuation magnetoconductivity in YBa₂Cu₃O₇: Gaussian, three-dimensional XY, beyond three-dimensional XY, and lowest-Landau-level scaling

Systematic measurements of the in-plane fluctuation magnetoconductivity in a YBa₂Cu₃O₇ 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 T_c, the mean-field fluctuation spectrum is effectively two dimensional. Decreasing the temperature towards T_c, 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 YBa₂Cu₃O₇ 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 T_c, 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 XY universality class. Still closer to T_c, evidence is found for a fluctuation regime beyond 3D XY scaling. This new scaling raises the interesting possibility for the ultimate weakly first-order character of the superconducting transition in YBa₂Cu₃O_7-δ. The whole set of data is condensed on H-T diagrams, which display the regions of stability for the observed fluctuation regimes, in fields oriented parallel or perpendicular to the Cu-O layers.