Fluctuation magnetoconductivity in YBa2Cu3O7: Gaussian, three-dimensional XY, beyond three-dimensional XY, and lowest-Landau-level scaling - art. no.214513

Systematic measurements of the in-plane fluctuation magnetoconductivity in a YBa2Cu3O7 single crystal are presented. Fields between 0 and 14 T were ap plied 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 a bove T-c, the mean-held fluctuation spectrum is effectively two dimensional . Decreasing the temperature towards T-c, a crossover to a three-dimensiona l (3D) Gaussian regime is seen at low applied fields. The analysis of these results allows the estimation of the coherence length perpendicular and pa rallel 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 d istance between the double Cu-O layers plays an important role in the 3D fl uctuation spectrum. In fields above 5 T applied parallel to the c axis, the fluctuation magnetoconductivity scales as predicted by the 3D lowest-Landa u-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 oc currence of a genuine critical regime, where the exponent is consistent wit h the predictions of the full dynamic 3D XY universality class. Still close r 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 YBa2Cu3O7-delta . The whole set of data is condensed on H-T diagrams, which display the reg ions of stability for the observed fluctuation regimes, in fields oriented parallel or perpendicular to the Cu-O layers.