Three-dimensional and two-dimensional high field vortex fluctuations in deoxygenated YBa2Cu3O7−x depending on the vicinity of Tc(H)

We analyzed reversible magnetization data, M versus T curves, of three single crystals of YBa2Cu3O7−x (Y123), with superconducting transition temperatures Tc = 62.5 (x = 0.35), 52 (x = 0.5), and 41 K (x = 0.6). M versus T curves of each sample exhibited a field independent crossing point, M(T∗), occurring close to the superconductor critical temperature. These crossing points were shown to be due to fluctuations of vortices. Besides the reversible data of each sample were shown to obey a two-dimensional diamagnetic lowest-Landau-level (LLL) fluctuation theory, it is shown here that the data, within a temperature region where the crossing points occur for two samples (62.5 K and 52 K), are also explained by a three-dimensional version of this fluctuation theory. Since the crossing points for these two samples occur close to Tc, these are interpreted as been due to three-dimensional vortex fluctuations instead two-dimensional ones. An expression for the field independent magnetization, M(T∗), which is expected to occur at the crossing point of the various M versus T curves, is obtained for the case of three-dimensional vortex fluctuations, and compared to the experimental values of M(T∗). This comparison produced consistent values for the coherence length along the c-axis of the samples with Tc = 62.5 and 52 K, solving an inconsistent result previously published, when experimental values of M(T∗) were compared with an expression obtained from two-dimensional vortex-fluctuations. The results of the present work show that, despite the fact that two-dimensional LLL fluctuations scaling is obeyed in a much wider temperature range for two studied samples (Tc = 52 (x = 0.5), and 62.5 K (x = 0.35)) when compared to the 3D-LLL scaling form, these systems behave as three-dimensional for temperatures close to Tc(H).