A Comparative Study of High-Field Diamagnetic Fluctuations in Deoxygenated YBa2Cu3O7-x and Polycrystalline (Bi–Pb)2Sr2Ca2Cu3O10

We studied three single crystals of YBa₂Cu₃O_7-x, (Y123) with superconducting transition temperature, T_c=62.5, 52, and 41 K, and a highly textured polycrystalline specimen of (Bi–Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223), with T_c=108K. Isofield magnetization data were obtained as a function of temperature, with the magnetic field applied parallel to the c axis of each sample. The reversible magnetization data for all samples exhibited a rounded transition as magnetization tended toward zero. The reversible data were interpreted in terms of two-dimensional diamagnetic lowest-Landau-level (LLL) fluctuation theory. The LLL scaling analysis yielded consistent values of the superconducting transition temperatures T_c(H) for the various samples. The resulting scaling data were fit well by the two-dimensional LLL expression for magnetization obtained by Tesanovic and colaborators, producing reasonable values of κ but the fitting parameter ∂H_c2/∂T produced values that were larger than the experimentally determined ones. We performed simultaneous scaling of Y123 data and Bi2223, obtaining a single collapsed curve. The single curve was obtained after multiplying the x and y axis of each scaling curve by appropriate sample-dependent scaling factors. An expression for the two-dimensional x-axis LLL scaling was extracted from theory, allowing comparison of theoretical values of the x-axis scaling factors with the experimental values. The comparison between the values of the x-axis produced a deviation of 40% which suggests that the hypothesis of universality of the two-dimensional LLL fluctuations is not supported by the studied samples. We also observe that Y123 magnetization data for temperatures above T_c obbey a universal scaling obtained for the diamagnetic fluctuation magnetization from a theory considering non-local field effects. The same scaling was not obbeyed by the corresponding magnetization calculated from the two-dimensional LLL theory.