Study of diamagnetic fluctuations in YBa₂Cu₃O₇₋ₓ and Bi₂Sr₂CaCu₂O₈₊ₓ: Possible observation of phase correlations persisting above Tc

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We report on isofield curves of √-M vs. T, where M is the reversible magnetization, of YBa₂Cu₃O₆.₉₅, YBa₂Cu₃O₆.₆₅, and Bi₂Sr₂CaCu₂O₈₊ₓ with the magnetic field, H, applied parallel to the c-axis of the samples (and also parallel to the ab- planes for YBaCuO). For temperatures close to the critical temperature, Tc, the quantity √-M is proportional to the order parameter amplitude |ψ|. Curves of √-M vs. T allowed to study the asymptotic behavior of the form (Tₐ-T)ᵐ of |ψ| near Tc, as a function of field. Results for the studied samples produced values of Tₐ(H) lying above Tc, suggesting that the magnetic field gradually allows to probe a region of temperatures where phase correlations persist above Tc. The study performed here in YBaCuO samples allowed to study how phase correlations evolve with doping in the pseudo-gap region of YBaCuO. √-M vs. T curves for all samples show a rather large amplitude fluctuation with no phase correlation extending well above Tₐ(H) which is interpreted in terms of a Gaussian Ginzburg-Landau approach with a total-energy cutoff in the fluctuation spectrum. Resulting values for the exponent m found for all samples, 0.5 < m < 0.7, are interpreted as due to phase fluctuations of the d-wave pairing symmetry of the order parameter in the ab- planes.