Diamagnetic fluctuations inYBa2Cu3O7−xandBi2Sr2CaCu2O8+x: Possible observation of phase correlations persisting aboveTc

Abstract

We report on isofield curves of -M vs T, where M is the reversible magnetization, of YBa2 Cu3 O6.95, YBa2 Cu3 O6.65, and Bi2 Sr2 CaCu2 O8+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 pseudogap 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. © 2009 The American Physical Society.