Magnetization and 63 Cu NMR-NQR relaxation measurements are used to study the superconducting fluctuations in YBa 2 Cu 3 O 6+x (YBCO) oriented powders. In optimally doped YBCO the fluctuating negative magnetization M f l (H, T ) is rather well described by an anisotropic Ginzburg-Landau (GL) functional and the curves M f l / √ H cross at T c . In underdoped YBCO, instead, over a wide temperature range an anomalous diamagnetism is observed, stronger than in the optimally doped compound by about an order of magnitude. The field and temperature dependences of M f l cannot be described either by an anisotropic GL functional or on the basis of scaling arguments. The anomalous diamagnetism is more pronounced in samples with a defined order in the Cu(1)O chains. The 63 Cu(2) relaxation rate shows little, if any, field dependence in the vicinity of the transition temperature T c (H = 0). It is argued how the results in the underdoped compounds can be accounted for

No abstract

No abstract

No abstract