Superconducting fluctuations and anomalous diamagnetism in underdoped YBa2Cu3O6+x from magnetization and Cu NMR-NQR relaxation measurements

Magnetization and 63Cu NMR-NQR relaxation measurements are used to study the superconducting fluctuations in YBa2Cu3O6+x (YBCO) oriented powders. In optimally doped YBCO the fluctuating negative magnetization Mfl(H,T ) is rather well described by an anisotropic Ginzburg-Landau (GL) functional and the curves Mfl/ √ H cross at Tc. 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 Mfl 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 63Cu(2) relaxation rate shows little, if any, field dependence in the vicinity of the transition temperature Tc(H = 0). It is argued how the results in the underdoped compounds can be accounted for ∗On leave of absence from Institute of Steel and Alloys, University of Moscow