Superconducting fluctuations and anomalous diamagnetism in underdopedYBa2Cu3O6+xfrom magnetization and63CuNMR-NQR relaxation measurements

Magnetization and ${}^{63}\mathrm{Cu}$ nuclear magnetic resonance-nuclear quadrupole resonance relaxation measurements are used to study the superconducting fluctuations in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ (YBCO) oriented powders. In optimally doped YBCO the fluctuating negative magnetization $M_{\mathrm{fl}}\mathrm{}(H,T)\mathrm{}$ is rather well described by an anisotropic Ginzburg-Landau (GL) functional and the curves $M_{\mathrm{fl}}/\sqrt{H}$ 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 dependence of $M_{\mathrm{fl}}$ 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 ${}^{63}\mathrm{Cu}\left(2\right)\mathrm{}$ relaxation rate shows little, if any, field dependence in the vicinity of the transition temperature $T_c\mathrm{}(H=0\mathrm{}).$ It is argued how the results in the underdoped compounds can be accounted for by the presence of charge inhomogeneities, favored by chains ordering.