Fluctuation effects and anomalous diamagnetism in YBCO124 and in underdoped YBCO123 from susceptibility and Cu-63 nuclear relaxation

The effects of superconducting fluctuations on the diamagnetic susceptibili ty and on the dynamical spin susceptibility involved in Cu-63 NMR-NQR relax ation rate are investigated in oriented powders of underdoped YBCO123 and o f YBCO124 and compared with the ones in optimally doped YBCO123. While in t his latter compound the fluctuation diamagnetism is well described by an an isotropic Ginzburg-Landau (GL) functional, in underdoped YBCO123 an anomalo us diamagnetism is observed, with a strong enhancement of the susceptibilit y, in a wide temperature range. The magnetization curves cannot be describe d by any GL anisotropic functional. Also in YBCO124 the fluctuation diamagn etism is hard to describe by GL-type approach, although the enhancement is not as marked as in underdoped YBCO123. In YBCO124, and in underdoped YBCO1 23, the temperature and field dependences of the Cu-63 relaxation rates W a ppear different from the ones in optimally doped YBCO123. No field-induced decrease of W is observed, as it is expected in the case of a changeover fr om s to d of the orbital simmetry of the fluctuating pairs or when the char acter of the fluctuations is different from the GL one.