Y-89 NMR relaxation and fluctuating diamagnetism in the overdoped regime of calcium-doped YBCO

The overdoped regime in Y1-xCaxBa2Cu3O7, for x=0.05 and x=0.10 (T-c(0)=82 K and T-c'(0)=73.8 K respectively) is studied by means of Y-89 NMR relaxatio n and SQUID magnetization measurements. It is shown that for hole concentra tion close to 0.19, where a critical point is suspected and where the antif erromagnetic correlations should disappear, 1/(T1T) is strictly constant fr om room temperature down to T-c. This behavior is also observed in the samp le at Ca-content 0.10, where the hole concentration is 0.203. The magnetiza tion as a function of temperature and the isothermal magnetization curves, reveal the occurrence of a marked enhancement of the fluctuating diamagneti sm in the overdoped regime, with respect to optimally doped YBCO. The magne tization data cannot be justified by anisotropic Ginzsburg-Landau functiona l for the free energy or by scaling arguments. These findings are tentative ly related to superconducting "drops" created above T-c or to phase fluctua tions in restricted dimensionality.