89Y NMR RELAXATION AND FLUCTUATING DIAMAGNETISM IN THE OVERDOPED REGIME OF CALCIUM-DOPED YBCO

Abstract

The overdoped regime in Y 1-x Ca x Ba 2 Cu 3 O 7, for x=0.05 and x=0.10 (Tc(0)=82 K and Tc(0) = 73.8 K respectively) is studied by means of 89 Y NMR relaxation and SQUID magnetization measurements. It is shown that for hole concentration close to 0.19, where a critical point is suspected and where the antiferromagnetic correlations should disappear, 1/(T1T) is strictly constant from room temperature down to Tc. This behavior is also observed in the sample at Ca-content 0.10, where the hole concentration is 0.203. The magnetization as a function of temperature and the isothermal magnetization curves, reveal the occurrence of a marked enhancement of the fluctuating diamagnetism in the overdoped regime, with respect to optimally doped YBCO. The magnetization data cannot be justified by anisotropic Ginzburg-Landau functional for the free energy or by scaling arguments. These findings are tentatively related to superconducting "drops" created above Tc or to phase fluctuations in restricted dimensionality.