Thermodynamic Properties of Superconducting YBa2(Cu1-xMx)3 06+y (M = Zn and Ni) Studied by Reversible Magnetization Measurements

Specific heat jumps ΔC at the superconducting transition temperature Tc have been estimated from reversible magnetizations of YBa2(Cul-xMx)3O6+y (M = Zn and Ni). The y-dependence of ΔC/Tc(≡ yM) is in good agreement with that of Loram et al. [Phys. Rev. Lett. 71 1740 (1993)] taken by direct specific heat measurements. The significant y-dependence of γM(x)/γu(0)-Tc(x)/TC(0) curves cannot be explained by conventional theories of pair breaking due to magnetic or nonmagnetic scatterings for d-wave order parameters. The present results and other experimental results reported previously indicate that electron localization effects play an important role in determining the behaviors of impurity-doped high-Tc oxides.