Possible bulk inhomogeneity of superconductivity in Y _1-xCaxBa₂Cu₃O_7-δ (x = 0-0.2)

In order to investigate the possible inhomogeneity of superconductivity, measurements reflecting bulk properties of a sample, namely, magnetic susceptibility, χ, and specific heat have been carried out for Y _1-xCaxBa₂Cu₃O_7-δ with x=0.1 and 0.2. From the χ measurements, both the absolute value of χ at 2 K on field cooling, |χ2K|, reflecting the superconducting (SC) volume fraction and Tc have been found to increase systematically with an increase of the hole concentration in the CuO₂ plane, p, for each x. From the specific heat measurements, on the other hand, it has been found that the residual Sommerfeld constant, γ0, is not zero but has a finite value even in the SC state. Moreover, γ0 in each x shows a p dependence similar to that observed in La_2-xSr_xCuO₄. These results strongly suggest that a phase separation into SC and normal-state regions occurs at least in the underdoped regime of Y_1-xCaxBa₂Cu ₃O_7-δ irrespective of the Ca content, and is a generic feature in the high-Tc cuprates.