Possible bulk inhomogeneity of superconductivity in Y1-xCaxBa2Cu3O7-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 Y1-xCaxBa2Cu3O7-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 La2-xSrxCuO4. These results strongly suggest that a phase separation into SC and normal-state regions occurs at least in the underdoped regime of Y1-xCaxBa2Cu3O7-δ irrespective of the Ca content, and is a generic feature in the high-Tc cuprates.