Meissner Effect in La_2−xBa_xCuO_y as Functions of x and y

抄録

The ac susceptibility was measured for the powdered samples of La_2−xBa_xCuO_y with various Ba and oxygen contents. Double maxima at x=0.08 and 0.16 and a large dip at x=0.12 were observed in both the Meissner volume fraction f and the superconducting transition temperature T_c′ where the Meissner signals start to show a large drop. We have newly found that for x=0.12 the decrease of the oxygen concentration y leads to a rise of T_c′ and an increase of f, suggesting that the decrease of y suppresses the low-temperature phase transition between orthorhombic and tetragonal structures which makes the superconductivity unstable around x=0.12. Furthermore, we observed that the onset temperature of the Meissner effect T_c^os of about 30 K is almost independent of x and y, while the superconducting transition becomes broad and f decreases with lowering T_c. These results indicate that some optimum composition which shows the intrinsic superconductivity exists in the range of 0.08≤x≤0.16.