Abstract
The full superconducting transition of a single-phase low-T c specimen was investigated as a function of a quenching temperature T q. The midpoint T c varied from 68.8 K to 95.9 K. A surprising feature was that the T c had a steep minimum at T c=300°C. The oxygen stoichiometry did not seem to be a dominant factor for the determination of T c. We speculate that the (BiO)2 layer was responsible for controlling the hole concentration of the CuO2 plane.