A point-contact Josephson junction between a bulk Ba-Y-Cu-O compound ($T_c\sim 90$ K) and a Nb needle has been made. The $I-V$ characteristics under microwave radiation show Shapiro steps, confirming the Josephson nature of the formed contact. Spin-singlet superconductivity in the Ba-Y-Cu-O system is implied from the voltage of the steps. The temperature dependence of the critical currents suggests that the Josephson junction if formed between Nb and another material with much higher $T_c$. Internal Josephson structure within the bulk ceramics is not found in the Ba-Y-Cu-O system.

• Received 31 March 1987

DOI:https://doi.org/10.1103/PhysRevLett.58.1979