Macroscopic Quantum Interference Effect for High-T_c YBaCuO and GdBaCuO Josephson Junctions

DC SQUID operation utilizing all ceramic YBa 2 Cu 3 O 7-δ and GdBa 2 Cu 3 O 7-δ Josephson junctions is reported. It is confirmed that the macroscopic quantum interference effect is observable up to 90 K, almost the transition temperature of bulk cermaics. The periodic patterns of GdBa 2 Cu 3 O 7-δ SQUID are rather deformed; probably due to the presence of magnetic element Gd. The temperature dependence of Josephson critical current shows a nearly linear behavior down to low temperatures, in disagreement with Ambegaokar-Baratoff's calculation.

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