Electrical transport across grain boundaries in bicrystals of YBa₂Cu₃O_7$δ$

The superconducting properties of about 20 bulk-scale flux-grown bicrystals of YBa₂Cu₃O_7-δ have been measured in order to deduce the nature of the coupling across their grain boundaries. Bicrystals with two types of misorientation relationship were studied: (i) θ [001] (C-type) and (ii) 90° [010] plus an additional rotation about the a_I | c_II axis (P-type). A general trend for a transition from flux pinning (FP) to Josephson Junction (JJ) to resistive (R) behavior was observed as the misorientation angle increased. However, FP character was observed well into the high-angle regime, in particular, for P-type bicrystals, and JJ behavior was observed at as low as 10° [001]. The J_α values of two bicrystals were increased by a second oxygen anneal. In one case (28° [001] boundary), extended oxygenation produced a change form JJ to mixed JJ/FP behavior, markedly raising the transport critical current density (J_α) at all fields. HRTEM has been performed on two of the same bicrystals which were electromagnetically characterized. A thin second phase layer unobservable by conventional TEM was observed in a resistive boundary, whereas no such phase was detected at a JJ boundary. We conclude that the character of the boundary cannot be predicted either from its misorientation angle or the magnitude of its J_c.