SUPERCONDUCTING TRANSPORT-PROPERTIES OF GRAIN-BOUNDARIES IN YBA2CU3O7 BICRYSTALS

Previous work on the superconducting transport properties of individual grain boundaries in thin-film bicrystals of YBa2Cu3O7 has been extended to provide a more comprehensive picture of their weak-link characteristics. Grain boundaries with three different geometries have been studied; the transport properties of all three types of boundaries are essentially identical, which implies that the poor superconducting coupling between grains is a result of the intrinsic structural disorder at the boundary. The grain-boundary critical current densities in bicrystal films prepared by evaporation and postannealing and by laser ablation are also in good agreement; this result demonstrates that the transport properties are insensitive to preparation technique and, thus, are not dominated by the diffusion of substrate impurities into the boundary region. High grain-boundary resistivities and low IcRn products imply that the boundaries act as strong barriers to current flow with locally depressed order parameters. Strong magnetic hysteresis, associated with trapped intragranular flux, is observed; this hysteretic behavior is also responsible for an increase in the grain boundary Jc for Hp/rapp>300500 Oe. © 1990 The American Physical Society.