Superconducting transport properties of grain boundaries inYBa2Cu3O7bicrystals
Previous work on the superconducting transport properties of individual grain boundaries in thin-film bicrystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ 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 ${\mathit{I}}_{\mathit{c}}$${\mathit{R}}_{\mathit{n}}$ 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 ${\mathit{J}}_{\mathit{c}}$ for Hp/${\mathrm{r}}_{\mathit{a}\mathit{p}\mathit{p}}$>300--500 Oe.