Orientation dependence of grain-boundary critical current densities in high-Tc bicrystals

The critical current density across grain boundaries of twinned thin-film Bi2Sr2CaCu2O8+x bicrystals has been measured as a function of the tilt angle Θ. For Θ=0°to 45°, the ratio of intergrain to intragrain critical current density decreases exponentially with increasing tilt angle. Surprisingly, this orientation dependence is very similar to that observed for YBa2Cu3O7 bicrystals. Microstructural investigations of plan view samples show a wavy grain boundary of the superconductor with a roughness of 100 nm to 1 μm which does not originate from the roughness of the substrate grain boundary (1-3 nm) but is caused by an island-plus-layer growth of twin domains. In general, one of the adjacent grains has a low-indexed habit plane at the boundary.