Orientation dependence of grain-boundary critical current densities in high-Tcbicrystals

The critical current density across grain boundaries of twinned thin-film ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{x}}$ 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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ 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.