Faceting and critical current densities of [001] high-angle tilt boundaries in YBCO films

The [001] high-angle tilt boundaries in YBa₂Cu₃O_7-x (YBCO) thin films deposited on SrTiO₃ (STO) bicrystal substrates have been studied by Scanning Squid Microscopy (SSM) and transmission electron microscopy (TEM). Generally critical current density across a boundary decreases with increasing misorientation. However, misorientation angle is found to be insufficient to predict the J_c behavior of a YBCO grain boundary. The two inclination angles of the adjacent grains are found to be important parameters. In two separate cases of same misorientation but different inclinations, the boundaries exhibit drastically different superconducting behaviors and this has been observed by SSM imaging of Josephson vortices. The YBCO film boundary is not an exact copy of the design boundary of the substrate bicrystal template. YBCO boundaries meander due to island formation during film growth. These microscopically wavy boundaries contain straight segments of atomistic facets. Three energetic criteria that affect faceting have been identified. The particular criterion that a facet plane cannot be too deviated from the design boundary plane (φ<28°) explains the strong effect of the design inclination angles on J_c despite extensive faceting. The original scattered values of J_c each calculated from individual fluxes of the same design boundary are likely to be a bona fide effect corresponding to individual facet having its own J_c and λ_j values.