In-plane and out-of-plane orientations of YBCO and their correlation with a/b-axis twin structures

This study employed the low-fluorine metal-organic deposition method to fabricate YBCO films with both c-axis and a/b-axis orientations, systematically investigating their lattice structure, in-plane and out-of-plane orientation relationships, phase formation, and morphology. First, the study focused on the coexistence of in-plane and out-of-plane orientations of the a- and b-crystal axes in YBCO films, clarifying their distribution within the texture and the factors influencing them, which in turn lead to the formation of a/b-axis twins. Secondly, the experiment revealed the impact of residual stress within the film on the a/b-axis twins and confirmed that the number of twins can be directionally regulated by applying external stress, thereby improving the critical current density of the film at low temperatures. Further investigation showed that the misfit stress between the substrate and the YBCO lattice significantly affects the in-plane and out-of-plane orientations, as well as the growth parameter window. By using substrates with different lattice structures, the issue of inconsistent in-plane orientation in a/b-axis oriented YBCO films was successfully resolved, reducing high-angle grain boundaries and enhancing current flow along the Cu-O planes, thus challenging the conventional understanding of their low performance. This work fills a research gap in the field of YBCO high-temperature superconducting coated conductors regarding the in-plane and out-of-plane orientations of the a/b-axis, providing theoretical foundations and experimental guidance, and laying a material foundation for applications in superconducting weak current technologies.