Clean grain boundaries and weak links in high T c superconducting YBa2Cu3O7−x thin films

It is demonstrated that polycrystalline thin films of high Tc superconducting YBa2Cu3O7−x can be grown with clean grain boundaries, i.e., without a boundary layer of segregation or different phase. In clean stoichiometric samples, angular misorientations of grains may be the origin of weak link behavior. High-resolution scanning transmission electron microscope images of films grown on ZrO2 and MgO by reactive evaporation, reactive sputtering, and laser ablation show atomic lattice images of clean grain boundaries. X-ray microanalysis with a 10 Å spatial resolution also indicates no composition deviation at the grain boundaries. Grain sizes and epitaxial relations of samples prepared by different methods are characterized.