Evidence for a ‘‘brick wall’’ microstructure in epitaxial YBa2Cu3Ox films

We have measured the in-plane (ab plane) critical current density Jc in sputtered, c-axis oriented YBa2Cu3Ox films as a function of the oxygen content x. The important reduction of Jc with decreasing x can be explained in terms of a ‘‘brick wall’’ microstructure, where the critical current density is dominated by the coupling strength (along the c axis) between the superconducting CuO2 planes. Scanning tunneling microscopy images of the film surfaces are consistent with the presence of the brick wall structure, which apparently results from the overlap between terraces belonging to adjacent spiral-shaped islands.