Superconducting weak link generated on a linear array of substrate steps in ultrathin films of YBa₂Cu₃O_7−δ

We have studied a three-unit-cell-thick YBa₂Cu₃O_7−δ (YBCO) superconducting film grown on a slightly vicinal SrTiO₃ substrate that was polished from the (001) plane by a small miscut angle (less than 0.5°). This produced, on its surface, a linear array of single-unit-cell-high steps with a period larger than 40 nm. A characteristic fault, with a displacement corresponding to one third of the c-axis length of YBCO, is generated in the film on each substrate step, as observed by atomic force microscopy. Transport measurements with current flows parallel and perpendicular to the fault show that the fault serves as a superconducting weak link, as well as a unique one-dimensional flux-pinning center in an applied magnetic field. In addition, a characteristic fine structure is found in the flux-flow resistivity as a function of magnetic field, the origin of which may be ascribed to a matching effect between the periodically aligned faults and the flux lattice.