Tuning of the critical current in YBa₂Cu₃O _7-x thin films by controlling the size and density of Y ₂O₃ nanoislands on annealed SrTiO₃ substrates

We introduced high density columnar defects, as artificial pinning centres (APCs) for quantized vortices, into YBa₂Cu₃O_7-x (YBCO) films during the film deposition procedure. APCs were introduced perpendicular to the film surface using nanosized Y₂O₃ islands prepared on SrTiO₃(100) substrates by pulsed laser deposition. Varying the deposition parameters and the substrate annealing conditions allowed strong changes in the shape and density of the Y ₂O₃ islands to be induced. The best performance among the APC samples as compared to the pure YBCO film was obtained for that grown on the Y₂O₃-deposited substrate with five laser pulses, corresponding to 0.2 Y₂O₃ monolayers (ML). Even when the 0.2 ML APC films were prepared using the same deposition conditions, the columnar defects enhanced J_c at 77 K from 1.8 to 2.7 MA cm ^-2 (self-field) and from 0.06 to 0.10 MA cm^-2 (H ≤ 5 T).