High-temperature superconductivity in ultrathin films of Y1Ba2Cu3O7−<i>x</i>

We have grown ultrathin films of Y1Ba2Cu3O7−x in situ on (001) SrTiO3 by pulsed laser deposition. The zero resistance transition temperature (Tc0) is &amp;gt;90 K for films &amp;gt;300 Å thick. The critical current density (Jc at 77 K) is 0.8×106 A/cm2 for a 300 Å film and 4–5×106 A/cm2 for a 1000 Å film. The Tc0 and Jc deteriorate rapidly below 300 Å, reaching values of 82 K and 300 A/cm2 at 77 K, respectively, for a 100 Å film. Films only 50 Å thick exhibit metallic behavior and possible evidence of superconductivity without showing zero resistance to 10 K. These results are understood on the basis of the defects formed at the film-substrate interface, the density of which rapidly decreases over a thickness of 100 Å. We have studied these defects by ion channeling measurements and cross-section transmission electron microscopy. Our results suggest that the superconducting transport in these films is likely to be two dimensional in nature, consistent with the short coherence length along the c axis of the crystals.