Scanning tunneling microscopy suggests that epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films grow unit cell by unit cell, by a terraced-island-growth mode. Although films grown at low temperatures exhibit a spiral-growth surface microstructure, films with high critical current densities (grown at high temperatures on nearly-lattice-matched substrates) do not. The terraced microstructure explains the steps found in ultrathin ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ layers in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superlattices. These steps may act as superconducting weak links, providing support for Josephson-coupled-array models of superconducting superlattices.

• Received 10 June 1991

DOI:https://doi.org/10.1103/PhysRevB.44.9760