Effects of epitaxial strain on the growth mechanism in YBa₂Cu₃O_7-x thin films in YBa₂Cu₃O_7-x/PrBa₂ Cu₃O₇ superlattices

We report on the growth mechanism of YBa₂Cu₃O_7-x. Our study is based on an analysis of ultrathin YBa₂Cu₃O_7-x (YBCO) layers in c-axis-oriented YBa₂Cu₃O_7-x/PrBa ₂Cu₃O₇ superlattices. We have found that the release of epitaxial strain in very thin YBCO layers triggers a change in the dimensionality of the growth mode. Ultrathin epitaxially strained YBCO layers with thickness below 3 unit cells grow in a block-by-block two-dimensional mode that is coherent over large lateral distances. Meanwhile, when the thickness increases and the strain relaxes, layer growth turns three-dimensional, resulting in rougher layers and interfaces.