Direct Evidence for Block-by-Block Growth in High-Temperature Superconductor Ultrathin Films

Charge neutrality and stoichiometry impose severe restrictions on the mechanisms of epitaxial growth of complex oxides. The fundamental question arises of what is the minimum growth unit when sample thickness is reduced beyond the size of the unit cell. We have investigated the growth mechanism of YBa 2 Cu 3 O 7 cuprate superconductor, using a consistent approach based on the growth of noninteger numbers of YBa 2 Cu 3 Cu 3 O 7 layers in YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices. Ex situ chemical and structural analysis evidence a 2D block-by-block mechanism in which the minimum growth units are complete unit cell blocks, growing coherently over large lateral distances.