Direct evidence for block-by-block growth in high-temperature superconductor ultrathin films

Charge neutrality and stoichiometry impose severe restrictions on the mecha nisms of epitaxial growth of complex oxides. The fundamental question arise s of what is the minimum growth unit when sample thickness is reduced beyon d the size of the unit cell. We have investigated the growth mechanism of Y Ba2Cu3O7 cuprate superconductor, using a consistent approach based on the g rowth of noninteger numbers of YBa2Cu3O7 layers in YBa2Cu3O7/PrBa2Cu3O7 sup erlattices. Ex situ chemical and structural analysis evidence a 2D block-by -block mechanism in which the minimum growth units are complete unit cell b locks. growing coherently over large lateral distances.