Nucleation Mechanism of YBa₂Cu₃O_7-δ on SrTiO₃(001)

Using UHV-scanning-tunneling microscopy and spectroscopy we show that submonolayer coverages of YBa₂Cu₃O_7-δ deposited on SrTiO₃(001) nucleate as cubic, semiconducting (Y,Ba)CuO_3-x phase as proven by the height of the nuclei and their density of states. Nuclei are observed at upper step edges and on the substrate terraces exclusively. Minimization of surface and interface energies forces this nucleation behavior. At about one monolayer, the film transforms into the orthorhombic, metallic YBa₂Cu₃O_7-δ phase indicating that the formation enthalpy of the bulk phases now dominates the growth process.