Structure of the growth interface of Y-Ba-Cu-O analogs onSrTiO3(001) substrates

The interface structures of c-axis-oriented ultrathin Gd-Ba-Cu-O and Eu-Ba-Cu-O films on ${\mathrm{SrTiO}}_3$ (001) substrates have been determined using x-ray standing waves. Rare-earth fluorescence profiles observed near the 001 Bragg condition of ${\mathrm{SrTiO}}_3$ show a dip without a marked peak, which can only be explained by either a BaO layer interfacing the ${\mathrm{TiO}}_2$ plane of ${\mathrm{SrTiO}}_3$ or the CuO or ${\mathrm{CuO}}_2$ layer interfacing the SrO plane. Calculations of x-ray field intensities using a simple rigid-sphere-ion model show that for both Gd-Ba-Cu-O and Eu-Ba-Cu-O neither the CuO layer nor the ${\mathrm{CuO}}_2$ layer can be the first grown layer on the ${\mathrm{TiO}}_2$ surface of ${\mathrm{SrTiO}}_3$. Similarly, neither the BaO layer nor the rare-earth layer can be the first layer on the SrO surface.