Characteristics of YBCO|CeO$_2$|Al$_2$O$_3$ structures with decreasing thickness of the cerium oxide sublayer

The transport properties, surface morphology and structure of epitaxial Y$_1$Ba$_2$Cu$_3$O$_{7-x}$ (YBCO) films have been studied with decreasing thickness of the CeO$_2$ epitaxial buffer layer on sapphire. It has been shown that the critical temperature and critical current density of YBCO films retain high values ($T_c>$ 87 K, $J_c$ (77 K)$>$ 2 MA/cm$^2$) as the thickness of the CeO$_2$ sublayer decreases from 100 nm to 1.2 nm. YBCO films remain superconducting on a CeO$_2$ layer with an equivalent thickness of 0.1 nm.