Effect of adjacent insulating oxide layers on superconductivity of one unit cell thick YBa2Cu3O7−δ layers in PrBa2Cu3O7−δ/YBa2Cu3O7−δ/insulating oxide trilayers

Abstract One unit cell thick (1-UCT) YBa 2 Cu 3 O 7−δ (YBCO) adjacent to nonsuperconducting PrBa 2 Cu 3 O 7−δ (PrBCO) buffer layers have been grown epitaxially on SrTiO 3 (100) using molecular beam epitaxy (MBE). The layer thickness was accurately controlled by monitoring reflection high-energy electron diffraction (RHEED) specular intensity oscillations. Various oxide caps were grown epitaxially on the 1-UCT YBCO layer to form PrBCO/1-UCT YBCO/insulating oxide trilayers. It was found that superconducting 1-UCT YBCO layers were achieved when BaO with a good lattice match to YBCO was used as an insulating oxide in the trilayers. As the terminating atomic layer of 1-UCT YBCO layer is CuO 1−δ , the addition of an atomic BaO layer means the completion of the charge reservoir block of BaO-CuO 1−δ -BaO positioned above the CuO 2 bilayer (CuO 2 YCuO 2 ). The PrBCO buffer layers provide another charge reservoir block positioned below the CuO 2 bilayer interposing Y. Therefore, the minimum unit for superconductivity in YBCO is the CuO 2 bilayer sandwiched between the charge reservoir blocks. In the present study, the systematic substitution of various AO layers for the one-sided atomic BaO layer of the charge reservoir block has been made by epitaxial growth of cap oxides such as rock-salt-type and perovskite-type oxides on the CuO 1−δ -terminated surface of 1-UCT YBCO layers. It is found that the small lattice mismatch between YBCO and the cap oxides, in addition to divalent A ions such as Ba 2+ , Sr 2+ , Ca 2+ , Pb 2+ and Cd 2+ in atomic AO layers subsequent to the CuO 1−δ -terminated surface, is a requirement for superconductivity in 1-UCT YBCO layers.