Evidence forc-axis correlated vortex pinning in Y Ba2Cu3O7 films on sapphire buffered with an atomically flat CeO2layer having a high density of nanodots

Experimental evidence of c-axis correlated vortex pinning in superconducting Y Ba2Cu3O7−δ (YBCO) thin films deposited on annealed CeO2-buffered sapphire was obtained. YBCO films were grown by pulsed laser deposition on CeO2-buffered () sapphire substrates. Prior to the superconducting film deposition, a self-assembly process was initiated where high-temperature (1025 °C) O2 annealing induced surface reconstruction of CeO2 on sapphire substrates. The result was an atomically flat CeO2 layer, self-assembled formation of nanodots on top of the CeO2 wetting layer, and high critical current density of the YBCO films deposited on such buffered sapphire substrates. The magnetic field dependence of Jc at various temperatures and the dependence of Jc on the orientation of the magnetic field were investigated. YBCO films with an annealed CeO2 buffer layer showed a high Jc peak when the applied field was directed along the c-axis of the YBCO. The surface morphology and microstructure of YBCO films on sapphire with as-grown and annealed CeO2 buffer layers were investigated using transmission electron microscopy. The results suggest that the Jc peaks in YBCO with an annealed CeO2 buffer layer are caused by c-axis correlated pinning sites, such as threading dislocations possibly induced by CeO2 nanodots and a high density of egg-shaped nanometre-sized precipitates that are elongated along the c-axis of YBCO.