Evidence for c-axiscorrelated vortex pinning in Y Ba2Cu3O7−δ films on sapphire buffered with an atomically flatCeO2 layer having a high density of nanodots

Abstract

Experimental evidence of c-axis correlated vortex pinning in superconductingY Ba2Cu3O7−δ (YBCO) thin filmsdeposited on annealed CeO2-buffered sapphire was obtained. YBCO films were grown by pulsed laser deposition onCeO2-buffered () sapphire substrates. Prior to the superconducting film deposition,a self-assembly process was initiated where high-temperature(1025 °C)O2 annealing inducedsurface reconstruction of CeO2 on sapphire substrates. The result was an atomically flatCeO2 layer, self-assembled formation of nanodots on top of theCeO2 wetting layer, and high critical current density of the YBCO films depositedon such buffered sapphire substrates. The magnetic field dependence ofJc at various temperatures and the dependence ofJc on the orientation of the magnetic field were investigated. YBCO films with an annealedCeO2 buffer layershowed a high Jc peak when the applied field was directed along thec-axisof the YBCO. The surface morphology and microstructure of YBCO films on sapphire with as-grown andannealed CeO2 buffer layers were investigated using transmission electron microscopy. The results suggest that theJc peaks in YBCO withan annealed CeO2 bufferlayer are caused by c-axis correlated pinning sites, such as threading dislocations possibly induced byCeO2 nanodots and a high density of egg-shaped nanometre-sized precipitates that are elongated along thec-axis of YBCO.