High density of nanodots on atomically flat CeO2 buffer layers for inducing effective vortex-pinning centers in YBa2Cu3O7-δ films on sapphire

Epitaxial CeO2 buffer layers were fabricated by pulsed laser deposition (PLD) on r-cut sapphire substrates. An atomically flat CeO2 surface with a high density of nanodots was formed by a self-assembly process. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy investigation showed that the nanodots were CeO2 other than impurities. YBa2Cu3O7-δ (YBCO) thin films were then grown on the annealed and the as-grown CeO2-buffered sapphires by PLD. The transport measurement results showed that the nanodots enhanced the effective pinning potential and significantly increased critical current density (J c). Especially, YBCO films with an annealed CeO2 buffer layer showed a high J c peak when the applied field was directed along the c-axis of YBCO. Cross-section transmission electron microscopy investigation revealed that the J c peaks in YBCO with annealed CeO2 buffer layer is caused by c-axis correlated pinning sites, such as threading dislocations possibly induced by CeO2 nanodots and a high density of elongated precipitates