Growth of self-assembled CeO2 nanostructures on r-cut sapphire for high-current-density YBa2Cu3O7-δ films

We report on a self-assembly process, where high-temperature O2 annealing induced a surface reorganization in CeO2 deposited on r-cut sapphire substrates. When the CeO2 film was thin (t < 2 nm), a highly ordered phase was formed as 3D isotropic islands of CeO2 with bare sapphire surface exposed. With increase in t, some of the isolated 3D islands started to be coalesced into large rectangular islands and finally connected with each other forming an ordered network. When the CeO2 film exceeded a critical thickness of 10 nm, an atomically-flat surface with a high density of nanodots was formed by the reorganization. YBa2Cu3O7-δ (YBCO) films grown on the atomically-flat CeO2 surface had a high critical current density (Jc > 3.0 x 106 A/cm2 at 77.3 K and 0 T). A correlation between the 'matching field' and the mean density of nanodots was also demonstrated. The results showed that the nanodots possibly induced effective vortex-pinning centers in YBCO films