Evidence for enhancement of vortex matching field above 5 T and oxygen-deficient annuli around barium-niobate nanorods

We report the dependence of critical temperature (Tc), the irreversibility line, the microstructure of nanorods with perovskite-based barium niobates (BNOs), and the c-axis length of the YBa2Cu3Oy (Y123) superconducting matrix on the BNO-doping level and growth temperature for Y123 thin films with BNO nanorods. The characteristic field (Bcr) determined from the vortex-Bose-glass-like irreversibility lines in Y123+BNO films is strongly correlated to the BNO nanorod density and the growth temperature. Despite a monotonic decrease in Tc with increasing Bcr and nanorod density, the irreversibility fields (Birr) were enhanced up to Bcr ∼ 5 T. From the Bcr value and the mean diameter of the BNO nanorod (∼10 nm), we estimate that a lower Tc matrix annulus with a diameter of 12–14 nm exists around each BNO nanorod due to the strong interface strains. Our present study suggests that generation of this lower Tc region around each BNO nanorod increases the vortex-pinning strength significantly and, moreover, may offer a new way of enhancing Birr for REBa2Cu3Oy film with nanorods.