Complex vortex pinning landscape in YBCO: temperature- and orientation-dependent $J_{c}$ studies

Studying the temperature and field-orientation dependence of critical current density $J_{c}$ gives insight into vortex pinning. The orientation dependence of $J_{c}$ in YBa$_{2}$Cu$_{3}$O$_{\sim 7}$, containing both correlated disorder and localized `isotropic' defects, was determined at temperatures $T$ = 77 K to 5 K in magnetic fields up to $\mu _{0}H$ = 6 T. The inductive measurements ensured very low dissipation levels, with electric fields $E \quad \sim $ 10$^{-10}$ V/cm in the highly textured YBCO thin layers. The study revealed systematic changes in the orientation dependent $J_{c}$. At high temperatures, $J_{c}$ exhibited a \textit{peak} for \textbf{H} $\vert \vert $ $c$-axis; with decreasing $T$, the $J_{c}$ became almost independent of angle and at still lower temperatures, it exhibited a \textit{minimum} for \textbf{H} $\vert \vert $ $c$-axis. These findings are interpreted in terms of a competition between different types of pinning sites in a complicated pinning landscape. Research at ORNL sponsored by Div. Materials Sciences and Engineering, USDOE.