films and coated conductors*

Large thermal fluctuations in high T_c superconductors give rise to fast vortex dynamics that promotes the time relaxation of the metastable supercurrents, which is detrimental for applications. We are pursuing a general understanding of the lowest achievable flux creep rate (S) for a superconductor at any temperature (T) and magnetic field (H). Initially, we found that there is a universal lower limit for S in the Anderson-Kim (A-K) regime at T<<T_c (Eley et al., Nat. Mat. 2017). Later, we expanded our quest to higher T and H outside the A-K limit, where the universality is lost and different regimes occur. We will present results on (Y,Gd)Ba₂Cu₃O_y coated conductors with randomly distributed BaHfO₃ nanoparticles. These samples exhibit extremely strong vortex pinning, with critical current densities among the highest in any known superconductor. We identify several glassy and plastic dynamics regimes, the boundaries among which are determined either by intrinsic vortex properties or by thickness effects. In particular, we find a thickness-controlled “second A-K regime” at high T, which sets the lowest S(T,H) limit in thin samples in technologically relevant T-H conditions.