Dramatic Increase in Vortex Creep Rate with Decreasing Film Thickness in Disordered Superconductors

In type-II superconductors, the interplay between vortex elasticity, vortex motion, and material disorder is a primary determinant of electronic and magnetic properties, such as creep (thermally activated vortex motion) and the critical current. Here, we present a systematic study of the dependence of the creep rate S on film thickness d in Nb and (Y,Gd)Ba₂Cu₃O_7-x (YGdBCO). We observe a progression from a thickness-independent to a thickness-dependent S with decreasing d, suggestive of a magnetic 3D-to-2D transition. For the Nb film, the critical thickness d_c at which this crossover occurs coincides with the collective pinning length L_c, i.e., the size of vortex segments that bend to adjust to the energy landscape provided by point defects. However, for YGdBCO, we find that d_c»L_c because the dynamics are strongly influenced by sparse large precipitates within its microstructure. We proceed by leveraging the sensitivity of S to d to determine the effective pinning length in the YGdBCO film and find surprisingly high values, e.g., ~800 nm at T/T_c ~ 0.5 and a field of 1 T. Finally, we show evidence that the dramatic thickness-dependence of creep is due to a change from elastic to rigid vortex dynamics. [S. Eley et al., submitted, arxiv.org/abs/1709.02776]