Analysis of the critical-state related AC magnetic response of superconductors with complex, dense pinning structures reveals a thermally activated vortex hopping process with unexpectedly high activation energies around the DC irreversibility line, which cannot be explained by extending the standard flux creep model at short time scales. To elucidate this aspect, we investigated the DC and the AC magnetic response of superconducting YBCO films and YBCO/PrBCO superlattices at usual frequencies and amplitudes, with the DC and AC fields oriented perpendicular to the film surface. The YBCO films have a well characterized pinning structure (embedded non-superconducting nanorods and nanoparticles), where the average spacing between the pinning centres can be determined. The considered superlattices (with the thickness of superconducting and non-superconducting blocks of several unit cells) are almost decoupled. In such superlattices, the presence of a disentangled vortex system is assured, at least for relatively low external DC fields, whereas the effective length of the hopping vortex segment is fixed by the thickness of superconducting layers. It is shown that the large vortex activation energies determined in the proximity of the DC irreversibility line (where the critical-state related AC magnetic response is generated) result from thermally activated vortex hopping events over a mean distance which overcomes the average distance between the pinning centres. In these conditions, the smearing of the vortex pinning potential by thermally induced vortex fluctuations is reduced, leading to a pinning-enhanced viscous drag coefficient.
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