Abstract
Vortex activation energy UAC in the critical-state related AC magnetic response of superconductors (appearing in the vicinity of the DC irreversibility line) takes large values, as often reported, which is not yet understood. This behavior is essentially different from that of the vortex-creep activation energy at long relaxation time scales, and may become important for AC applications of superconductors. To elucidate this aspect, we investigated the AC signal of almost decoupled [Y Ba2Cu3O7]n/[PrBa2Cu3O7]4 superlattices (with n = 11 or 4 units cells) in perpendicular DC and AC magnetic fields. In these model samples, the length of the hopping vortex segment is fixed by the thickness of superconducting layers and vortices are disentangled, at least at low DC fields. It is shown that the high UAC values result from the large contribution of the pinning enhanced viscous drag in the conditions of thermally activated, non-diffusive vortex motion at short time scales, where the influence of thermally induced vortex fluctuations on pinning is weak.
Highlights
Vortex pinning potential of superconducting materials is widely investigated by measuring the DC magnetization relaxation over a relatively large time interval[1], as well as by analyzing the AC magnetic response.[2]
It is shown that the high UAC values result from the large contribution of the pinning enhanced viscous drag in the conditions of thermally activated, non-diffusive vortex motion at short time scales, where the influence of thermally induced vortex fluctuations on pinning is weak
The DC relaxation results are well described by a vortex diffusion process, where, at a constant DC magnetic field H, the induced current density J and temperature T dependence of the vortex-creep activation energy U is given by the general vortex-creep relation,[3] U(J, T) = Tln(t/t0), for the relaxation time t larger than the time scale for creep t0
Summary
Vortex pinning potential of superconducting materials is widely investigated by measuring the DC magnetization relaxation over a relatively large time interval[1] (of the order of 103 s), as well as by analyzing the AC magnetic response.[2]. It is argued that the repeatedly observed linear Arrhenius plots[5,6,7,8,9,10,11,12] reflect a thermally activated, non-diffusive vortex motion process at short t = 1/ f scales, where the thermal smearing of the pinning potential below IL (or the smoothening of the vortex structure above IL)[4] is not completed In these conditions, the high UAC values result from the large contribution of the pinning enhanced viscous drag and the dynamic critical current density as the relevant (relaxation free) critical current density for the AC magnetic response
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