Nonmonotonic temperature dependence of the experimentally determined vortex-creep activation energy in disordered high-temperature superconductors

Abstract

The temperature $T$ variation of the normalized vortex-creep activation energy $U*$ determined in standard magnetization relaxation experiments for ${\mathrm{Pb}}_2{\mathrm{Sr}}_2{\mathrm{Y}}_{0.53}{\mathrm{Ca}}_{0.47}{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$ single crystals with random point disorder exhibits a maximum, which moves to lower-$T$ values by increasing the external magnetic field oriented parallel to the $c$ axis. The nonmonotonic $U*\left(T\right)$ dependence is related to the change of the vortex pinning barriers involved in the creep process across the order-disorder transition in the vortex system (accompanied by the occurrence of the second magnetization peak), in a dynamic scenario. The decrease of $U*$ with decreasing $T$ in the low-$T$ region is caused by the shift of the current density $J$ range probed in standard magnetization measurements toward the critical current density, and the significant $U*\left(J\right)$ variation in the elastic-creep domain. The dynamic approach is confirmed by the behavior of highly disordered top-seeded melt-grown $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ crystals at low $T$, for which no second magnetization peak appears, and $U*$ does not depend on $T$.