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

The temperature T variation of the normalized vortex-creep activation energy U* determined in standard magnetization relaxation experiments for Pb₂Sr₂Y_0.53Ca_0.47Cu ₃O_8+δ 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* (T) 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* (J) variation in the elastic-creep domain. The dynamic approach is confirmed by the behavior, of highly disordered top-seeded melt-grown YBa₂Cu₃O_7-δ crystals at low T, for which no second magnetization peak appears,:and U* does not depend on T.