Absence of dynamical crossover in the vortex creep near by the second peak effect in superconducting Hg-1201 single crystals

DC magnetic relaxation measurements in HgBa₂CuO₄ single crystals are analyzed nearby the fishtail line. It is found that in this case, it is not necessary to introduce any crossover from plastic creep to elastic creep models at the fishtail line. This type of fishtail effect comes only from a competition between a critical current at low temperature which increases versus field and the activation energy, which decreases versus field. According to the doping level of the compound, the fishtail effect can be observed or not, without any correlation with a vortex phase transition. Moreover, in this type of fishtail effect, there is no history effects as recently observed in YBaCu₂O₃ by the partial magnetization loop technique, suggesting that the transition from plastic to elastic flow is here hidden by the disorder of these materials. Received 11 January 2000