Anomalous magnetic behavior of superconducting Tl2Ba2CaCu2O8 thin films in small magnetic fields close to the transition temperature

Using a specially designed SQUID magnetometer we measured the temperature dependence of the critical current density in a ring patterned Tl2Ba2CaCu2O8 thin film for magnetic fields 0.03Oe≤H<30Oe parallel to the c-axis. In addition, the temporal relaxation of the remanent state as prepared by field cooling in an external field of 100Oe at different temperatures T≤93 K<Tc is determined. The jc(T) data show a field-dependent anomalous kink close to Tc pointing to reduced dissipation with increasing temperature allowing to construct a corresponding H-T borderline. A similar behavior is observed for the normalized relaxation rate S(T) as extracted from the temporal behavior of the remanent state, which, at low temperatures, exhibits the expected increase for increasing T-values, while an anomalous decrease of S(T) is found for temperatures above 85 K. While the low-T regime is attributed to creep of 2D pinned single vortex lines, the high-T behavior is suggested to be dominated by collective motion with a more sluggish dynamics. This change in dynamics is also reflected by the activation barriers for flux creep U(j), which show a corresponding crossover in μ from 0.06 to 0.99. An additional scaling analysis of the E-j characteristics for T≥85 K according to vortex glass theory reveals quasi-2D collective creep behavior with Tg=96 K. (orig.)