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

Using a specially designed SQUID magnetometer we measured the temperature d ependence of the critical current density in a ring patterned Tl2Ba2CaCu2O8 thin film for magnetic fields 0.03 Oe less than or equal to H < 30 Oe para llel to the c-axis. In addition, the temporal relaxation of the remanent st ate as prepared by field cooling in an external held of 100 Oe at different temperatures T less than or equal to 93 K < T-c is determined. The j(c)(T) data show a field-dependent anomalous kink close to T-c pointing to reduce d dissipation with increasing temperature allowing to construct a correspon ding H-T borderline. A similar behavior is observed for the normalized rela xation rate S(T) as extracted from the temporal behavior of the remanent st ate, which, at low temperatures, exhibits the expected increase for increas ing 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 sin gle vortex lines, the high-ir behavior is suggested to be dominated by coll ective motion with a more sluggish dynamics. This change in dynamics is als o reflected by the activation barriers for flux creep U(j): which show a co rresponding crossover in mu from 0.06 to 0.99. An additional scaling analys is of the E-j characteristics for T greater than or equal to 85 K according to vortex glass theory reveals quasi-2D collective creep behavior with T-g = 96 K.