Critical currents and thermally activated flux motion in high-temperature superconductors

We have measured the resistance below Tc of single crystals of the high-temperature superconductors Ba2YCu3O7 and Bi2.2Sr2Ca0.8Cu2O8+δ in magnetic fields up to 12 T. The resistive transition of both compounds is dominated by intrinsic dissipation which is thermally activated, resulting in an exponential temperature dependence of the resistivity well below Tc. The dissipation is significantly larger and of different character in the Bi-Cu compound than in Ba2YCu3O7. The relation between the activated behavior and the depinning critical current is discussed.