Pinning and thermal fluctuations of a flux line in high-temperature superconductors

Using a simple model of single vortex depinning, we show that the temperature dependence of both resistivity and ac susceptibility in the superconducting state in moderate magnetic fields can be understood in terms of the activation of vortices from homogeneous and densely distributed pinning centers. Furthermore, the field dependence of the activation energy is explained by including the interaction of vortices above H{~}1 T, below which the distribution of pinning strengths becomes important.