Simulations of the onset of diffusion in a flux-line lattice in a random potential

We simulate the finite-temperature dynamics of flux lines in a thin film. It is shown that a weak random potential significantly reduces the temperature, ${\mathit{T}}_{\mathit{D}}$, at which flux lines start to diffuse and dissipation sets in. The diffusion starts to take place along grain boundaries in the flux-line lattice. These findings are discussed in relation to the magnetic properties of the high-temperature superconductors.