Elastic string in a random potential.

We have studied numerically the dynamics of a directed elastic string in a twodimensional array of quenched random impurities. The string is driven by a constant transverse force and thermal fluctuations are neglected. There is a transition from pinned to unpinned behavior at a critical value F T of the driving force. At the transition the average string velocity scales with the driving force. The scaling is equally well described by a power law υ d ∼(F-F T ) ζ , with ζ=0.24±0.1, or by a logarithm, υ d ∼1/ln(F-F T )