ELASTIC STRING IN A RANDOM POTENTIAL

We have studied numerically the dynamics of a directed elastic string in a two-dimensional 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 v(d) approximately (F - F(T))zeta, with zeta = 0.24 +/- 0.1, or by a logarithm, v(d) approximately 1/ln(F - F(T)). The divergence of the velocity-velocity correlation length at threshold is characterized by an exponent nu = 1.05 +/- 0.1.