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 [ital F][sub [ital 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 [ital v][sub [ital d]][similar to]([ital F][minus][ital F][sub [ital T]])[sup [zeta]], with [zeta]=0.24[plus minus]0.1, or by a logarithm, [ital v][sub [ital d]][similar to]1/ln([ital F][minus][ital F][sub [ital T]]). The divergence of the velocity-velocity correlation length at threshold is characterized by an exponent [nu]=1.05[plus minus]0.1.