ELASTIC STRING IN A RANDOM MEDIUM

We consider a one-dimensional elastic string as a set of massless bead s interacting through springs characterized by anisotropic elastic con stants. The string, driven by an external force, moves in a medium wit h quenched disorder. We find that longitudinal fluctuations lead to no nlinear behavior in the equation of motion that is kinematically gener ated by the motion of the string. The strength of the nonlinear effect s depends on the anisotropy of the medium and the distance from the de pinning transition. On the other hand, the consideration of restricted solid-on-solid conditions imposed on the string leads to a nonlinear term with a diverging coefficient at the depinning transition.