  We consider a moving interface that is coupled to an elliptic equation in a
heterogeneous medium. The problem is motivated by the study of displacive
solid-solid phase transformations. We show that a nearly flat interface is
given by the graph of the function $g$ which evolves according to the equation
$g_t (x) = -(-\Delta)^{1/2}g (x) + \varphi(x, g(x)) + F$. This equation also
arises in the study of dislocations and fracture. We show in the periodic
setting that such interfaces exhibit a stick-slip behavior associated with
pinning and depinning. Further, we present some numerical evidence that the
effective velocity of the phase boundary scales as the square-root of the
excess macroscopic force above the depinning transition.

Analysis of PDEs