Perturbation theory for ac-driven interfaces in random media

We study D-dimensional elastic manifolds driven by ac forces in a disordered environment using a perturbation expansion in the disorder strength and the mean-field approximation. We find that for D ≤ 4 , perturbation theory produces nonregular terms that grow unboundedly in time. The origin of these nonregular terms is explained. By using a graphical representation we argue that the perturbation expansion is regular to all orders for D > 4. Moreover, for the corresponding mean-field problem we prove that ill-behaved diagrams can be resummed in a way that their unbounded parts mutually cancel. Our analytical results are supported by numerical investigations. Furthermore, we conjecture the scaling of the Fourier coefficients of the mean velocity with the amplitude of the driving force h .