The dynamics of the discrete Gaussian model for the surface of a crystal
deposited on a disordered substrate is investigated by Monte Carlo simulations.
The mobility of the growing surface was studied as a function of a small
driving force F and temperature T. A continuous transition is found from
high-temperature phase characterized by linear response to a low-temperature
phase with nonlinear, temperature dependent response. In the simulated regime
of driving force the numerical results are in general agreement with recent
dynamic renormalization group predictions.Comment: 10 pages, latex, 3 figures, to appear in Phys. Rev. E (RC