Adatom dynamics in a periodic potential under time-periodic bias

We study the dynamics of a Brownian particle in a ID external potential under the influence of a time-periodic bias with an amplitude small. with respect to the potential barriers. We consider both a periodic potential corresponding to a smooth crystal surface and a regular array of steps with an extra Ehrlich-Schwoebel barrier for step crossing. For the smooth surface, we extend our previous work in the high friction limit to the low friction case and find that the oscillating bias enhances the diffusion coefficient D-T due to the broadening of the jump length distribution. In the case of a stepped surface with terraces of length L, the bias induces a non-zero average current J(ave) in the direction of descending steps as long as the driving frequency is smaller than a threshold frequency Omega(T)approximate to L-1. The current shows a maximum as a function of temperature for fixed L. However, no evidence of stochastic resonance type of enhancement can be found either in D-T or J(ave). (C) 2000 Elsevier Science B.V. All rights reserved.