  The influence of out-of-plane and in-plane contact vibrations and temperature
on the friction force acting on a sharp tip elastically pulled on a crystal
surface is studied using a generalized Prandtl- Tomlinson model. The average
friction force is significantly lowered in a frequency range determined by the
"washboard" frequency of the stick-slip motion and the viscous damping
accompanying the tip motion. An approximately linear relation between the
actuation amplitude and the effective corrugation of the surface potential is
derived in the case of in-plane actuation, extending a similar conclusion for
out-of-plane actuation. Temperature causes an additional friction reduction
with a scaling relation in formal agreement with the predictions of reaction
rate theory in absence of contact vibrations. In this case the actuation
effects can be described by the effective energy or, more accurately, by
introducing an effective temperature.
