The force exerted by the electrons on the nuclei of a current-carrying
molecular junction can be manipulated to engineer nanoscale mechanical systems.
In the adiabatic regime a peculiarity of these forces is negative friction,
responsible for Van der Pol oscillations of the nuclear coordinates. In this
work we study the robustness of the Van der Pol oscillations against
high-frequency bias and gate voltage. For this purpose we go beyond the
adiabatic approximation and perform full Ehrenfest dynamics simulations. The
numerical scheme implements a mixed quantum-classical algorithm for open
systems and is capable to deal with arbitrary time-dependent driving fields. We
find that the Van der Pol oscillations are extremely stable. The nonadiabatic
electron dynamics distorts the trajectory in the momentum-coordinate phase
space but preserves the limit cycles in an average sense. We further show that
high-frequency fields change both the oscillation amplitudes and the average
nuclear positions. By switching the fields off at different times one obtains
cycles of different amplitudes which attain the limit cycle only after
considerably long times.Comment: 12 pages, 7 figure