Nonadiabatic Van der Pol oscillations in molecular transport

TL;DR

This study investigates the stability of Van der Pol oscillations in molecular junctions under high-frequency bias, demonstrating their robustness beyond the adiabatic approximation through advanced simulations.

Contribution

It introduces a full Ehrenfest dynamics simulation approach to analyze nonadiabatic effects on Van der Pol oscillations in molecular transport.

Findings

Van der Pol oscillations are highly stable under nonadiabatic conditions.

High-frequency fields alter oscillation amplitudes and average nuclear positions.

Limit cycles are preserved in the average despite nonadiabatic distortions.

Abstract

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.