Current-induced atomic dynamics, instabilities, and Raman signals: Quasi-classical Langevin equation approach

TL;DR

This paper develops a semi-classical Langevin equation framework to analyze current-induced atomic dynamics, instabilities, and Raman signals in molecular junctions, highlighting the roles of various electronic forces and Joule heating.

Contribution

It introduces a path-integral derived Langevin approach to model non-equilibrium ionic dynamics, incorporating multiple current-induced forces and analyzing their effects on stability and Raman signals.

Findings

Current-induced forces can destabilize molecular junctions.

Joule heating significantly affects atomic stability.

Raman signals reveal signatures of current-induced instabilities.

Abstract

We derive and employ a semi-classical Langevin equation obtained from path-integrals to describe the ionic dynamics of a molecular junction in the presence of electrical current. The electronic environment serves as an effective non-equilibrium bath. The bath results in random forces describing Joule heating,current-induced forces including the non-conservative wind force,dissipative frictional forces, and an effective Lorentz-like force due to the Berry phase of the non-equilibrium electrons. Using a generic two-level molecular model, we highlight the importance of both current-induced forces and Joule heating for the stability of the system. We compare the impact of the different forces, and the wide-band approximation for the electronic structure on our result. We examine the current-induced instabilities (excitation of runaway "waterwheel" modes) and investigate the signature of these in the Raman signals.