Title: Current-induced atomic dynamics, instabilities, and Raman signals: Quasiclassical Langevin equation approach
Author(s): Lü J.T., Brandbyge M., Hedegård P., Todorov T.N., Dundas D.,
Physical Review B, 85, pp. 245444- (25 June 2012)
We derive and employ a semiclassical 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 nonequilibrium bath. The bath results in random forces describing Joule heating, current-induced forces including the nonconservative wind force, dissipative frictional forces, and an effective Lorentz-type force due to the Berry phase of the nonequilibrium 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.
Title: An ignition key for atomic-scale engines
Author(s): Dundas D., Cunningham B., Buchanan C., Terasawa A., Anthony T Paxton A.T., Todorov T.N.
Journal of Physics: Condensed Matter, 24, pp. 402203-1-402203-6 (2012)
A current-carrying resonant nanoscale device, simulated by non-adiabatic
molecular dynamics, exhibits sharp activation of non-conservative
current-induced forces with bias. The result, above the critical bias,
is generalized rotational atomic motion with a large gain in kinetic energy.
The activation exploits
sharp features in the electronic structure, and constitutes, in effect, an
ignition key for atomic-scale motors. A controlling factor for the effect
is the non-equilibrium dynamical response matrix for small-amplitude atomic motion
under current. This matrix can be found from the steady-state electronic structure
by a simpler static calculation, providing a way to detect the likely appearance, or
otherwise, of non-conservative dynamics, in advance of real-time modelling.