Recent Publications

Tchavdar Todorov

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  1. 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)

    doi: 10.1103/PhysRevB.85.245444
    Abstract



    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.

  2. 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)

    doi: 10.1088/0953-8984/24/40/402203
    Abstract



    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.