Dr. Tchavdar Todorov

Reader in Applied Mathematics and Theoretical Physics

Tchavdar

DBB.01.011
+44 (0) 28 9097 6030
t.todorov@qub.ac.uk

Atomistic Simulation Centre School of Mathematics and Physics Queen's University Belfast University Road Belfast BT7 1NN Northern Ireland

Degrees, Awards and Honours

  • 2003 | Maxwell Medal and Prize (Institute of Physics, 2003)

Interests

  • Transport in nanoscale conductors
  • Current-induced forces and current-driven atomic motion
  • Time-dependent tight binding
  • Electron-nuclear dynamics

Most Recent Publications

  1. Electron-phonon thermalization in a scalable method for real-time quantum dynamics, Physical Review B, 2016, 93, No. 2
    doi: 10.1103/PhysRevB.93.024306 Abstract Full Text

    We present a quantum simulation method that follows the dynamics of out-of-equilibrium many-body systems of electrons and oscillators in real time. Its cost is linear in the number of oscillators and it can probe time scales from attoseconds to hundreds of picoseconds. Contrary to Ehrenfest dynamics, it can thermalize starting from a variety of initial conditions, including electronic population inversion. While an electronic temperature can be defined in terms of a nonequilibrium entropy, a Fermi-Dirac distribution in general emerges only after thermalization. These results can be used to construct a kinetic model of electron-phonon equilibration based on the explicit quantum dynamics.

  2. Length Matters: Keeping Atomic Wires in Checkhttp://dx.doi.org/10.1103/PhysRevB.93.024306, MRS Proceedings, 2015, 1753
    doi: 10.1557/opl.2015.197 Abstract
    Dynamical effects of non-conservative forces in long, defect free atomic wires are investigated. Current flow through these wires is simulated and we find that during the initial transient, the kinetic energies of the ions are contained in a small number of phonon modes, closely clustered in frequency. These phonon modes correspond to the waterwheel modes determined from preliminary static calculations. The static calculations allow one to predict the appearance of non-conservative effects in advance of the more expensive real-time simulations. The ion kinetic energy redistributes across the band as non-conservative forces reach a steady state with electronic frictional forces. The typical ion kinetic energy is found to decrease with system length, increase with atomic mass, and its dependence on bias, mass and length is supported with a pen and paper model. This paper highlights the importance of non-conservative forces in current carrying devices and provides criteria for the design of stable atomic wires.

  3. Nonconservative current-driven dynamics: beyond the nanoscalehttp://dx.doi.org/10.1557/opl.2015.197, Beilstein Journal of Nanotechnology, 2015, 6, pp. 2140
    doi: 10.3762/bjnano.6.219 Abstract Full Text

    Long metallic nanowires combine crucial factors for nonconservative current-driven atomic motion. These systems have degenerate vibrational frequencies, clustered about a Kohn anomaly in the dispersion relation, that can couple under current to form nonequilibrium modes of motion growing exponentially in time. Such motion is made possible by nonconservative current-induced forces on atoms, and we refer to it generically as the waterwheel effect. Here the connection between the waterwheel effect and the stimulated directional emission of phonons propagating along the electron flow is discussed in an intuitive manner. Nonadiabatic molecular dynamics show that waterwheel modes self-regulate by reducing the current and by populating modes in nearby frequency, leading to a dynamical steady state in which nonconservative forces are counter-balanced by the electronic friction. The waterwheel effect can be described by an appropriate effective nonequilibrium dynamical response matrix. We show that the current-induced parts of this matrix in metallic systems are long-ranged, especially at low bias. This nonlocality is essential for the characterisation of nonconservative atomic dynamics under current beyond the nanoscale.

All of Tchavdar's publications

 

"If we take the view that quantisation of energy levels, tunnelling and interference are where quantum mechanics departs
most violently from classical notions, we may ask where do the two come closest? Nowhere is this proximity more tangible
than in the realm of particle interactions."
Undated, Anon

 

This we can put to a simple test. In a random array of barriers, a combination of
quantisation, tunnelling and interference generates trapped quantum states:

 

 

But couple this to another set of degrees of freedom, and the picture washes out:

 

 

 

Without electron-phonon interactions life would crash. For starters we’d lose our sense of smell (PRL 98 (2007) 038101), not
always a bad thing of course. Electrical and electronic equipment would seize up. Even if we could keep electrical appliances going,
we wouldn't be getting the energy in the forms and places needed. On the downside, the laptop armies at meetings are among
today's biggest energy wasters. E-phonon scattering is how electrons and atomic vibrations exchange energy and momentum,
which often controls charge and energy transport in molecular and condensed matter systems, and enforces thermal equilibrium.
Despite their fundamental role e-phonon interactions remain a challenge. Stepping beyond the Born-Oppenheimer approximation
opens a many-body problem (even ignoring e-e interactions). The most difficult are situations involving the simultaneous coupled
dynamics of the two subsystems. We have worked on this problem in nanoscale conductors and systems under irradiation, with
support by EPSRC and the Leverhulme Trust. In both contexts - "molecular electronics" and radiation damage - electrons and
the ionic motion can be driven very far out of equilibrium, resulting in violent momentum and energy exchange. Recently we have
developed a method for coupled e-phonon dynamics that can reach significant size- and time-scales. Take a 2d quantum dot
on a substrate, following electronic excitation. We can then track the relaxation back towards equilibrium in time. The process
can be monitored via the dot state occupancies. Notice the interplay between transitions. A vital process in this is spontaneous
phonon emission. It's often the hardest to capture, and is where mean-field electron-nuclear dynamics breaks down.

 

 

Valerio Rizzi, Tchavdar Todorov, Jorge Kohanoff and Alfredo Correa, PRB 93 (2016) 024306