Dr. Tchavdar Todorov
Reader in Applied Mathematics and Theoretical Physics
+44 (0) 28 9097 6030
Atomistic Simulation Centre
School of Mathematics
Queen's University Belfast
Belfast BT7 1NN
Degrees, Awards and Honours
- 2003 | Maxwell Medal and Prize (Institute of Physics, 2003)
- Transport in nanoscale conductors
- Current-induced forces and current-driven atomic motion
- Time-dependent tight binding
- Electron-nuclear dynamics
Most Recent Publications
- Length Matters: Keeping Atomic Wires in Check, 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.
- 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.
- Nonconservative dynamics in long atomic wireshttp://dx.doi.org/10.3762/bjnano.6.219, Physical Review B, 2014, 90, pp. 115430
doi: 10.1103/PhysRevB.90.115430 Abstract
The effect of nonconservative current-induced forces on the ions in a defect-free metallic nanowire is investigated using both steady-state calculations and dynamical simulations. Nonconservative forces were found to have a major influence on the ion dynamics in these systems, but their role in increasing the kinetic energy of the ions decreases with increasing system length. The results illustrate the importance of nonconservative effects in short nanowires and the scaling of these effects with system size. The dependence on bias and ion mass can be understood with the help of a simple pen and paper model. This material highlights the benefit of simple preliminary steady-state calculations in anticipating aspects of brute-force dynamical simulations, and provides rule of thumb criteria for the design of stable quantum wires.
"If we take the view that quantisation of energy levels, tunnelling and
interference are the effects where quantum mechanics departs most violently
from classical notions, we may ask where do the two come closest?
Nowhere is this proximity more compelling than in the realm of