Real time simulation of transport and
current driven dynamics in nanoscale devices
This problem has numerous attractive features as an area for
fundamental research. At its most basic level it involves the
non-equilibrium exchange of energy between current-carrying electrons
and the atoms in the nanoscale device. From a practical point of view
it has major ramifications in understanding the functioning of next
generation electronic devices. In addition, the breadth of scope of
this problem facilitates contact with the numerous experiments in the
field of conduction in atomic scale systems.
Over the last several years, we have developed a number of theoretical
and computational techniques at the ASC which has allowed us to make
major advances in the understanding of the fundamental processes
affecting the functioning of these devices. These techniques include
The use of the Wigner function for the numerical simulation of
correlated electron transport in molecules
The calculation of current noise to investigate molecular
rotation
The development of a correlated electron-ion dynamics
formalism for describing the proper exchange of energy between
electrons and ions in molecular-scale electronic devices.
In addition, these methods, together with associated parallel computer
codes developed in the ASC, have allowed us to open up new avenues of
research. One example that illustrates all of the above is the problem
- of great current interest both in our group and in other groups,
from experiment and theory - of non conservative current-induced
forces and their capacity to drive atomic-scale motors. We have shown
conclusively (mathematically and computationally) that current-induced
forces are fundamentally non-conservative in nature. Since its
publication, less than a year ago, this work has been echoed in two
New and Views articles in the Nature journals, and has sparked off
further work in Leiden (experiment) and in Copenhagen (theory).
Current-driven atomic waterwheel
An Ehrenfest dynamical simulation of a bent atomic wire was performed. The dynamics of the
corner atom in the 2D plane was investigated using a nearest-neighbour single-orbital
orthogonal tight-binding model. For an applied bias of 1V, a bend of 70o and
specific for the onsite energies of the corner atoms and its nearest neighbours, we observe
the atom spiralling outwards with its kinetic energy increasing. This is a signature of the
non-conservative nature of current-induced forces.
Bond currents in azulene
These cartoons show the development of bond currents: corresponding to those bonds
highlighted. These currents flow as the time dependent Schroedinger equation is solved
starting with coherent mixtures of one of the three states illustrated, which are
highest occupied (|0>), and lowest (|1>) and next lowest (|2>) unoccupied molecular
orbitals in azulene. In this case, all three states are made up exclusively from
electrons in the \pi-system.
The coherent mixtures are |0> + |1> (first 6 frames); |0> + |1> (next 6 frames) and
|0> + |1> (final 6 frames).
In terms of its place in the work of ASC as a whole, this subject
combines several threads of research in our Centre:
time-dependent tight binding theory
time-dependent density functional theory
the theory of transport
the use of molecular dynamics (in this case, under
non-equilibrium conditions with electronic open
boundaries)
to help understand the behaviour of individual atoms and groups of
atoms, in the problem at hand.
