Lecturer in Atomistic Simulation
David Bates Building (DBB)
Atomistic Simulation Centre (ASC)
Department of Physics and Astronomy
School of Mathematics and Physics
Queen's University Belfast
Belfast BT7 1NN
Tel: +44 (0) 28 9097 6022
Fax: +44 (0) 28 9097 5359
1. Correlated electron-ion dynamics (CEID) and electronic decoherence
As long with many other properties, the colour of molecules and solids can be probed with light sources, from lamps to lasers.
In fact, the colour of matter can be understood and predicted from an atomistic point of view, i.e, by studying the motion
of the electrons and the nuclei molecules and solids are made of.
In most cases, it is fine to neglect the nuclear motion, as the atomic nuclei are much heavier than the electrons.
However, this is not always the case, as neglecting the nuclear motion might lead to very wrong predictions, e.g., for
the 'smart' materials organic solar cells and organic LEDs are made of!
To fix this issue, here at the ASC we have developed a novel and powerful numerical techniques, called
Correlated Electron-Ion Dynamics (CEID), which I like to apply to all the difficult cases in which the electronic
and nuclear motion cannot be easily disentangled.
The variant of the CEID I have devised employs a convergent expansion of the quantum fluctuations of the nuclei about their mean-field motion
to provide an accurate, yet affordable, numerical solution of the nonadiabatic Schroedinger problem.
This variant of CEID has been successfully tested for semi-empirical models of short molecules and conjugated polymers (in collaboration
with A.P. Horsfield and
Recently, CEID has been also found useful to study the electronic
decoherence in low dimensional systems from an atomistic point of view (in collaboration with
I. Franco and
An open-source CEID code based on my approach
can be downloaded from this repository.
Title: Analog of Rabi oscillations in resonant electron-ion systems
Author(s): Stella L., Miranda R.P., Horsfield A.P., Fisher A.J.,
The Journal of Chemical Physics, 134, No. 19, Art. No. 194105 (21 May 2011)
Title: Robust nonadiabatic molecular dynamics for metals and insulators
Author(s): Stella L., Meister M., Fisher A. J., Horsfield A. P.
Journal of Physical Chemistry, 127, No. 21, Art. No. 214104 (4 December 2007)
Evolution of the electronic purity (a measure of the electronic coherence) for a model conjugated molecule.
The electronic coherence gradually decreases with time (decoherence) through a series of repeated 'revivals', see inset.
By CEID one can model this subtle electron-nuclear correlation effect.
2. Quantum plasmonics
We are all familiar with electronic devices, which exploit the possibility to control the electron flux
in metals and semiconductors.
Plasmonic devices also take advantage of the electron motion in metals. However, this is a very peculiar
kind of motion which originates from the coherent oscillations of many electrons, also known as plasmons.
Surface plasmons are oscillations which take place at the interface between a metal and a dielectric (i.e., air)
and they can be easily generated and controlled using a laser. For this reason, plasmonic devices are good
candidates for faster (i.e., terahertz) and smaller (i.e., nanoscale) components for cheap and
reliable communication technologies.
To faithfully model the plasmonic response at the nanoscale, we use a simple and robust
jellium approximation along with a Time-Dependent Density-Functional Theory (TDDFT) description of the electron dynamics.
Hence, we can include the contributions from the quantum delocalisation of the electron in a transferable and
unbiased way (in collaboration with P. Zhang,
F.J. García Vidal,
and P. García Gonzalez).
Our method scales favourably with the size of the system and is suited to model the plasmonic response of systems
beyond the possibilities of atomistic simulations.
Title: Performance of Nonlocal Optics When Applied to Plasmonic Nanostructures
Author(s): Stella L., Zhang P., Garcia-Vidal F.J., Rubio A., Garcia-Gonzalez P.,
The Journal of Physical Chemistry C, 117, No. 17, pp. 8941- 8949 (2 May 2013)
Field enhancement between a pair of sodium nanowires.
At variance with classical local and semi-local approximations, the jellium/TDDFT approach yields an enhancement
localised in the gap between the nanowires.
This is a consequence of the quantum delocalisation of the electrons.
3. Generalised Langevin Equation (GLE) and out-of-equilibrium molecular dynamics
For the sake of simplicity, when we observe a physical system, e.g., a piece of solid, we quietly assume that this system is
just weakly coupled to the rest of the universe.
However, this assumption is just an approximation and in principle there is always an exchange of energy from the system to its environment,
e.g., a heated piece of solid, however well isolated, will eventually cool down to room temperature.
This unavoidable exchange of energy between the system and its environment also acts at molecular and atomic scales.
The Generalised Langevin Equation formalism provides an invaluable tool to model realistic environments in an efficient, yet
For a rather wide class of solids, a GLE can be derived from the classical Lagrangian of the system and its environment.
I have recently devised an efficient molecular dynamics algorithm to integrate such atomistic GLE to model the microscopic dynamics
of systems driven out-of-equilibrium
(in collaboration with L. Kantorovich
and C. Lorenz).
In the near future, I plan to apply this method to investigate the impact of non-trivial system-environment correlations
to the system's dissipative dynamics, e.g., to accurately model heat generation and transfer in nanostructures.
Title: Generalized Langevin equation: An efficient approach to nonequilibrium molecular dynamics of open systems
Author(s): Stella L., Lorenz C.D., Kantorovich L.,
Physical Review B, 89, pp. 134303- (7 April 2014)