Research
Overview
My research centres on the use of computational simulations of condensed
matter systems. I use quantum mechanical methods to understand
the properties of materials, molecules and nanostructures - in fact,
anything made of atoms.. which means pretty much everything in the world!
So called "ab initio" techniques centre on the solution of the Schrodinger
equation, which describes the behaviour of electrons around atoms, without
making any empirical assumptions. By understanding the quantum mechanical
behaviour of complex systems containing large numbers of electrons and
nuclei, we can gain an understanding of their structure, energetics
and dynamics and understand how their properties can be improved for
use in fields as diverse as electronics, chemical engineering,
nanotechnology and medicine.
Linear-Scaling DFT
The main technique I use is Density Functional Theory (DFT). DFT makes
a set of approximations about the way electrons interact with each
other and with the nuclei of atoms. In most cases these approximations
turn out to be surprisingly accurate and well-justified ones. Traditional
approaches to DFT are capable of describing systems of hundreds of atoms
with a high accuracy. However, they involve solving for delocalised
single-particle orbitals which span the entire system studied. Such approaches
inevitably scale cubically with the system size for large numbers of atoms,
so become pretty much unfeasible with current computational hardware beyond
around 1000 atoms. The group I work with (alongside Peter Haynes and Arash
Mostofi) is one of a few around the world developing so-called
"Linear-Scaling" approaches to DFT. Linear-Scaling DFT reformulates the
problem so as to avoid the calculation of eigenstates, and is therefore
able to scale linearly with system size up to very large numbers of atoms.
I am a developer of the
ONETEP Linear-Scaling DFT code. Current research into
applications of linear-scaling DFT centres around biological physics,
strongly-correlated systems, nanostructures, defects, interfaces and
solvation models. These advances promise to bring the power of
quantum-mechanical simulations to bear on systems of an unprecedented
scale, for use in applications as diverse as the design of new drug
molecules to specifically target particular diseases to the
characterisation of nanomaterials for photovoltaic solar cells.
Defects in Metal Oxides
One of my main simulation interests is defects in crystals, particularly
in metal oxides.With Matthew Foulkes and Mike Finnis, I have investigated
the properties of point defects (particularly vacancies) in aluminium
oxide (Al2O3). We have developed new techniques for extrapolation
to the so-called "dilute limit", of well separated defects,
which is particularly hard to achieve in calculations in a
periodic crystal. We have also investigated the way concentrations of
defects depend on the level of aliovalent doping, and developed a new framework
for the self-consistent calculation of defect concentrations under doping.
With Crispin Barnes and Massimo Barbagallo, I have investigated the properties
of Europium Monoxide. Using DFT+U simulations of EuO with and without oxygen
vacancies, we explained the enhancement of the magnetic moment of EuO under
oxygen deficient conditions - a surprising result which may have implications
for the use of EuO in spintronic devices.
Nanorods
With Phil Avraam, Peter Haynes and Paul Tangney, I am working on understanding
the behaviour of polar semiconductor nanorods. We have carried out very
large-scale simulations of GaAs nanorods to try to understand the behaviour of
the dipole moment of such systems. The dipole moment is central to the unusual
electrical and optical properties of nanorods, and is influenced by a wide
array of factors, including the intrinsic polarisation of the underlying crystal,
the range of possible shapes and surfaces, the surface termination by ligand
species, and solution in a variety of solvents. Only simulation-based
techniques are able to disaggregate the many factors and provide an understanding
of how we can control and improve the properties of nanorods.
Quantum Monte Carlo
During my PhD (supervised by Professor Matthew Foulkes) I worked on Quantum
Monte Carlo methods. These involve direct solution of the Many-Body Schrodinger
equation using statistical methods, in which the outcomes of a large number of
computer "experiments" are averaged to give quantum mechanical expectation values.
I worked on calculations of the surface energy of the electron gas (with Ben
Wood), on the properties of defects in Al2O3 (with Kilian Frensch), and on the
behaviour of polarisation and localisation in many-body systems.