My research is dedicated to the application and development of theory and computational simulation tools for solving problems in condensed matter systems. I work predominantly at the atomistic length-scale, using quantum mechanics to describe systems of interacting electrons and nuclei. Such theory and simulation is often called ab initio, or first-principles, and is invaluable for understanding the structure of matter, providing microscopic insight into the behaviour of materials. The state-of-the-art computational tools that are developed in my group are shared with the wider scientific community, either through commercial, academic or general public (open-source) license, to benefit the pursuit and dissemination of knowledge in this field.
- Thermoelectric properties of semiconducting nanostructures
- Defects in nanostructures and at interfaces
- Multi-functional oxide materials, ferroelectrics
- Polymer networks and melts
- Spin transport in organic thin films
- Development of linear-scaling density-functional theory and local orbital methods for large-scale simulations
About electronic structure theory and density-functional theory
Electronic structure theory aims to describe the behaviour of electrons in matter. We study systems ranging in size from a single atom up to a fraction of a micron (a thousandth of a millimetre) in length. Our main objective is to achieve a greater understanding of the properties of materials. Armed with this understanding, materials can be designed and engineered to behave in technologically useful ways. Electronic structure theory has a direct impact on our everyday lives (transistors, magnetic drives, drug discovery, catalysis, superconductivity, alloys...).
In order to calculate the properties of a system of atoms, one must solve the equations of quantum mechanics. It is currently impossible to do this exactly for more than a few atoms. Density-functional theory (DFT) provides a way of finding an approximate solution to the problem, and has made possible the study of many systems through ab initio (i.e., no prior assumptions) computer simulations. Atomistic simulations based on DFT have had an immense impact on the way in which materials are studied. As these techniques have become more sophisticated, well-understood and robust, their reach has extended beyond the realm of condensed matter physics into such diverse disciplines as materials science, chemistry, earth sciences, biochemistry and biophysics. There are many tools available for doing DFT calculations, e.g, CASTEP and PWSCF.
About the codes we develop in my group
ONETEP: Conventional DFT calculations work with delocalised wavefunctions (Bloch bands) that extend over the entire system. This means that the computer effort required to solve the problem scales as the cube of the system-size. As a result, even with the most powerful supercomputers, systems of no greater than a few hundred atoms may be studied. Reformulating the problem in terms of localised functions and the single-particle density matrix reduces this scaling to only linear with the system-size, opening up the possibility of bringing to bear the predictive power of DFT on modelling systems of scientific and technological interest that are beyond the capabilities of conventional approaches. I am one of the authors of the ONETEP linear scaling DFT code.
Wannier90: The delocalised Bloch bands that describe the electronic ground state of a system can be transformed into a set of maximally localised Wannier functions (MLWFs). These are philosophically similar to the localised molecular orbitals used in quantum chemistry. Working with MLWFs has a number of advantages over using extended Bloch states: they provide chemical insight into bonding; their centres contain information about the local polarisation via the modern theory of polarlisation; they can be used as a natural and very accurate minimal basis set for large-scale calculations of, for example, transport properties in nanostructures. I am one of the authors of the Wannier90 code for calculating MLWFs. Read this review article [PDF] or go to the Wannier90 website for more details.