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EPSRC Reference: EP/C006739/1
Title: Modelling Non-Adiabatic Processes In Materials with Correlated Electron-Ion Dynamics: The Next Frontier in Quantum Modelling of Materials
Principal Investigator: Todorov, Dr TN
Other Investigators:
Paxton, Professor AT Finnis, Professor MW
Researcher Co-Investigators:
Dr C Sanchez
Project Partners:
Department: Sch of Mathematics and Physics
Organisation: Queen's University of Belfast
Scheme: Standard Research (Pre-FEC)
Starts: 01 October 2005 Ends: 31 October 2009 Value (£): 347,715
EPSRC Research Topic Classifications:
Eng. Dynamics & Tribology Materials Characterisation
EPSRC Industrial Sector Classifications:
Manufacturing Electronics
Related Grants:
EP/C524381/1 EP/C524403/1
Panel History:  
Summary on Grant Application Form
When scientists model materials at the atomic scale it is very common for them to make an approximation that enables them to make an enormous simplification. The approximation stems from the observation that the mass of an electron is at least three orders of magnitude less than the mass of an atom. It would seem a very reasonable step to say that whatever the positions of the atomic nuclei the very light electrons are able almost instantaneously to find a configuration which minimizes the energy of the system. This approximation is known as the Born-Oppenheimer (B-O) approximation, and it is invoked in the overwhelming majority of all simulations of materials at the atomic scale.One of the consequences of making the B-O approximation is that while there may be energy exchanges between electrons and ion cores those exchanges must be reversible. Irreversible flow of energy between electrons and ions cannot be treated if the B-O approximation is invoked. This is a severe limitation because it rules out many processes and phenomena in materials that are a consequence of such irreversible flows of energy. For example, it rules out the possibility of being able to model the heating of a metallic wire caused by a current flowing through it. Although we know that in metals heat is conducted primarily by electrons we must avoid the B-O approximation if we wish to model how the electrons receive the heat from hot vibrating ions in the first place. This is a long-standing major problem for atomistic simulations of processes in metals at ambient temperatures such as friction, plastic deformation and especially radiation damage. There are also many examples in polymers where injected electrons distort the polymer chain forming a polaron. To form a polaron an excited electron relaxes by an irreversible exchange of energy with the ions. For an electron to move its motion must be correlated with that of the ions.It is very tempting to think that if we write down quantum equations of motion for electrons and couple them to classical equations of motion for ions we will be able to treat irreversible exchanges of energy between the electrons and ions. This approach is known as Ehrenfest dynamics. We have shown recently that whereas Ehrenfest dynamics can describe the heating of cold electrons by hot ions, it provides a wholly incorrect description of the heating of cold ions by hot electrons. We have analysed carefully the reasons for this surprising failure and discovered that it is a result of smearing out the electrons into a structureless fluid and ignoring the correlations between quantum fluctuations of the positions and momenta of the ions and those of the electrons. This insight led us to develop a new way to couple equations of motion of electrons and ions, which we call correlated electron-ion dynamics (CEID). In CEID we respect the true quantum nature of the ions, and include the correlations of quantum fluctuations of their positions and momenta with those of the electrons. We have written a code called DINAMO which solves the CEID equations.In this proposal we have selected three exemplar areas of fundamental and strategic significance in materials science where irreversible exchanges of energy between electrons and ions play central roles. They are (a) transport in nanostructures, (b) radiation damage in metals and (c) excited states in polymers. In each case we shall apply DINAMO to solve long-standing problems in these areas. Our selection is driven by the wish to make as broad an impact in modelling materials as we can, to interest the widest possible community. We see this proposal as opening up a new area in materials modelling at the quantum level, and we have included plans to inform and involve broad communities of scientists, broad both in age-range and discipline.
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Organisation Website: http://www.qub.ac.uk