Tony Harker
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- Electronic structure of defects in solids
- Safety/Environmental aspects of nanoparticles
- Nondestructive evaluation of advanced engineering composites
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Contact details:
Office: Room 4C4
Tel: +44 (0)20 7679 3404
Ext: 33404
Fax: +44 (0)20 7679 0595
Email: a.harker ucl.ac.uk |
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Research Interest
Having moved from industry to academia, I have breadth of activities which is unusual. There are several consistent underlying themes, in particular the behaviour of elastic waves in materials (with application to materials characterisation and materials testing) and the electronic structure of solids (especially defect centres). Each of these has been developed over several years. Interwoven with these have been both recurring motifs (such as the application of statistical methods to pattern recognition, classification, and extremal values) and isolated investigations (safety studies, applications of microwaves to characterisation of materials and to concrete demolition). Most recently I have been involved with investigations of the electronic structure of defects in the context of a quantum information processing scheme, and the propagation of elastic waves in fibre-reinforced metal matrix composites.
Other activities
I teach two courses on computation and simulation based on the Mathematica programming language, and keep a watching brief on other computing courses within the Department. I have been involved with the new MSc course in Nanotechnology since it was first conceived. I am also deputy Head of Department of Physics and Astronomy
Recent Publications
T C Choy, A M Stoneham, A H Harker, Dynamical resonance tunnelling -- a theory of giant emission from carbon field emitters, J. Phys.: Condens. Matter 17 (2005) 1505-1528. http://www.cmmp.ucl.ac.uk/~ahh/research/Papers/ChoStoHar2005.pdf
Certain carbon field emitters show enormously enhanced electron emission. We link this observation to dynamic resonant tunnelling and to a generalized image interaction. The behaviour is very different from that predicted by Fowler and Nordheim. The key factor is a field dependence of the inverse potential enhancement kappa(omega): this is studied through the non-linear second-order susceptibility gamma(omega) which couples the static applied field to the dynamic field. We derive the criterion for this mechanism to operate and demonstrate can provide a linear field dependence of kappa(omega). We further provide a link between gamma(omega) and other microscopic parameters of the surface plasmon model, notably the anharmonicity coefficient, via a Duffing oscillator model. Through the use of a one-dimensional fluctuating barrier model with a self-consistent approach, we further assess the significance of other non-linear damping effects.
A Lange, A Harker and N Saffari. A multilevel multipole method for modelling elastic-wave multiple scattering in fibre-reinforced composites. Review of Progress in Quantitative Non-Destructive Evaluation, 2003; vol. 22, 85-92.
http://www.cmmp.ucl.ac.uk/~ahh/research/Papers/LanHarSaf2003.pdf
The multilevel multipole method is a scheme for reducing the complexity of a scattering problem from a dependency on the square of the number of scatterers to being linear in the number of scatterers. We develop such a method for studying the propagation of elastic waves in a two-phase material of rods in a solid matrix, and apply it to the study of elastic waves in a metal-matrix composite.
A Kerridge, A H Harker, A M Stoneham Quantum behaviour of hydrogen and muonium in vacancy-containing complexes, J Phys: Condens Matter 16 (2004) 8743
http://www.cmmp.ucl.ac.uk/~ahh/research/Papers/KerHarSto2004.pdf
Most solid-state calculations either omit quantum mechanical zero point motion and tunnelling, or estimate it in an subsidiary step. Such quantum effects are especially significant for light nuclei, such as the proton or its analogue, the muon. We propose a simple approach to including such quantum behaviour, in a form readily integrated with standard electronic structure calculations. This approach is demonstrated for a number of vacancy-containing defect complexes in diamond. Our results suggest that for the NHV− complex, quantum motion of the proton between three equivalent potential energy minima is sufficiently rapid to time-average measurements at X-band frequencies.
Biography
Dec 1948 Born Reading, England.
1967 BA (First Class Honours), Theoretical Physics, Cambridge University
1974 D. Phil., Theoretical Physics, Oxford University
1974 Royal Society European Exchange Fellow, Technische Hochschule, Stuttgart
1974 Research Fellow, Atomic Energy Research Establishment, Harwell
1977 Employed by UKAEA, latterly AEA, Technology, at Harwell
1990 Head, Theoretical Studies Department, AEA, Industrial Technology, Harwell
1 April 1995 Product Development Group, AEA Technology, to 31 July 1995, Harwell
1 October 1995 Principal Research Fellow, Department of, Physics and Astronomy, University College, London
1 October 1998 Reader, Department of, Physics and Astronomy, University College, London
Research
Ultrasonic inspection of advanced engineering material.
The figure shows the horizontal (u), vertical (v) and total displacements in an elastic wave travelling in the direction of the arrow through a piece of metal reinforced with silicon carbide fibres (circles). Red corresponds to a large amplitude, blue to a small one. The two columns of figures are for different wave frequencies. Metal-matrix composites of this kind are used in critical components such as the turbine rings of jet aircraft, and it is important to be able to test them for flaws. If there is a defect such as a crack or a void, it will reflect a sound wave, and the reflection will be stronger if the displacement in the wave is large near the defect. The figures show that by altering the frequency one can be sensitive to defects in different parts of the structure.
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