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Biomedical subjects

A M Stoneham

Publications and source records attributed to A M Stoneham.

5 recordsLinked to original sources
PubMed2005

Importance of quantum tunneling in vacancy-hydrogen complexes in diamond.

Our ab initio calculations of the hyperfine parameters for negatively charged vacancy-hydrogen and nitrogen-vacancy-hydrogen complexes in diamond compare static defect models and models which account for the quantum tunneling behavior of hydrogen. The static models give rise to hyperfine splittings that are inconsistent with the experimental electron paramagnetic resonance data. In contrast, the hyperfine parameters for the quantum dynamical models are in agreement with the experimental observations. We show that the quantum motion of the proton is crucial to the prediction of symmetry and hyperfine constants for two simple defect centers in diamond. Static a priori methods fail for these systems.

Journal Article↗
PubMed2003

Quantum dots as dynamical systems.

Quantum dots show a range of time-dependent behaviours. We show that the polarity of II-VI nanoparticles has important dynamical implications for electronic, vibrational and other phenomena. Polarity-dependent phenomena are found even for nearly spherical stoichiometric clusters of ZnO and ZnS in studies based on interatomic potentials or on a plane-wave density-functional approach. We find a substantial dipole moment for free nanoparticles, whether of the zinc blende or wurtzite structure. This dipole causes a highly non-uniform spin-density distribution on electronic excitation or after a change in the dot's electronic charge state. The spin density of the triplet exciton shows that the dipole aligns so as to reduce the dipole moment in the electronically excited state. The polarity of II-VI dots also affects their vibrational properties. High- and low-frequency tails of the vibration density of states arise from modes strongly localized at surface atoms, near the poles of the dipole. These features, first noted for free clusters, also hold for particles embedded in a wide-gap dielectric, a-SiO(2). We present the results of molecular dynamics of the ZnS particle embedded into the silica glass, and consider the role played by the soft modes in energy-dissipation processes such as dephasing during non-radiative recombination of excitons, and energy transfer from the dot to the matrix.

Journal Article↗
PubMed2002

Computational physics: a perspective.

Computing comprises three distinct strands: hardware, software and the ways they are used in real or imagined worlds. Its use in research is more than writing or running code. Having something significant to compute and deploying judgement in what is attempted and achieved are especially challenging. In science or engineering, one must define a central problem in computable form, run such software as is appropriate and, last but by no means least, convince others that the results are both valid and useful. These several strands are highly interdependent. A major scientific development can transform disparate aspects of information and computer technologies. Computers affect the way we do science, as well as changing our personal worlds. Access to information is being transformed, with consequences beyond research or even science. Creativity in research is usually considered uniquely human, with inspiration a central factor. Scientific and technological needs are major forces in innovation, and these include hardware and software opportunities. One can try to define the scientific needs for established technologies (atomic energy, the early semiconductor industry), for rapidly developing technologies (advanced materials, microelectronics) and for emerging technologies (nanotechnology, novel information technologies). Did these needs define new computing, or was science diverted into applications of then-available codes? Regarding credibility, why is it that engineers accept computer realizations when designing engineered structures, whereas predictive modelling of materials has yet to achieve industrial confidence outside very special cases? The tensions between computing and traditional science are complex, unpredictable and potentially powerful.

Computer Simulation↗
PubMed1989

Computer modelling: future directions.

Recent developments in computing and in the theory of simulation have extended greatly the successes of the modelling of ionic crystals pioneered by Mott and Littleton. This has changed the way in which computer experiments are brought to bear on an increasing range of solid-state phenomena. Yet applied science creates new demands, both in the form of new types of system and in terms of the complexity and subtlety of what is studied. The author's brief survey looks at some of the successes and gaps from interfaces and catalysts to neurotransmitters and from superconductors to slags.

Computer Simulation↗