6,441 research outputs found
Dynamical stabilization of classical multi electron targets against autoionization
We demonstrate that a recently published quasiclassical M\oller type approach
[Geyer and Rost 2002, J. Phys. B 35 1479] can be used to overcome the problem
of autoionization, which arises in classical trajectory calculations for many
electron targets. In this method the target is stabilized dynamically by a
backward--forward propagation scheme. We illustrate this refocusing and present
total cross sections for single and double ionization of helium by electron
impact.Comment: LaTeX, 6 pages, 2 figures; submitted to J. Phys.
The role of collaboration in the South African tooling industry
Published ArticleThe efficient design and manufacturing of injection moulds (and other tooling), requires collaboration between multi-disciplinary, geographically separated units and companies, including the company that orders plastic parts, the ones that design and/or produce the moulds and/or mould-components, and those that use injection moulding machines to produce components. Distributed collaborative tooling design and manufacturing, while being a great concept, is still difficult to implement in today's complex environment. This is clear as more functional groups become involved in the total product development process earlier to meet time-to-market and cost objectives. Although internal design and manufacturing groups may have experience in collaborating with different degrees of success, the global trend is towards greater outsourcing of activities and inclusion of complementary organisations. The requirement for extended collaboration - the process of securely and selectively exchanging, reviewing, and managing the change of product information with other internal participants and external groups such as customers, design partners, suppliers and distributed manufacturing companies - presents huge challenges. Delivering these capabilities in today's global product development processes requires a different approach to doing business than a few years ago. Companies must be willing to share more information, which "is hard because we are competitors." The toolmaking companies' in South Africa (of which 90% are small and medium size enterprises (SMEs)) concern for success in delivering these capabilities, can also be greatly impacted by the underlying software or information technology infrastructure required for supporting the entire product development process. This article provides a perspective for SMEs and others that might participate in collaborative tool design and manufacture in South Africa today or in the future
Detecting multivariate interactions in spatial point patterns with Gibbs models and variable selection
We propose a method for detecting significant interactions in very large
multivariate spatial point patterns. This methodology develops high dimensional
data understanding in the point process setting. The method is based on
modelling the patterns using a flexible Gibbs point process model to directly
characterise point-to-point interactions at different spatial scales. By using
the Gibbs framework significant interactions can also be captured at small
scales. Subsequently, the Gibbs point process is fitted using a
pseudo-likelihood approximation, and we select significant interactions
automatically using the group lasso penalty with this likelihood approximation.
Thus we estimate the multivariate interactions stably even in this setting. We
demonstrate the feasibility of the method with a simulation study and show its
power by applying it to a large and complex rainforest plant population data
set of 83 species
Modified NASA-Lewis chemical equilibrium code for MHD applications
A substantially modified version of the NASA-Lewis Chemical Equilibrium Code was recently developed. The modifications were designed to extend the power and convenience of the Code as a tool for performing combustor analysis for MHD systems studies. The effect of the programming details is described from a user point of view
Entropic effects in large-scale Monte Carlo simulations
The efficiency of Monte Carlo samplers is dictated not only by energetic
effects, such as large barriers, but also by entropic effects that are due to
the sheer volume that is sampled. The latter effects appear in the form of an
entropic mismatch or divergence between the direct and reverse trial moves. We
provide lower and upper bounds for the average acceptance probability in terms
of the Renyi divergence of order 1/2. We show that the asymptotic finitude of
the entropic divergence is the necessary and sufficient condition for
non-vanishing acceptance probabilities in the limit of large dimensions.
Furthermore, we demonstrate that the upper bound is reasonably tight by showing
that the exponent is asymptotically exact for systems made up of a large number
of independent and identically distributed subsystems. For the last statement,
we provide an alternative proof that relies on the reformulation of the
acceptance probability as a large deviation problem. The reformulation also
leads to a class of low-variance estimators for strongly asymmetric
distributions. We show that the entropy divergence causes a decay in the
average displacements with the number of dimensions n that are simultaneously
updated. For systems that have a well-defined thermodynamic limit, the decay is
demonstrated to be n^{-1/2} for random-walk Monte Carlo and n^{-1/6} for Smart
Monte Carlo (SMC). Numerical simulations of the LJ_38 cluster show that SMC is
virtually as efficient as the Markov chain implementation of the Gibbs sampler,
which is normally utilized for Lennard-Jones clusters. An application of the
entropic inequalities to the parallel tempering method demonstrates that the
number of replicas increases as the square root of the heat capacity of the
system.Comment: minor corrections; the best compromise for the value of the epsilon
parameter in Eq. A9 is now shown to be log(2); 13 pages, 4 figures, to appear
in PR
Carrier multiplication yields in PbS and PbSe nanocrystals measured by transient photoluminescence
We report here an assessment of carrier multiplication (CM) yields in PbSe
and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton
dynamics in transient photoluminescence decays. Interest in CM, the generation
of more than one electron and hole in a semiconductor after absorption of one
photon, has renewed in recent years because of reports suggesting greatly
increased efficiencies in nanocrystalline materials compared to the bulk form,
in which CM was otherwise too weak to be of consequence in photovoltaic energy
conversion devices. In our PbSe and PbS NC samples, however, we estimate using
transient photoluminescence that at most 0.25 additional e-h pairs are
generated per photon even at energies hv > 5Eg, instead of the much higher
values reported in the literature. We argue by comparing NC CM estimates and
reported bulk values on an absolute energy basis, which we justify as
appropriate on physical grounds, that the data reported thus far are
inconclusive with respect to the importance of nanoscale-specific phenomena in
the CM process.Comment: 10 pages, 7 figure
A Modified Scheme of Triplectic Quantization
A modified version of triplectic quantization, first introduce by Batalin and
Martnelius, is proposed which makes use of two independent master equations,
one for the action and one for the gauge functional such that the initial
classical action also obeys that master equation.Comment: 8 page
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