582 research outputs found
Energy landscape of relaxed amorphous silicon
We analyze the structure of the energy landscape of a well-relaxed 1000-atom
model of amorphous silicon using the activation-relaxation technique (ART
nouveau). Generating more than 40,000 events starting from a single minimum, we
find that activated mechanisms are local in nature, that they are distributed
uniformly throughout the model and that the activation energy is limited by the
cost of breaking one bond, independently of the complexity of the mechanism.
The overall shape of the activation-energy-barrier distribution is also
insensitive to the exact details of the configuration, indicating that
well-relaxed configurations see essentially the same environment. These results
underscore the localized nature of relaxation in this material.Comment: 8 pages, 12 figure
Liquid-liquid phase transition in Stillinger-Weber silicon
It was recently demonstrated that the Stillinger-Weber silicon undergoes a
liquid-liquid first-order phase transition deep into the supercooled region
(Sastry and Angell, Nature Materials 2, 739 (2003)). Here we study the effects
of perturbations on this phase transition. We show that the order of the
liquid-liquid transition changes with negative pressure. We also find that the
liquid-liquid transition disappears when the three-body term of the potential
is strengthened by as little as 5 %. This implies that the details of the
potential could affect strongly the nature and even the existence of the
liquid-liquid phase.Comment: 13 page
Village health workers : proceedings of a workshop
Meeting: Workshop on Village Health Workers, 6-13 Mar. 1976, Shiraz, IRCompilation of seminar papers on health services in rural areas of Iran, Nepal, the Philippines, Thailand and Papua New Guinea, with emphasis on the training of auxiliary health workers - includes (1) information on attitudes, motivations and educational level of health workers; training courses, curriculum (2) list of participants, statistical data
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A Comparative Study of the Harmonic and Arithmetic Averaging of Diffusion Coefficients for Non-linear Heat Conduction Problems
We perform a comparative study for the harmonic versus arithmetic averaging of the heat conduction coefficient when solving non-linear heat transfer problems. In literature, the harmonic average is the method of choice, because it is widely believed that the harmonic average is more accurate model. However, our analysis reveals that this is not necessarily true. For instance, we show a case in which the harmonic average is less accurate when a coarser mesh is used. More importantly, we demonstrated that if the boundary layers are finely resolved, then the harmonic and arithmetic averaging techniques are identical in the truncation error sense. Our analysis further reveals that the accuracy of these two techniques depends on how the physical problem is modeled
Strain Relaxation Mechanisms and Local Structural Changes in Si_{1-x}$Ge_{x} Alloys
In this work, we address issues pertinent to the understanding of the
structural and electronic properties of Si_{1-x} Ge_{x}alloys, namely, (i) how
does the lattice constant mismatch between bulk Si and bulk Ge manifests itself
in the alloy system? and (ii) what are the relevant strain release mechanisms?
To provide answers to these questions, we have carried out an in-depth study of
the changes in the local geometric and electronic structures arising from the
strain relaxation in Si_{1-x} Ge_{x} alloys using an ab initio molecular
dynamics scheme. The optimized lattice constant, while exhibiting a general
trend of linear dependence on the composition (Vegard's law), shows a negative
deviation from Vegard's law in the vicinity of x=0.5. We delineate the
mechanisms responsible for each one of the above features. We show that the
radial-strain relaxation through bond stretching is responsible for the overall
trend of linear dependence of the lattice constant on the composition. On the
other hand, the negative deviation from Vegard's law is shown to arise from the
angular-strain relaxation.Comment: 21 pages, 7 figure
Energetics and atomic mechanisms of dislocation nucleation in strained epitaxial layers
We study numerically the energetics and atomic mechanisms of misfit
dislocation nucleation and stress relaxation in a two-dimensional atomistic
model of strained epitaxial layers on a substrate with lattice misfit.
Relaxation processes from coherent to incoherent states for different
transition paths are studied using interatomic potentials of Lennard-Jones type
and a systematic saddle point and transition path search method. The method is
based on a combination of repulsive potential minimization and the Nudged
Elastic Band method. For a final state with a single misfit dislocation, the
minimum energy path and the corresponding activation barrier are obtained for
different misfits and interatomic potentials. We find that the energy barrier
decreases strongly with misfit. In contrast to continuous elastic theory, a
strong tensile-compressive asymmetry is observed. This asymmetry can be
understood as manifestation of asymmetry between repulsive and attractive
branches of pair potential and it is found to depend sensitively on the form of
the potential.Comment: 11 pages, 9 figures, to appear in Phys. Rev.
Political strategies of external support for democratization
Political strategies of external support to democratization are contrasted and critically examined in respect of the United States and European Union. The analysis begins by defining its terms of reference and addresses the question of what it means to have a strategy. The account briefly notes the goals lying behind democratization support and their relationship with the wider foreign policy process, before considering what a successful strategy would look like and how that relates to the selection of candidates. The literature's attempts to identify strategy and its recommendations for better strategies are compared and assessed. Overall, the article argues that the question of political strategies of external support for democratization raises several distinct but related issues including the who?, what?, why?, and how? On one level, strategic choices can be expected to echo the comparative advantage of the "supporter." On a different level, the strategies cannot be divorced from the larger foreign policy framework. While it is correct to say that any sound strategy for support should be grounded in a theoretical understanding of democratization, the literature on strategies reveals something even more fundamental: divergent views about the nature of politics itself. The recommendations there certainly pinpoint weaknesses in the actual strategies of the United States and Europe but they have their own limitations too. In particular, in a world of increasing multi-level governance strategies for supporting democratization should go beyond preoccupation with just an "outside-in" approach
Structural Transitions and Global Minima of Sodium Chloride Clusters
In recent experiments on sodium chloride clusters structural transitions
between nanocrystals with different cuboidal shapes were detected. Here we
determine reaction pathways between the low energy isomers of one of these
clusters, (NaCl)35Cl-. The key process in these structural transitions is a
highly cooperative rearrangement in which two parts of the nanocrystal slip
past one another on a {110} plane in a direction. In this way the
nanocrystals can plastically deform, in contrast to the brittle behaviour of
bulk sodium chloride crystals at the same temperatures; the nanocrystals have
mechanical properties which are a unique feature of their finite size. We also
report and compare the global potential energy minima for (NaCl)NCl- using two
empirical potentials, and comment on the effect of polarization.Comment: extended version, 13 pages, 8 figures, revte
Maternal environment shapes the life history and susceptibility to malaria of Anopheles gambiae mosquitoes
BACKGROUND: It is becoming generally recognized that an individual's phenotype can be shaped not only by its own genotype and environmental experience, but also by its mother's environment and condition. Maternal environmental factors can influence mosquitoes' population dynamics and susceptibility to malaria, and therefore directly and indirectly the epidemiology of malaria. METHODS: In a full factorial experiment, the effects of two environmental stressors - food availability and infection with the microsporidian parasite Vavraia culicis - of female mosquitoes (Anopheles gambiae sensu stricto) on their offspring's development, survival and susceptibility to malaria were studied. RESULTS: The offspring of A. gambiae s.s. mothers infected with V. culicis developed into adults more slowly than those of uninfected mothers. This effect was exacerbated when mothers were reared on low food. Maternal food availability had no effect on the survival of their offspring up to emergence, and microsporidian infection decreased survival only slightly. Low food availability for mothers increased and V. culicis-infection of mothers decreased the likelihood that the offspring fed on malaria-infected blood harboured malaria parasites (but neither maternal treatment influenced their survival up to dissection). CONCLUSIONS: Resource availability and infection with V. culicis of A. gambiae s.s. mosquitoes not only acted as direct environmental stimuli for changes in the success of one generation, but could also lead to maternal effects. Maternal V. culicis infection could make offspring more resistant and less likely to transmit malaria, thus enhancing the efficacy of the microsporidian for the biological control of malaria
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