Comparison between classical potentials and ab initio for silicon under large shear

The homogeneous shear of the {111} planes along the <110> direction of bulk silicon has been investigated using ab initio techniques, to better understand the strain properties of both shuffle and glide set planes. Similar calculations have been done with three empirical potentials, Stillinger-Weber, Tersoff and EDIP, in order to find the one giving the best results under large shear strains. The generalized stacking fault energies have also been calculated with these potentials to complement this study. It turns out that the Stillinger-Weber potential better reproduces the ab initio results, for the smoothness and the amplitude of the energy variation as well as the localization of shear in the shuffle set

La conservation d'une espèce patrimoniale en déclin au sein d'un agrosystème. Le cas de la Guifette noire Chlidonias niger (Linnaeus, 1758) dans le Marais poitevin

National audienceThe Marais poitevin, the second largest wetland in France, has been progressively cultivated, and can be considered as an agrosystem. Humid grasslands (essentially submitted to cattle grazing), are considered as the last remaining wetlands of the area, and biodiversity conservation measures are now focusing on them. Among the patrimonial grassland species, we interested in the Black tern Chlidonias niger (Linnaeus, 1758). Several conservation measures (protected areas, environmental management) have been deployed in the area to conserve this threatened species. However, despite some positive effects of these measures, the species is still signifi cantly declining and is now concentrating in areas benefi ting from an environmental management. The example of such a wetland species living in an agrosystem raises the question of the conservation policies to adopt in the current context of large-scale land-use changes. Should we develop interventionist management measures or preferring the restoration of habitat functionalities to conserve threatened species?Le Marais poitevin, deuxième zone humide de France, a été progressivement mis en culture au point de devenir un agrosystème. Les prairies humides (pour l'essentiel pâturées), considérées comme des vestiges de l'ancienne zone humide, sont les espaces sur lesquels se concentrent les mesures de conservation de la biodiversité du marais. Parmi les espèces patrimoniales associées à ces milieux, nous présentons l'exemple de la Guifette noire Chlidonias niger (Linnaeus, 1758). Cette espèce menacée a été à l'origine de multiples mesures de conservation (aires protégées et aménagements lui étant spécialement consacrés) dans le marais. Cette étude révèle que de telles mesures, bien qu'ayant des effets positifs, ne parviennent toutefois pas à enrayer le déclin signifi catif de la population qui se concentre de plus en plus sur les espaces bénéfi ciant d'une gestion environnementale. L'exemple de cette espèce de zone humide, aujourd'hui présente au sein de milieux de substitution qu'offre un agrosystème, pose la question des choix de politiques conservatoires qui doivent être opérés dans un contexte de changement d'occupation du sol à large échelle. Doit-on encore développer des mesures de gestion interventionniste ou privilégier la restauration de la fonctionnalité de milieux pour conserver des espèces menacées

Dislocation formation from a surface step in semiconductors: an ab initio study

The role of a simple surface defect, such as a step, for relaxing the stress applied to a semiconductor, has been investigated by means of large scale first principles calculations. Our results indicate that the step is the privileged site for initiating plasticity, with the formation and glide of 60$^\circ$ dislocations for both tensile and compressive deformations. We have also examined the effect of surface and step termination on the plastic mechanisms

A fully molecular dynamics-based method for modeling nanoporous gold

International audienceModels that can be used to describe nanoporous gold are often generated either by phase-field or Monte-Carlo methods. It is not ascertained that these models are closely matching experimental systems, and there is a need for other variants. Here is proposed an original approach to generate alternative models, which is solely based on molecular dynamics simulations. Structures obtained with this method are structurally characterized by determining the ligaments diameter distributions, the scaled genus densities and the interfacial shape distributions. Selected mechanical characterizations are also done by deforming the structures in tension and in compression. Structural and mechanical properties are in good agreement with experimental and theoretical published results

La conservation d’une espèce patrimoniale en déclin au sein d’un agrosystème. Le cas de la Guifette noire Chlidonias niger (Linnaeus, 1758) dans le Marais poitevin

Le Marais poitevin, deuxième zone humide de France, a été progressivement mis en culture au point de devenir un agrosystème. Les prairies humides (pour l’essentiel pâturées), considérées comme des vestiges de l’ancienne zone humide, sont les espaces sur lesquels se concentrent les mesures de conservation de la biodiversité du marais. Parmi les espèces patrimoniales associées à ces milieux, nous présentons l’exemple de la Guifette noire Chlidonias niger (Linnaeus, 1758). Cette espèce menacée a été à l’origine de multiples mesures de conservation (aires protégées et aménagements lui étant spécialement consacrés) dans le marais. Cette étude révèle que de telles mesures, bien qu’ayant des effets positifs, ne parviennent toutefois pas à enrayer le déclin significatif de la population qui se concentre de plus en plus sur les espaces bénéficiant d’une gestion environnementale. L’exemple de cette espèce de zone humide, aujourd’hui présente au sein de milieux de substitution qu’offre un agrosystème, pose la question des choix de politiques conservatoires qui doivent être opérés dans un contexte de changement d’occupation du sol à large échelle. Doit-on encore développer des mesures de gestion interventionniste ou privilégier la restauration de la fonctionnalité de milieux pour conserver des espèces menacées ?The Marais poitevin, the second largest wetland in France, has been progressively cultivated, and can be considered as an agrosystem. Humid grasslands (essentially submitted to cattle grazing), are considered as the last remaining wetlands of the area, and biodiversity conservation measures are now focusing on them. Among the patrimonial grassland species, we interested in the Black tern Chlidonias niger (Linnaeus, 1758). Several conservation measures (protected areas, environmental management) have been deployed in the area to conserve this threatened species. However, despite some positive effects of these measures, the species is still significantly declining and is now concentrating in areas benefiting from an environmental management. The example of such a wetland species living in an agrosystem raises the question of the conservation policies to adopt in the current context of large-scale land-use changes. Should we develop interventionist management measures or preferring the restoration of habitat functionalities to conserve threatened species

Quantifier les besoins énergétiques des limicoles.

National audienceLa plupart des limicoles de l'hémisphère nord nichent dans la toundra circumpolaire et passent la période de non-reproduction sur les escales migratoires et les sites d'hivernage en domaine tempéré à tropical. Pendant cette longue période internuptiale, si quelques limicoles peuvent exploiter des habitats continentaux humides, la majorité dépend des domaines intertidaux de substrat meuble ou dur, et plus particulièrement des grands sites estuariens. Au sein du compartiment benthique d'un estuaire européen classique, si on exclut les pinnipèdes, on trouve schématiquement trois grands échelons trophiques : les oiseaux et les poissons, le macrozoobenthos et les algues (microalgues et macroalgues). Cette chaîne trophique courte s'explique en grande partie par la grande diversité d'oiseaux s'alimentant en domaine intertidal. Puisque les limicoles comptent parmi les principaux prédateurs du compartiment benthique des systèmes estuariens, l'évaluation de l'énergie qu'ils prélèvent par rapport à la ressource disponible est essentielle dans l'étude des réseaux trophiques intertidaux. Dans ce chapitre, nous proposons une courte revue des méthodes classiquement utilisées pour estimer l'énergie prélevée par une communauté de limicoles en fonction de la ressource et de l'espace benthiques disponibles au sein d'un domaine intertidal donné. Nous détaillerons successivement comment quantifier les besoins énergétiques des limicoles, puis comment quantifier la valeur énergétique du macrobenthos et de sa fraction récoltable par les limicoles, et enfin prendre en compte la superficie intertidale réellement disponible pour les limicoles

BENTHIC MACROFAUNA CONSUMPTION BY WATER BIRDS

The diversity and abundance of birds present in intertidal coastal ecosystems are closely related to the biomass of benthic invertebrates. The assessment of energy consumed compared with the available resource is one of the fundamental aspects of intertidal foodweb studies. The feeding of birds on benthic invertebrates was studied in the bay of Saint-Brieuc, a 2900ha tidal bay located on the Côtes d'Armor coast (Brittany). The consumption of the nine most numerous wader and duck species present on the site was valued at 1,9gAFDW/m²/yr. This is comparable with results observed in the Mont Saint-Michel bay but much lower than results from the Wadden sea. In order to further the understanding of the predator-prey system, the taking into account of local hypsometric singularities is relevant. Thus it is possible to determine an "average available foraging area" which takes into account the foreshore exposure frequency according to tidal conditions, which represents the real usage of the foreshore by birds. In the Bay of Saint-Brieuc foreshore, the available average surface is about 1115ha, i.e. a consumption of 4,9g/m²/yr. The use of this space by birds depends on the type, density and the accessibility of prey, the sediment characteristics, and the presence of events which can cause a disturbance. The distribution of the benthic macrofauna and spatial distribution of four wader species were studied to map the main feeding areas and the benthos they host. This data set will allow the study of potential feeding habitat compared with currently exploited habitat, and to analyse the compatibility of a complex benthic resources/birds/human activities system

Toward the understanding of the brittle to ductile transition at low size in silicon: Experiments and simulations

While bulk silicon is brittle at temperatures below 600-700K, the compression of nanopillars has shown that a decrease of the diameter below few hundreds of nanometers could change the silicon behavior from brittle to ductile [1,2]. This size effect cannot be explained by the initial defect density like in metals, because pristine silicon nano-objects do not contain residual defects. In these conditions the cracks and/or the dislocations nucleation should take origin at the surface. The identification of the parameters governing the brittle to ductile transition in size and the understanding of the mechanisms are the key points to further develop the MEMS and NEMS technology or to prevent the failure of microelectronic components based on the silicon strained technology. Nowadays the respective improvements in simulations and experiments allow to investigate the mechanical properties of objects of similar sizes, close to hundreds of nanometers. We have then used both approaches - experiments and simulations – to understand the mechanisms at the origin of cracks and dislocations nucleation in such nanopillars. Experimentally,nanopillars with diameters of 100 nm and heights of 300 nm are obtained by lithography. They are deformed in compression by a flat punch nano-indentor under controlled-displacement mode at room temperature, and analyzed by scanning electron microscopy and high resolution transmission electron microscopy. In simulation, nanopillars up to 44 nm in diameter and height are investigated under compression and tension in controlled-displacement too, with a temperature ranging from 1 to 600K. The atomic interactions in silicon are modeled by two different semi-empirical potentials, Stillinger Weber and a Modified Embedded-Atom-Method (MEAM), both fitted to better reproduce the ductile and brittle properties of bulk silicon. Under compressive load (Fig. 1), both approaches reveal a ductile behavior with similar stress-strain curves, and large shear bands of amorphous silicon along the slip plane. In addition the simulations enlighten the formation of stacking fault plane in the anti-twining shear stress direction at the onset of plasticity, not yet confirmed by experiments (work in progress). The simulations under tensile load (Fig. 2) show the nucleation of perfect dislocations from the surface that can lead to cavity opening when they interact [3]. We observe first that the height of the nanopillars must be higher than 20 nm to allow the cavity opening, and second that the brittle to ductile transition is controlled by the diameter of the nanopillars, as observed experimentally in compression. The deformation of pillars with large diameters operates by cavity expansion leading to the brittle fracture, while pillars with smaller diameters are deformed by dislocations gliding leading to ductile fracture. Finally, the simulations in temperature seem to corroborate the fact that the size of the brittle to ductile transition could increase with temperature, as presumed experimentally [2]

Glissile dislocations with transient cores in silicon

We report an unexpected characteristic of dislocation cores in silicon. Using first-principles calculations, we show that all the stable core configurations for a non-dissociated 60$^\circ$ dislocation are sessile. The only glissile configuration, previously obtained by nucleation from surfaces, surprinsingly corresponds to an unstable core. As a result, the 60$^\circ$ dislocation motion is solely driven by stress, with no thermal activation. We predict that this original feature could be relevant in situations for which large stresses occur, such as mechanical deformation at room temperature. Our work also suggests that post-mortem observations of stable dislocations could be misleading, and that mobile unstable dislocation cores should be taken into account in theoretical investigations