Self-sustained oscillations in a Large Magneto-Optical Trap

We have observed self-sustained radial oscillations in a large magneto-optical trap (MOT), containing up to $10^{10}$ Rb$^{85}$ atoms. This instability is due to the competition between the confining force of the MOT and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms $N>10^9$ are required to observe this unstable behavior.Comment: submitted to Phys. Rev. Lett. 4 pages, 3 figure

Enhancing the area of a Raman atom interferometer using a versatile double-diffraction technique

IIn this paper we demonstrate a new scheme for Raman transitions which realize a symmetric momentum-space splitting of $4 \hbar k$, deflecting the atomic wave-packets into the same internal state. Combining the advantages of Raman and Bragg diffraction, we achieve a three pulse state labelled interferometer, intrinsically insensitive to the main systematics and applicable to all kind of atomic sources. This splitting scheme can be extended to $4N \hbar k$ momentum transfer by a multipulse sequence and is implemented on a $8 \hbar k$ interferometer. We demonstrate the area enhancement by measuring inertial forces

Mechanisms for Lasing with Cold Atoms as the Gain Medium

We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding respectively to Mollow, Raman and Four Wave Mixing mechanisms. We measure an output power of up to 300 $\mu$W and present the main properties of these different lasers in each regime

Sediment reworking by marine benthic species from the Gullmar Fjord (Western Sweden): Importance of faunal biovolume

In order to compare and quantify sediment reworking activities by different species/functional groups of macrofauna, a laboratory experiment was carried out with species from the Gullmarsfjord (Western Sweden). Monospecific communities of Amphiura filiformis, Echinocardium cordatum, Scalibregma inflatum and Abra nitida were introduced in experimental mesocosms, with identical densities (795 ind. m−2), for 10 days. Sediment reworking was studied by quantifying downward and upward movements of fluorescent inert tracers (luminophores). Luminophores with different colour were initially deposited both at the sediment surface and within the sediments. Population biomass and biovolume were also determined. Surface tracers reworking coefficients ranged from 0.6 to 2.2 cm2 y−1 and 0.9 to 4.1 y−1, respectively for the biodiffusive-like and non-local transports. Calculated biodiffusive-like coefficient was between 1.0 and 2.3 cm2 y−1 for the deep tracers. For both tracers, the E. cordatum population presented the highest reworking coefficients. Among the morphological and/or ethological parameters that could determine overall patterns of reworking and differences between species, results have shown a direct relationship between the apparent biodiffusive mixing and the biovolume of the individuals (Db=0.35 ⁎ Biovolume). This suggests that the biovolume of macrofauna may allow a rough estimate of the biodiffusive-like reworking intensity of particles deposited on the sediment surface

When a collective outcome triggers a rare individual event: a mode of metastatic process in a cell population

A model of early metastatic process is based on the role of the protein PAI-1, which at high enough extracellular concentration promotes the transition of cancer cells to a state prone to migration. This transition is described at the single cell level as a bi-stable switch associated with a subcritical bifurcation. In a multilevel reaction-diffusion scenario, the microenvironment of the tumor is modified by the proliferating cell population so as to push the concentration of PAI-1 above the bifurcation threshold. The formulation in terms of partial differential equations fails to capture spatio-temporal heterogeneity. Cellular-automata and agent-based simulations of cell populations support the hypothesis that a randomly localized accumulation of PAI-1 can arise and trigger the escape of a few isolated cells. Far away from the primary tumor, these cells experience a reverse transition back to a proliferative state and could generate a secondary tumor. The proposed role of PAI-1 in controlling this metastatic cycle is candidate to explain its role in the progression of cancer

Use of axial tomography to follow temporal changes of benthic communities in an unstable sedimentary environment (Baie des Ha! Ha!, Saguenay Fjord)

In the upper layer of the sediment column, organic matter recycling is greatly influenced by bioturbation. However, there are many physical changes in the nature of the sediment that may disturb benthic communities and create a biogeochemical imbalance. Following a very heavy rainfall between 26 and 29 July 1996, an intense flash flood in the Saguenay Fjord caused discharges of 6 million cubic metres of sediments into Baie des Ha! Ha!. Unstable sediment deposits located at the top of the delta of the Rivie`re des Ha! Ha! were sporadically exported to the deep basin. After this physical disturbance, meiobenthic and macrobenthic organisms progressively re-colonised the sediment column. To determine the impacts of such sedimentary depositions on benthic fauna, two stations, one at the head and one at the mouth of the Baie des Ha! Ha!, have been monitored since 1996. During this survey, we developed a new method for the quantification of biogenic structures using computer axial tomography (CAT-Scan). Benthic fauna analysis showed that the two stations were characterised by different temporal changes in the benthic dynamics according to their geographic location. Using CAT-Scan analysis of sediment cores, we were able to characterise the stability of the sediment column for the two stations in 1999 and 2000. Scan results suggest that colonisation processes were closely linked with the stability of the sediment column. Erosion and redeposition of surficial sediments caused a succession in the formation of biogenic structures. These variations were characterised for the first time using CAT-Scan, which is a nondestructive, rapid, and precise method. Tomographic analysis showed the importance of the production and destruction rates of biogenic structures and the sedimentation rate for the preservation of burrows and potentially reactive components. This study finally demonstrated that each erosional event could be followed by a rapid formation of biogenic structures, allowing the re-oxidation of old deposits

Rhéologie de panneaux composites bois/thermoplastiques sous chargement thermomécanique : aptitude au postformage

Ce travail est une étude exploratoire sur la possibilité de réaliser un composite bois polymère ou WPC, rencontrant certaines exigences, notamment la post-thermoformabilité. Nous avons élaboré un panneau composite fibre de bois/thermodurcissable/thermoplastique dans la logique d’un « éco-matériau ». Ce matériau innovant est destiné à un usage structurel ou décoratif. Il est fabriqué selon un procédé de pressage à chaud d’un matelas fibreux. La problématique de l’incompatibilité d’interface entre le bois et le thermoplastique, récurrente pour bon nombre de WPC, a été résolue grâce au développement d’une technique originale d’amélioration de l’adhésion par enrobage. Les améliorations ont permis d’obtenir des caractéristiques physico-mécaniques semblables aux panneaux de densité moyenne. La composante thermoplastique nous a permis d’octroyer une aptitude de post-thermoformabilité à ce matériau. L’analyse détaillée de la composition des panneaux WPC, de leurs propriétés à différentes températures, ainsi qu’un travail de modélisation expliquent, en partie, le comportement complexe de ce nouveau WPC.This project deals with a new type of Wood Polymer Composite (WPC) able to be post moulded. Based on wood industry processing we developed different wood/adhesive/thermoplastic fibreboards. This exploratory work leads to an innovative WPC fibreboard dedicated to structural or decorative use. In order to reach sufficient mechanical properties we managed to improve the cohesion properties. In fact, weak interfacial adhesion is a recurrent problem of all Wood/Thermoplastic products. An original coating process of MAPP onto PP fibres gave great interfacial adhesion enhancement and mechanical properties similar to MDF panels. The post thermoforming ability was measured with an original thermoforming test. These investigations, through characterisation and modelling, allowed us to determine the role of different factors and their interactions. Our post mouldable composite WPC fibreboards are porous and made-up of a fibre network which showed a complex rheological behaviour. Morphology, concentration, distribution of each components and surface interactions between them explain this behaviour

Metaheuristic methods applied to the environmentally conscious optimization of wood-plastic composite

This paper addresses the optimization of the quality of wood plastic composites(WPC) designed for outdoor uses such as decking, taking into account theenvironmental impact during the life cycle of the product, from production to end oflife. In a context where several conflicting objectives must be satisfied simultaneouslyin the design process, meta-heuristic approaches provide efficient methods foroptimization. Particle swarm optimization (PSO) has been chosen here to solve acomplex problem in which physical properties such as creep and duration of load,water absorption and swelling, need to be improved with a limited impact onenvironment. This requires to get reliable information on material properties as relatedto its composition, environmental impacts through life cycle analysis (LCA), and toimplement this information through analytical or probabilistic models in the PSOalgorithm in order to obtain a set of optimal solutions for the composite. This papershows the feasibility of this approach, which can be generalized in the design of anytype of composite structures, provided objective functions can be specified

Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones

We perform an extensive test of theoretical stellar models for main-sequence stars in ugriz, using cluster fiducial sequences obtained in the previous paper of this series. We generate a set of isochrones using the Yale Rotating Evolutionary Code (YREC) with updated input physics, and derive magnitudes and colors in ugriz from MARCS model atmospheres. These models match cluster main sequences over a wide range of metallicity within the errors of the adopted cluster parameters. However, we find a large discrepancy of model colors at the lower main sequence (Teff < ~4500 K) for clusters at and above solar metallicity. We also reach similar conclusions using the theoretical isochrones of Girardi et al. and Dotter et al., but our new models are generally in better agreement with the data. Using our theoretical isochrones, we also derive main-sequence fitting distances and turn-off ages for five key globular clusters, and demonstrate the ability to derive these quantities from photometric data in the Sloan Digital Sky Survey. In particular, we exploit multiple color indices (g - r, g - i, and g - z) in the parameter estimation, which allows us to evaluate internal systematic errors. Our distance estimates, with an error of sigma(m - M) = 0.03-0.11 mag for individual clusters, are consistent with Hipparcos-based subdwarf fitting distances derived in the Johnson-Cousins or Stromgren photometric systems.Comment: 26 pages, 28 figures. Accepted for publication in ApJ. Version with high resolution figures available at http://spider.ipac.caltech.edu/~deokkeun/sdss_iso.pd

Optimisation multi-objectifs de propriétés d'un matériau par essaim particulaire : application aux composites à base de bois

L’optimisation multi-objectifs, appliquée aux propriétés d'un matériau, permet l'exploration de l'espace de recherche et l’obtention d’un ensemble de solutions Pareto-optimales. Un espace de recherche étant défini par des équations algébriques, des connaissances qualitatives et des algorithmes, il peut être modélisé dans un même système d'optimisation par essaim particulaire. Nous présenterons les principes de modélisation que nous illustrerons avec l'écoconception de composite bois-plastique