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Exciton band structure in two-dimensional materials
No full textInternational audienceLow-dimensional materials differ from their bulk counterpart in many respects. In particular, thescreening of the Coulomb interaction is strongly reduced, which can have important consequencessuch as the significant increase of exciton binding energies. In bulk materials the binding energyis used as an indicator in optical spectra to distinguish different kinds of excitons, but this is notpossible in low-dimensional materials, where the binding energy is large and comparable in size forexcitons of very different localization. Here we demonstrate that the exciton band structure, whichcan be accessed experimentally, instead provides a powerful way to identify the exciton character.By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane andsingle-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensionalmaterials, highlighting the different role played by the exchange electron-hole interaction and by theelectronic band structure. Our interpretation is substantiated by a prediction for phosphorene
Measurement of top quark polarisation in t-channel single top quark production
No full textSubmitted to J. High Energy Phys ; see paper for full list of authorsInternational audienceA first measurement of the top quark spin asymmetry, sensitive to the top quark polarisation, in t-channel single top quark production is presented. It is based on a sample of pp collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7 inverse-femtobarns. A high-purity sample of t-channel single top quark events with an isolated muon is selected. Signal and background components are estimated using a fit to data. A differential cross section measurement, corrected for detector effects, of an angular observable sensitive to the top quark polarisation is performed. The differential distribution is used to extract a top quark spin asymmetry of 0.26 +/- 0.03 (stat) +/- 0.10 (syst), which is compatible with a p-value of 4.6% with the standard model prediction of 0.44
Search for a very light NMSSM Higgs boson produced in decays of the 125 GeV scalar boson and decaying into tau leptons in pp collisions at sqrt(s) = 8 TeV
No full textsee paper for full list of authorsInternational audienceA search for a very light Higgs boson decaying into a pair of tau leptons is presented within the framework of the next-to-minimal supersymmetric standard model. This search is based on a data set corresponding to an integrated luminosity of 19.7 inverse-femtobarns of proton-proton collisions collected by the CMS experiment at a centre-of-mass energy of 8 TeV. The signal is defined by the production of either of the two lightest scalars, h[1] or h[2], via gluon-gluon fusion and subsequent decay into a pair of the lightest Higgs bosons, a[1] or h[1]. The h[1] or h[2] boson is identified with the observed state at a mass of 125 GeV. The analysis searches for decays of the a[1] (h[1]) states into pairs of tau leptons and covers a mass range for the a[1] (h[1]) boson of 4 to 8 GeV. The search reveals no significant excess in data above standard model background expectations, and an upper limit is set on the signal production cross section times branching fraction as a function of the a[1] (h[1]) boson mass. The 95\% confidence level limit ranges from 4.5 pb at m(a[1]) (m(h[1])) = 8 GeV to 10.3 pb at m(a[1]) (m(h[1])) = 5 GeV
A robust inversion method according to a new notion of regularization for Poisson data with an application to nanoparticle volume determination
No full textInternational audienceIn this paper we present an efficient method for the reconstruction of the volume distribution of diluted polydisperse noninteracting nanoparticles with identical shapes from small angle X-ray scattering measurements. The described method solves a maximum likelihood problem with a positivity constraint on the solution by means of an expectation maximization iterative scheme coupled with a robust stopping criterion. We prove that this is a regularization method according to an innovative notion of regularization specifically defined for inverse problems with Poisson data. Such a regularization, together with an upper bound to the largest retrievable particle size given by the Shannon theorem, results in high fidelity quantitative reconstructions of particle volume distributions, making the method particularly effective in real applications. We test the performance of the method on synthetic data in the case of uni- and bi-modal particle volume distributions. Moreover, we show the reliability of the method on real data provided by a Xenocs device prototype
Computational Fluid Dynamic Simulations of Maternal Circulation: Wall Shear Stress in the Human Placenta and Its Biological Implications
No full textInternational audienceIn the human placenta the maternal blood circulates in the intervillous space (IVS). The syncytiotrophoblast (STB) is in direct contact with maternal blood. The wall shear stress (WSS) exerted by the maternal blood flow on the STB has not been evaluated. Our objective was to determine the physiological WSS exerted on the surface of the STB during the third trimester of pregnancy
Experimental and theoretical evidence of vortex breakdown in turbulent flow past the 25◦ Ahmed body
No full textInternational audienceno abstrac
Supercritical CO2 extraction of molybdenum-ligand complexes from sulfuric solutions
No full textInternational audienceThe development of environmental-friendly process for strategic metal extraction, limiting organic solvent use and effluent production, appears to be quite a challenging purpose. Production of pure molybdenum using supercritical CO2 extraction process, from sulfuric solution obtained by ore or used catalyst leaching, has been evaluated. Two organic ligands, trioctylamine and 2-ethylhexyl 2-ethylhexylphosphonic acid, were studied to extract Mo as metal complexes solubilized in supercritical CO2. Extraction with trioctylamine revealed to be non-efficient due to a lack of Mo-trioctylamine complex solubility in CO2, even if extraction selectivity of molybdenum versus iron impurity was interesting. On the contrary, extraction with 2-ethylhexyl 2-ethylhexylphosphonic acid leads to high Mo collection efficiency (up to 90% in 7 h). However, no selectivity was observed between molybdenum and zirconium. The selectivity of 2-ethylhexyl 2-ethylhexylphosphonic acid towards molybdenum in presence of iron was better but clearly decreased in presence of zirconium
Climate Policy Must Favour Mitigation Over Adaptation
No full textIn climate change policy, adaptation tends to be viewed as beingas important as mitigation. In this article we present a simple yet generalargument for which mitigation must be preferred to adaptation.The argument rests on the observation that mitigation is a public goodwhile adaptation is a private one. This implies that the more one disaggregatesthe units in a social welfare function, i.e. the more one teasesout the public good nature of mitigation, the lower is average incomeand thus less money (per region, country or individual) is available foradaptation and mitigation. We show that, while this reduces incentivesto invest in the private good adaptation, it increases incentivesto invest in the public good mitigation since even small contributionsof everyone can have significant impacts at the large. Conclusively,private adaptation thus must be viewed as a significant loss to globalwelfare. When taking this result to the data we find that a representativepolicy maker who relies on world-aggregated data would invest inboth adaptation and mitigation, just as the previous literature recommends.However, a representative policy maker who relies on countryleveldata, or data at further levels of disaggregation, would optimallyonly invest in mitigation
Quasi-configurations: building blocks for point-line configurations
No full textInternational audienceWe study point-line incidence structures and their properties in the projective plane. Our motivation is the problem of the existence of configurations, still open for few remaining values of . Our approach is based on quasi-configurations: point-line incidence structures where each point is incident to at least lines and each line is incident to at least points. We investigate the existence problem for these quasi-configurations, with a particular attention to -configurations where each element is -or -valent. We use these quasi-configurations to construct the first and configurations. The existence problem of finding , , and configurations remains open
Propagation laser en plasma sous-dense et modélisation de déflectométrie protonique
No full textThe understanding and the control of high-power laser propagation into under-dense plasma is important to achieve inertial confinement fusion. During this process, the interaction of the laser with the plasma filling the hohlraum can lead to significant losses of laser energy which prevent ignition. Self-focusing or filamentation of the laser light is one of these phenomena which are desired to be mitigated since they also affect the uniformity of the laser illumination on the hohlraum wall.In order to improve our understanding of the laser-plasma interaction phenomena at play, we describe an experimental and numerical study involving an intense laser pulse between 1014 W.cm-2 and 1016 W.cm-2 , and which interacts with millimetric and under-dense plasma (having density of few % of the critical density). This work presents two experiments fielding a series of diagnostics aimed at well characterizing the laser propagation (Hisac camera) together with heat deposition in plasmas using Thomson scattering. Experimental results will be presented and discussed in the light of detailed simulations performed with the 3D laser propagation code Hera. In order to take into account the temperature gradients within the plasma during the laser propagation, Hera (laser propagation code) and FCI2 (radiation-hydrodynamic code) have been coupled. Besides, proton radiography has been used in order to access to electric fields. The measurements led to the implementation of a new and promising numerical tool using the Hera and Diane codes (Diane is a Monte Carlo particle tracing code). 3D proton radiography modelling opens new possibilities for users of this temporally and spatially resolved diagnostic.Dans le cadre de la Fusion par Confinement Inertiel, la maîtrise de la propagation des faisceaux laser intenses, qui se propagent dans le plasma sous-dense d'une cavité d'ignition, reste un enjeu majeur. En effet, durant leur propagation, les faisceaux vont modifier les paramètres du plasma, ces paramètres contrôlant eux-mêmes la propagation. Cette rétroaction entre le plasma et le laser est potentiellement instable et produit de l'autofocalisation, de la filamentation, etc. Ces mécanismes peuvent alors dégrader fortement la propagation et in fine l'uniformité du dépôt d'énergie recherché.Dans cette étude, expérimentale et numérique, nous nous intéressons à la propagation d'un filament laser (tache focale d'une dizaine de micromètres, impulsion de 1,5 ns et d'intensité variant de 1014 W.cm-2 à 1016 W.cm-2) dans un milieu très sous dense (quelques pour-cents de la densité électronique critique). Deux expériences ont été réalisées sur l'installation de puissance LULI2000 et ont permis à la fois d'observer la transmission laser et de caractériser les gradients de température. Ces gradients sont produits par le transport électronique et influencent la réponse du plasma au laser. Afin de modéliser le rôle des gradients de température dans les instabilités d'auto-focalisation et de filamentation, nous avons associé un code d'hydrodynamique-radiative (FCI2) à un code détaillé de propagation (Héra). Pour compléter les observables précédentes, nous avons aussi mis en œuvre un diagnostic de déflectométrie protonique. Ce diagnostic permet de mesurer les champs électriques présents dans le plasma, ces champs étant issus du chauffage et de l'interaction laser. Un nouvel outil, associant le code de propagation laser et un code Monte-Carlo de transport de protons, a donc été mis en place pour modéliser les déflexions des protons. Cet outil ouvre en plus de nouvelles perspectives pour discuter de l'influence des effets 3D dans l'exploitation de ce diagnostic. Les résultats obtenus confirment l'intérêt à sonder les champs électriques au cœur même du plasma pendant la propagation laser