L'alimentation durable

L’alimentation est un sujet passionnant et très actuel. Elle fait partie de notre quotidien et impacte une multitude de facettes de notre vie. Les liens entre la nourriture, la santé et l’environnement sont très étroits et c’est cette complexité qui m’a particulièrement intéressé et poussé à faire un travail de recherche sur cette thématique. L’Homme n’a, durant toute l’Histoire, jamais eu une compréhension aussi précise du fonctionnement de la nature qu’aujourd’hui. Paradoxalement, il n’a jamais autant détruit l’environnement dans lequel il vivait. L’industrialisation du secteur agricole n’a pas pris en compte la question de la durabilité et les conséquences qui en découlent sont aujourd’hui un nouveau défi pour l’agriculture. Comment donc faire face à la demande de nourriture de manière durable ? Les solutions sont nombreuses mais nécessitent une réelle remise en question de nos modes actuels de production et de consommation

Series which are both max-plus and min-plus rational are unambiguous

Consider partial maps from the free monoid into the field of real numbers with a rational domain. We show that two families of such series are actually the same: the unambiguous rational series on the one hand, and the max-plus and min-plus rational series on the other hand. The decidability of equality was known to hold in both families with different proofs, so the above unifies the picture. We give an effective procedure to build an unambiguous automaton from a max-plus automaton and a min-plus one that recognize the same series

Bond-order modulated staggered flux phase for the t−Jt{-}J model on the square lattice

Motivated by the observation of inhomogeneous patterns in some high-T$_c$ cuprate compounds, several variational Gutzwiller-projected wave-functions with built-in charge and bond order parameters are proposed for the extended $t-J-V$ model on the square lattice at low doping. First, following a recent Gutzwiller-projected mean-field approach by one of us (Phys. Rev. B. {\bf 72}, 060508(R) (2005)), we investigate, as a function of doping and Coulomb repulsion, the stability of the staggered flux phase with respect to small spontaneous modulations of squared unit cells ranging from $2\times 2$ to $\sqrt{32}\times\sqrt{32}$. It is found that a $4\times 4$ bond-order (BO) modulation appears spontaneously on top of the staggered flux pattern for hole doping around 1/8. A related wave-function is then constructed and optimized accurately and its properties studied extensively using an approximation-free variational Monte Carlo scheme. Finally, the competition of the BO-modulated staggered flux wave-function w.r.t. the d-wave RVB wave-function or the commensurate flux state is investigated. It is found that a short range Coulomb repulsion penalizes the d-wave superconductor and that a moderate Coulomb repulsion brings them very close in energy. Our results are discussed in connection to the STM observations in the under-doped regime of some cuprates.Comment: 10 pages, 9 figure

Topologically protected edge states in small Rydberg systems

We propose a simple setup of Rydberg atoms in a honeycomb lattice which gives rise to topologically protected edge states. The proposal is based on the combination of dipolar exchange interaction, which couples the internal angular momentum and the orbital degree of freedom of a Rydberg excitation, and a static magnetic field breaking time reversal symmetry. We demonstrate that for realistic experimental parameters, signatures of topologically protected edge states are present in small systems with as few as 10 atoms. Our analysis paves the way for the experimental realization of Rydberg systems characterized by a topological invariant, providing a promising setup for future application in quantum information.Comment: 6 pages, 6 figure

Utilisation de l'imagerie IR pour l'étude de la fatigue des aciers : Premières observations d'effets de surface

International audienceDe nouvelles méthodes de caractérisation de la tenue à la fatigue des matériaux métalliques par mesure thermique sont développées depuis plusieurs années. Elles donnent accès en quelques dizaines de minutes à une estimation de la limite d'endurance et de la dispersion des résultats de fatigue d'un matériau, là où plusieurs jours sont nécessaires traditionnellement. Un des axes de développement de ces méthodes consiste à caractériser à l'aide de ces techniques les effets de surface sur les propriétés à la fatigue. Pour capter l'information due à l'hétérogénéité locale provoquée par le traitement de surface, il est nécessaire de passer d'une vision moyenne à des mesures de champ. Deux points clefs peuvent être considérés pour ce passage. Le premier concerne la mesure et est traité dans cet article. Le second concerne l'identification du champ de sources locales à partir du champ de température mesurée et n'est pas traité ici. Un protocole expérimental de mesure du champ de température d'une tôle mince avec une précision suffisante (incertitude de l'ordre du milliKelvin) est présenté pour discerner des hétérogénéités de propriété à la fatigue (p. ex. surface vs. cœur). L'article expose les différentes sources d'artefacts rencontrés et les techniques utilisées pour les supprimer

Dissipation measurements in steel sheets under cyclic loading by use of infrared microthermography

WOSInternational audienceHeterogeneous dissipation in steel sheets due to cyclic loading is difficult to measure, especially in the transverse direction because of the high conductivity and low thickness of the sheets. The goal of this article is thus to develop an experimental protocol allowing for the dissipation field determination from infra-red thermography. The protocol is based on a specific differential measurement and an asynchronous acquisition. It reduces measurement artefacts due to coating, rigid body motion, convection, and optical deleterious effects. It is eventually applied to different specimens

Probabilistic multiscale models and measurements of self-heating under multiaxial high cycle fatigue

WOSInternational audienceDifferent approaches have been proposed to link high cycle fatigue properties to thermal measurements under cyclic loadings, usually referred to as “self-heating tests.” This paper focuses on two models whose parameters are tuned by resorting to self-heating tests and then used to predict high cycle fatigue properties. The first model is based upon a yield surface approach to account for stress multiaxiality at a microscopic scale, whereas the second one relies on a probabilistic modelling of microplasticity at the scale of slip-planes. Both model identifications are cost effective, relying mainly on quickly-obtained temperature data in self-heating tests. They both describe the influence of the stress heterogeneity, the volume effect and the hydrostatic stress on fatigue limits. The thermal effects and mean fatigue limit predictions are in good agreement with experimental results for in and out-of phase tension-torsion loadings. In the case of fatigue under non-proportional loading paths, the mean fatigue limit prediction error of the critical shear stress approach is three times less than with the yield surface approach

Rapid multiaxial high cycle fatigue limit predictions using self-heating-based probabilistic multiscale models

Thermal measurements under multiaxial cyclic loadings are used herein to predict multiaxial fatigue properties. Two models describing random microplasticity activation via a Poisson Point Process. The thermal response is interpreted as the “mean” behaviour of the microplastic activity, whereas the fatigue limit relies on the weakest link assumption. The first model is based upon a yield surface approach to account for stress multiaxiality at a microscopic scale. The second one relies on a probabilistic modelling of microplasticity at the scale of slip-planes. Both models are identified on thermal results and a uniaxial mean fatigue limit, and then validated using fatigue limits as well as thermal responses in the case of tension-torsion loadings on tubular specimens made of medium carbon steel. They predict well hydrostatic stress, volume and proportional multiaxial effects. The model with microplasticity described at the scale of slip-planes also offers a good prediction of nonproportional mean fatigue limits (~ 5% error) whereas the other model is less predictive (~ 17% error)