3D atom probe tomography of swift heavy ion irradiated multilayers

International audienceNanometer scale layered systems are well suited to investigate atomic transport processes induced by high-energy electronic excitations in materials, through the characterization of the interface transformation. In this study, we used the atom probe technique to determine the distribution of the different elements in an (amorphous-Fe$_2$Tb 5 nm/hcp-Co 3 nm)$_{20}$ multilayer before and after irradiation with Pb ions in the electronic stopping power regime. Atom probe tomography is based on reconstruction of a small volume of a sharp tip evaporated by field effect. It has unique capabilities to characterize internal interfaces and layer chemistry with sub-nanometer scale resolution in three dimensions. Depth composition profiles and 3D element mapping have been determined, evidencing for asymetric interfaces in the as-deposited sample, and very efficient Fe-Co intermixing after irradiation at the fluence $7\times10^{12}$ ion cm$^{-2}$. Estimation of effective atomic diffusion coefficients after irradiation suggests that mixing results from interdiffusion in a molten track across the interface in agreement with the thermal spike model

An abrupt change in the African monsoon at the end of the Younger Dryas

International audienceHigh-resolution studies of variations in the elemental and stable carbon- and nitrogen-isotope composition of organic matter in cores from Lakes Malawi, Tanganyika, and Bosumtwi (tropical Africa) indicate an abrupt change in the wind-driven circulation of these lakes that, within the limits of available chronologies, was contemporaneous with the end of the Younger Dryas in the northern hemisphere. The change was also coincident with shifts in surface winds recorded in cores from off the west and northeast coasts of Africa. A range of other proxies indicate that these changes in wind regime were accompanied by a marked increase in precipitation in the northern tropics. Africa south of ~5°-10°S, on the other hand, initially suffered drought conditions. Together, the evidence suggests an abrupt northward translation of the African monsoon system at circa 11.5 ± 0.25 ka B.P. The data assembled here contribute to a growing body of work showing that the Younger Dryas was a major climatic excursion in tropical Africa. Furthermore, they add substance to recent suggestions that climatic events in the southern hemisphere may have played a significant role in the abrupt demise of the Younger Drya

“Was Peirce a Genuine Anti-Psychologist in Logic?”

The aim of the paper is to try and make one’s ideas clearer about such concepts as “logic,” “psychology,” “mind,” “normativity,” rationality,” as they were conceived by Peirce, in order to elucidate his genuine position as far as the relationship between logic and pychology is concerned, whether he was or was not a straightforward “anti psychologist” in logic, and from such analyses, to make some suggestions about the contemporary relevance of Peirce’s original views on such isues

Métaphysique et philosophie de la connaissance

Enseignement Cours et séminaire Dispense d’enseignement au Collège de France pour l’année 2017-2018. Colloques Trois colloques ont été organisés, ainsi que de nombreux ateliers. Les principes de l’épistémologie 28-29 septembre 2017 : troisième colloque international du Groupe de recherche en épistémologie (GRÉ). Il s’agissait d’examiner simultanément 1) les grands principes que dégagent les épistémologues à propos de la croyance et de sa révision ainsi qu’à propos de la connaissance, de ses c..

La métaphysique et les sciences. Les nouveaux enjeux

After a few historical reminders about the tense links that have always existed between metaphysics and the sciences, one suggests a few rules of mutual good conduct in order to avoid the ever present risks of scientism and apriorism, before stating the majors issues both metaphyscians and scientists have to face, if they wish to pay attention to some crucial problems surrounding language, knowledge or ethics, and to avoid idealism. Such issues are based on betting on two possibilities : of scientific realism as such, on the one hand, and the possibility, on the other hand, of some scientific metaphysics, capable of telling us, just as well as the sciences, what is true, indeed, of what we think about reality, but also and mostly, of reality as such.  Après quelques rappels historiques sur les liens tendus qui ont toujours existé entre la métaphysique et les sciences, on suggère quelques règles de bonne conduite mutuelle pour éviter les risques toujours présents de scientisme et d’apriorisme avant d’exposer les enjeux majeurs qui se présentent aux métaphysiciens comme aux savants attentifs à certains problèmes cruciaux touchant au langage, à la connaissance ou encore à l’éthique, et désireux d’échapper à l’idéalisme. Ces enjeux reposent sur le pari d’une double possibilité : celle du réalisme scientifique comme tel, et celle d’une métaphysique scientifique capable de nous dire, sans rien avoir à envier aux sciences, ce qui est vrai, certes, de ce que nous pensons de la réalité, mais aussi et surtout de la réalité en soi

Multi-basin depositional framework for moisture-balance reconstruction during the last 1300 years at Lake Bogoria, central Kenya Rift Valley

Multi-proxy analysis of sediment cores from five key locations in hypersaline, alkaline Lake Bogoria (central Kenya Rift Valley) has allowed reconstruction of its history of depositional and hydrological change during the past 1300years. Analyses including organic matter and carbonate content, granulometry, mineralogical composition, charcoal counting and high-resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them. Thesepalaeolimnological data were supplemented with information on present-day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column. A new age model based on Pb-210, Cs-137 and C-14 dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake. An extensive set of chronological tie points between the equivalent high-resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins, enabling interbasin comparison of sedimentation dynamics through time. The resulting reconstruction demonstrates considerable moisture-balance variability through time, reflecting regional hydroclimate dynamics over the past 1300years. Between ca 690 and 950AD, the central and southern basins of Lake Bogoria were reduced to shallow and separated brine pools. In the former, occasional near-complete desiccation triggered massive trona precipitation. Between ca 950 and 1100AD, slightly higher water levels allowed the build-up of high pCO(2) leading to precipitation of nahcolite still under strongly evaporative conditions. Lake Bogoria experienced a pronounced highstand between ca 1100 and 1350AD, only to recede again afterwards. For a substantial part of the time between ca 1350 and 1800AD, the northern basin was probably disconnected from the united central and southern basins. Throughout the last two centuries, lake level has been relatively high compared to the rest of the past millennium. Evidence for increased terrestrial sediment supply in recent decades, due to anthropogenic soil erosion in the wider Bogoria catchment, is a reason for concern about possible adverse impacts on the unique ecosystem of Lake Bogoria

Quelle place pour les adventices dans l’épidémiosurveillance ? Rapport au Comité National d’Epidémiosurveillance

La gestion des adventices est à juste titre considérée comme un frein majeur à l’adoption deméthodes intégrées de conduite des cultures peu dépendantes en produits phytosanitaires.Leur dynamique d’infestation se construisant sur plusieurs saisons culturales du fait du stock desemences dans le sol, les plantes adventices ne peuvent être gérées comme le seraient d’autresravageurs dépendant des seuls paramètres saisonniers pour déclencher leur épidémie. Pourautant notre groupe conclut que l’épidémiosurveillance des adventices peut être utile à unerationalisation des moyens de lutte. Elle peut permettre, sur le pas de temps court, d’optimiseren saison le choix tactique et le timing des interventions de tout ordre ayant un impact sur lesadventices et doit pour cela apparaître clairement dans les Bulletins de Santé du Végétal (BSV).Sa compilation sur plusieurs saisons permettra de mieux saisir l’évolution de la flore et depointer du doigt les facteurs responsables sous-jacents. Ceux-ci constituent autant d’éléments àintégrer dans l’analyse du risque et les préconisations stratégiques qui peuvent en découler. (...

Plasmon-driven creation of magnetic topological structures

In the present research, we demonstrate the usage of plasmonic effects in thin film structures to control magnetic topological textures, specifically skyrmions and skyrmioniums. We investigate numerically the generation and alteration of these topological structures caused by hemisphere gold nanoparticle placed over a magnetic layer coated with a dielectric material. The electromagnetic and photothermal models are used to clarify the processes of producing heat and absorption, and the results were implemented in micromagnetic formalism to reveal the dynamics of magnetization under various conditions. Our findings demonstrate the significance of the laser pulse duration and the contact area between nanoparticles and the underlying magnetic layer in forming topological textures. In particular, we show how to generate a single skyrmion, multiple skyrmions, and skyrmioniums, and how to dynamically transition between these states. These results highlight the possibility of manipulating magnetic textures by using plasmonic effects, which presents significant opportunities for spintronics and non-conventional computer applications

Entretien avec Claudine Tiercelin, professeure au Collège de France

Dans cet entretien réalisé par Simon Levesque pour le compte du Cygne noir, Claudine Tiercelin discute de l’importance de Charles S. Peirce sur sa pensée et dans le cheminement de sa carrière, mais également de la valeur générale des travaux du philosophe étatsunien du point de vue de l’histoire de la philosophie. Elle offre aussi des précisions sur la notion de méthode au regard de l’enquête, qui motive et dirige toute entreprise de connaissance, et des applications ou approfondissements vers lesquels l’ont portée ces enquêtes, notamment dans le choix des thèmes pour ses cours des dernières années au Collège de France