Aperiodic quantum oscillations of particle-hole asymmetric Dirac cones

We report experimental measurements and theoretical analysis of Shubnikov-de Haas (SdH) oscillations in a Dirac cone system: the a-(BEDT-TTF)2I3 organic metal under hydrostatic pressure. The measured SdH oscillations reveal anomalies at high magnetic fields B where the 1/B oscillations periodicity is lost above 7 T. We interpret these unusual results within a theoretical model that takes into account intrinsic distortions of the a-(BEDT-TTF)2I3 Dirac cones such as a parabolic particle-hole asymmetric correction. Others possible causes, such as a cone tilting or a Zeeman effect, are carefully ruled out. The observations are consistent among a-(BEDT-TTF)2I3 samples with different Fermi levels

Haut parleur tout aimant guidé sur joint ferrofluide : caractérisation dynamique du guidage

Une étude expérimentale sur le comportement visqueux de ferrofluides, au sein d'une goutte insérée entre deux plans parallèles, en condition de cisaillement oscillatoire et sous champ magnétique homogène et stationnaire, permet d'alimenter un modèle pour les pertes d'un haut-parleur tout aimant guidé sur joint ferrofluide. Ces joints sont localisés dans les zones de fort gradient de champs magnétique. Le modèle de perte, fonction du volume, de la viscosité et de la saturation magnétique du ferrofluide, est alors validé par confrontation avec une expérience

A trait-based approach to explain weed species response to agricultural practices in a simulation study with a cropping system model

EA EcolDurInternational audienceThe objective of the present study was to use a cropping system model to evaluate the potential impacts of modified agricultural practices ex ante on weeds and to adapt ecological analytical methods to analyse simulation output to identify species traits selected by cropping techniques. Cropping systems typical of three French regions were determined from farm surveys and a French data base monitoring agricultural practices and weeds (”Biovigilance-Flore”). Current scenarios as well as recent and probable future changes were identified in surveys and by expert opinion were simulated in each region, using the FLORSYS model and a weed flora consisting of sixteen major annual weed species. Each scenario was simulated over 24 years and repeated 10 times with randomly chosen regional weather series. The simulated weed flora was analysed with RLQ-analyses and fourth-corner analyses. These showed that longer rotations selected weed species with elongated/flattened seeds; spring crops and late-sown autumn crops favoured round seeds and species with short emergence periods; frequent mouldboard ploughing selected species with thick-coated seeds; frequent tillage favoured monocotyledons and species with a late and/or short emergence period; glyphosate before sowing in direct-drilled fields selected seeds with low area/mass ratios. The identified selection pressures could be explained by effects on life-cycle processes (e.g. thick-coated seeds survive better in the soil after burial, low area/mass ratios delay germination because of slower imbibition). The trait x practice relationships were then used to predict the behaviour of additional weed species in cropping systems. (C) 2013 Elsevier B.V. All rights reserved

Two-dimensional organic superconductors studied by NMR under pressure.

International audienceThe (BEDT)2X family of layered superconductors is one of the largest among nearly 50 org. superconductors currently known. One of the advantages of the org. compds. as prototype materials for studying the mechanisms of supercond. is their relatively high sensitivity to hydrostatic pressure. We review recent NMR studies of these compds. using NMR under liq. and gas pressure. We focus on the low temp. part of the phase diagram where the physics is controlled by electronic correlations leading to a competition between magnetism and supercond. This interplay between different ground states is shown by the observation of a pseudo-gap and antiferromagnetic fluctuations and can be finely tuned by the application of pressure. Using a gas pressure system gives the unique possibility of sweeping the pressure at low temp. Recently we used this technique to study the AF-SC boundary and established the existence of a first order transition line between the superconducting and antiferromagnetic state

Plant diversity in understory vegetation strips of alley cropping agroforestry systems

National audienc