89 research outputs found
First evidence of resistive switching in polycrystalline GaV4S8 thin layers
No full textRecently a new type of reversible and non-volatile resistive
switching was discovered in single crystals of Mott insulators AM4X8 (A = Ga, Ge; M = V, Nb, Ta; X = S, Se). Here we report on the first synthesis of thin layers (thicknesses in the 100 to 1000 nm range) of GaV4S8 by RF magnetron sputtering process. Energy dispersive spectroscopy, X-ray diffraction and TEM analyses attest the high quality of polycrystalline GaV4S8 thin layers. Electrical measurements demonstrate that deposited GaV4S8 thin films exhibit a non-volatile reversible resistive switching at room temperature with writing/erasing times of ~10 μs and a memory window (Rhigh – Rlow)/ Rlow > 33%
Etude des mecanismes de conduction electrique et de l'anisotropie de resistivite dans les differentes phases magnetiques du compose La2CuO4
No full textSIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : TD 79312 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Percolation of single-walled carbon nanotubes in ceramic matrix nanocomposites
No full textThe percolation of carbon nanotubes (CNT) in an electrical insulating ceramic is studied for the first time. The in situ synthesis of the CNT (0.2–25 vol%) by a CCVD route allows to achieve their homogeneous distribution in the spinel matrix. Up to 11 vol% CNT, the DC electrical conductivity (σ) is well fitted by the scaling law of the percolation theory σ=k(p−pc)t with a low percolation threshold pc=0.64 vol%. At the threshold, σ jumps over seven order of magnitude (from 10−10 to 0.0040 S cm−1) and then reaches a maximum at 8.5 S cm−1. The results are discussed in relation with the characteristics of the CNT, their damaging during the hot-pressing at 1300 °C and the microstructure of the composites. CNT-ceramic composites become attractive materials not only for their enhanced mechanical properties, but also for the possibility to tailor the electrical conductivity through the CNT content
Localization, Coulomb interactions, and electrical heating in single-wall carbon nanotubes/polymer composites
No full textInternational audienc
Physics and Chemistry of Chalcogenide Quantum Materials with Lacunar Spinel Structure
No full textInternational audienceThis paper aims to review the physical properties and crystal chemistry of the family of correlated quantum materials AM4Q8 (A = Ga, Ge; M = V, Nb, Ta, Mo; Q = S, Se). These compounds exhibit a lacunar spinel structure with tetrahedral transition metal clusters which favor a correlated state. But compared to most other inorganic Mott insulators, the AM4Q8 compounds show very small Mott-Hubbard gaps (0.1-0.3 eV). These small values originate from the weak electronic repulsion occurring on the scale of the tetrahedral clusters and from the effect of spin-orbit coupling. As a consequence, AM4Q8 quantum materials show a great variety of ground states and astonishing electronic properties depending on cluster filling, compression, or distortion. For example, they exhibit a multiferroic behavior related to an orbital ordering on the clusters and a variety of magnetic ordering like skyrmions. Under external pressure they undergo a bandwidth-controlled insulator to metal transition (IMT) that leads to a superconducting state at low temperature. When chemically doped, the AM4Q8 compounds undergo a filling-controlled insulator to metal transition with appearance of a half ferromagnetic metallic behavior or colossal negative magnetoresistance. The AM4Q8 compounds are also sensitive to the electric field and exhibit a striking resistive switching above a threshold electric field of a few kV/cm which is related to the breakdown of the Mott insulating state at the nanoscale. This phenomenon unlocks new functionalities that may be used to build up a new type of resistive random-access memory (RRAM) or an artificial neuron. All these examples show the potential of this family of quantum materials whose exploration has only just begun
Localization of Charge Carriers and Magneto-Transport in Nanocomposites Based on Carbon Nanotubes
No full textApport des approches de métabolomique sur l'adaptation des poissons à une alimentation durable
No full textRésuméApport des approches de métabolomique sur l'adaptation des poissons à une alimentation durable. 7. Journées Scientifiques Internationales sur la Valorisation des Bioressource
Electric Transport Properties and Percolation in Carbon Nanotubes / PMMA Composites
No full textAbstractCarbon nanotubes (CNT) possess remarkable physical properties. However, because of their size, they are difficult to handle. Making composites with them gives an alternative way to handle these objects and to make use of their properties at a macroscopic scale. Here, we present a comparative study of the transport properties of PMMA / nanotubes composite films with both SWNTs and MWNTs. At room temperature, the conductivity of the composites follows a pure percolation behavior when increasing the nanotube content. The universal scaling law for random site percolation with a very low percolation threshold (0.3 weight %) is obeyed over two orders of magnitude in CNT content. At low temperature, SWNTs and MWNTs based composites behave differently. While MWNTs composites still obey the percolation law, deviations are observed for SWNTs. The transport properties are discussed in terms of tube-tube contact and charging energy of the tubes.</jats:p
Les lipides de dépôt chez les poissons d'élevage : contrôle cellulaire, métabolique et hormonal
No full textCet article a pour but de dégager les spécificités cellulaires, métaboliques et hormonales du dépôt de lipides chez les poissons sur la base de données bibliographiques existantes et de données personnelles. Il n’existe que très peu d’informations sur l’existence et les caractéristiques des tissus adipeux chez les poissons. Nous supposons indirectement que le développement des tissus adipeux se fait selon des mécanismes similaires à ceux observés chez les mammifères, c’est-à-dire à la fois par hypertrophie et par hyperplasie. Toutefois l’implication du phénomène d’hyperplasie doit être démontrée directement. Les lipoprotéines circulantes impliquées dans le transport des lipides provenant de l’aliment et des lipides provenant des dépôts sont similaires chez les poissons et chez les mammifères exceptée pour la vitellogénine qui apparaît lors du développement des gonades femelles. Le transport du cholestérol semble différent de celui des mammifères, mais la spécificité de ce phénomène reste à étudier. Les points de contrôle du transport et de la mobilisation des lipides se situent à deux niveaux : la composition en apoprotéines des lipoprotéines et les enzymes responsables de la métabolisation des lipoprotéines. Le dépôt de lipides est contrôlé directement par une hormone: l’insuline. Le contrôle indirect du dépôt de lipides et le contrôle direct de la mobilisation des lipides font appel à un complexe multi-hormonal impliquant les hormones hypophysaires, thyroïdiennes, pancréatiques et corticostéroïdes. Le poisson présente l’originalité d’être dépendant de l’environnement dans sa capacité de stockage des lipides et d’avoir une forte capacité physiologique de mobilisation et de redistribution de ses dépôts.Specificities of cellular, metabolic, and hormonal control of fat accretion and mobilisation are reviewed in the light of personnal data. Very little information on adipose tissues of fish is available. According to mechanisms observed in mammals, the involvment of hyperplastic processes together with hypertrophic processes in the development of adipose tissues in fish is emphasized but has yet to be demonstrated. The characteristics of lipoproteins involved in the transport of dietary lipid and of lipid from fat stores seemed to be similar in fish and in mammals except for the appearance of vitellogenin during female gonadal development. Cholesterol transport seems to be different from that of mammals but the specificities of such a process remain to be clarified. Transport of lipids and mobilisation are controlled both by the apoprotein composition of lipoproteins and by the existence of enzymes which metabolize the lipoproteins. The main hormone involved directly in the control of fat accretion is insulin. The indirect control of fat accretion (through the interaction with insulin action) and the direct control of fat mobilisation is mediated through a multi-hormonal process which include hypohyseal, thyroid, pancreatic and corticosteroid hormones. Fish seem to be original in their dependancy of lipid accretion from the environment and in their high physiological potencies for lipid mobilization and redistribution within tissues
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