Comportement asymptotique à haute conductivité de l'épaisseur de peau en électromagnétisme

International audienceOn étudie un modèle tridimensionnel décrivant l'effet de peau en électromagnétisme. On commence par donner un développement asymptotique multi-échelle de la solution des équations de Maxwell en régime harmonique posées dans un domaine formé de deux matériaux, l'un diélectrique et l'autre fortement conducteur, avec une interface régulière entre les deux. Afin de mesurer l'effet de peau on introduit une fonction ``épaisseur de peau'' définie sur l'interface, ce qui généralise la quantité scalaire classique. On donne alors un développement asymptotique à haute conductivité pour cette fonction, ce qui met en évidence l'influence de la géométrie de l'interface sur l'épaisseur de peau

Anomalous electronic transport in Quasicrystals and related Complex Metallic Alloys

We analyze the transport properties in approximants of quasicrystals alpha-AlMnSi, 1/1-AlCuFe and for the complex metallic phase lambda-AlMn. These phases presents strong analogies in their local atomic structures and are related to existing quasicrystalline phases. Experimentally they present unusual transport properties with low conductivities and a mix of metallic-like and insulating-like characteristics. We compute the band structure and the quantum diffusion in the perfect structure without disorder and introduce simple approximations that allow to treat the effect of disorder. Our results demonstrate that the standard Bloch-Boltzmann theory is not applicable to these intermetallic phases. Indeed their dispersion relation are flat indicating small band velocities and corrections to quantum diffusion that are not taken into account in the semi-classical Bloch-Boltzmann scheme become dominant. We call this regime the small velocity regime. A simple Relaxation Time Approximation to treat the effect of disorder allows us to reproduce the main experimental facts on conductivity qualitatively and even quantitatively.Comment: 14 page

Estimation of the hydraulic parameters of unsaturated samples by electrical resistivity tomography

In situ and laboratory experiments have shown that electrical resistivity tomography (ERT) is an effective tool to image transient phenomena in soils. However, its application in quantifying soil hydraulic parameters has been limited. In this study, experiments of water inflow in unsaturated soil samples were conducted in an oedometer equipped to perform three-dimensional electrical measurements. Reconstructions of the electrical conductivity at different times confirmed the usefulness of ERT for monitoring the evolution of water content. The tomographic reconstructions were subsequently used in conjunction with a finite-element simulation to infer the water retention curve and the unsaturated hydraulic conductivity. The parameters estimated with ERT agree satisfactorily with those determined using established techniques, hence the proposed approach shows good potential for relatively fast characterisations. Similar experiments could be carried out on site to study the hydraulic behaviour of the entire soil deposi

Simulation Monte-Carlo du Modèle de Hubbard à deux dimensions

14 pagesThe Quantum Monte-Carlo simulations of the two-dimensional Hubbard model are presented for the half filling. The method based on the direct-space proposed by Suzuki and al., and Hirsch and al. was used. The states generated by this method are basis states in occupation number representation built with Wannier states localised on each site of the square array. The configurations of fermions can be observed on the real 2D array. An antiferromagnetic factor is defined and calculated for each temperature. The curves of energy, specific heat, conducivity and antiferromagnetic factor are presented for different values of the repulsive coulombian on site interaction U. There is a metal-insulator transition at low temperature for small values of U. This transition corresponds with a paramagnetic-ferromagnetic first order transition. Indeed, for these interaction values, the energy curves show a gap which is a characteristic of a first order transition. An hysteresis phenomenon appears on the conductivity curves. There is a behaviour change for U/t=3.5. For the values U>3.5 there is ferromagnetic-paramagnetic change without observable effect on the energy and the specific heat. The metal-insulator transition does not exist any more, the conductivity stays very small. Isotherms of the physical quantities versus U/t show a transition which seems to be the metal-insulator Mott transition. These results allow to draw a phase diagram with two first order transition lines

Utilisation de la conductivité électromagnétique pour la mesure directe de la salinité des sols

La conductivite électromagnétique permet d'étudier in situ la variabilité de la salinité des sols. Dans cet article, les auteurs décrivent cette méthode et présentent trois exemples d'utilisatio

Dielectrophoretic separation of blood pathogens ou Bacterial extraction from biological samples using DEP forces

International audienceWe report here a method based on DEP separation to concentrate pathogens out of a biological sample by combining positive and negative DEP to separate pathogens from the sample matrix. In this approach, we take advantage from the large tolerance of micro-organisms towards osmotic shock to perform dielectrophoretic separation in a low electric conductivity medium. This condition enables to collect micro-organisms by positive DEP, while lysed blood cells are repelled from the electrodes by negative DEP. Our microfluidic device is designed based on numerical simulation. This approach is validated on a large range of microorganisms, since it has been tested with different species of bacteria (S. epidermidis (Gram +) and E. coli (Gram -)) and yeasts (C. albicans)