Magnetoelectric properties of the multiferroic CuCrO2_2 studied by means of ab initio calculations and Monte Carlo simulations

Motivated by the discovery of multiferroicity in the geometrically frustrated triangular antiferromagnet CuCrO$_2$ below its N\'eel temperature $T_N$, we investigate its magnetic and ferroelectric properties using ab initio calculations and Monte Carlo simulations. Exchange interactions up to the third nearest neighbors in the $ab$ plane, inter-layer interaction and single ion anisotropy constants in CuCrO$_2$ are estimated by series of density functional theory calculations. In particular, our results evidence a hard axis along the [110] direction due to the lattice distortion that takes place along this direction below $T_N$. Our Monte Carlo simulations indicate that the system possesses a N\'eel temperature $T_N\approx27$ K very close to the ones reported experimentally ($T_N = 24-26$ K). Also we show that the ground state is a proper-screw magnetic configuration with an incommensurate propagation vector pointing along the [110] direction. Moreover, our work reports the emergence of spin helicity below $T_N$ which leads to ferroelectricity in the extended inverse Dzyaloshinskii-Moriya model. We confirm the electric control of spin helicity by simulating $P$-$E$ hysteresis loops at various temperatures.Comment: 6 pages, 8 figure

Detection mechanism in highly sensitive ZnO nanowires network gas sensors

Metal-oxide nanowires are showing a great interest in the domain of gas sensing due to their large response even at a low temperature, enabling low-power gas sensors. However their response is still not fully understood, and mainly restricted to the linear response regime, which limits the design of appropriate sensors for specific applications. Here we analyse the non-linear response of a sensor based on ZnO nanowires network, both as a function of the device geometry and as a response to oxygen exposure. Using an appropriate model, we disentangle the contribution of the nanowire resistance and of the junctions between nanowires in the network. The applied model shows a very good consistency with the experimental data, allowing us to demonstrate that the response to oxygen at room temperature is dominated by the barrier potential at low bias voltage, and that the nanowire resistance starts to play a role at higher bias voltage. This analysis allows us to find the appropriate device geometry and working point in order to optimize the sensitivity. Such analysis is important for providing design rules, not only for sensing devices, but also for applications in electronics and opto-electronics using nanostructures networks with different materials and geometries

Semi-Blind Deconvolution for Resolution Enhancement in Ultrasound Imaging

International audienceIn the field of ultrasound imaging, resolution enhancement is an up-to-date challenging task. Many device-based approaches have been proposed to overcome the low resolution nature of ultrasound images but very few works deal with post-processing methods. This paper investigates a novel approach based on semi-blind deconvolution formulation and alternating direction method framework in order to perform the ultrasound image restoration task. The algorithm performance is addressed using optical images and synthetic ultrasound data for a various range of criteria. The results demonstrate that our technique is more robust to uncertainties in the a priori ultrasonic pulse than classical non-blind deconvolution methods

Amélioration de la résolution des images ultrasonores en mode B par déconvolution semi-aveugle

National audienceEn imagerie médicale, et plus précisément dans le domaine de l'imagerie ultrasonore, les problématiques liées à l'amélioration de la résolution font aujourd'hui l'objet de très nombreux travaux. Alors que beaucoup d'approches se consacrent à l'amélioration du dispositif d'acquisition des images échographiques (pré-traitement) pour pallier leur faible résolution, très peu de travaux se sont attachés à des techniques de post-traitement. Nous proposons ici une nouvelle approche pour la restauration d'image basée sur une formulation de type déconvolution semi-aveugle, résolue dans le cadre algorithmique de la méthode des directions alternées. Les performances de notre algorithme sont évaluées à l'aide de données synthétiques (fantôme de Shepp-Logan) et d'une image ultrasonore in vivo en mode B, sur la base de plusieurs critères quantitatifs. Dans le cas où la réponse impulsionnelle spatiale du système est mal connue, les résultats démontrent une robustesse accrue par rapport à une méthode de déconvolution classique (non aveugle)

Scale Normalization for the Distance Maps AAM

International audienceThe Active Apearence Models (AAM) are often used in Man-Machine Interaction for their ability to align the faces. We propose a new normalization method for AAM based on distance map in order to strengthen their robustness to differences in illumination. Our normalization do not use the photometric normalization protocol classically used in AAM and is much more simpler to implement. Compared to Distance Map AAM performances of {Leg06} and other AAM implementation which use CLAHE {Zuiderveld94} normalization or gradient information, our proposition is at the same time much robust to illumination and AAM initialization. The tests have been drive in the context of generalization: 10 persons with frontal illumination from M2VTS database {m2vts} were considered to build the AAM, and 17 persons under 21 different illuminations from CMU database {Sim02} were used for the testing base

Silicon on Nothing Mems Electromechanical Resonator

The very significant growth of the wireless communication industry has spawned tremendous interest in the development of high performances radio frequencies (RF) components. Micro Electro Mechanical Systems (MEMS) are good candidates to allow reconfigurable RF functions such as filters, oscillators or antennas. This paper will focus on the MEMS electromechanical resonators which show interesting performances to replace SAW filters or quartz reference oscillators, allowing smaller integrated functions with lower power consumption. The resonant frequency depends on the material properties, such as Young's modulus and density, and on the movable mechanical structure dimensions (beam length defined by photolithography). Thus, it is possible to obtain multi frequencies resonators on a wafer. The resonator performance (frequency, quality factor) strongly depends on the environment, like moisture or pressure, which imply the need for a vacuum package. This paper will present first resonator mechanisms and mechanical behaviors followed by state of the art descriptions with applications and specifications overview. Then MEMS resonator developments at STMicroelectronics including FEM analysis, technological developments and characterization are detailed.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/EDA-Publishing

Segmented subwavelength silicon gratings manufactured by high productivity microelectronic technologies for linear to radial/azimuthal polarization conversion

International audienceA polarization rotation is realized by subwavelength binary gratings, where the round trip phases of thesmallest grating modes are fixed to the smallest possible integer numbers of 2π allowing a phase difference of πbetween TE and TM polarizations and almost 100% transmission. The principle is applied to a polarization transformationin the 1030 to 1064-nm wavelength range, using a segmented polarization rotating element convertinga linearly polarized incidence to a radial or azimuthal polarization distribution. The elevated costs of such kindsof polarization transformers based on assembled birefringent crystals are avoided by using mass-fabricationcompatible silicon-on-insulator technology on a wafer scale. It shows the general potential of microelectronictechnology, concerning the batch manufacturing of wavelength-scale diffractive, grating-based elements forprocessing free space waves

Genotype characterization of cocoa into genetic groups through caffeine and theobromine content predicted by near infra red spectroscopy

Purine compounds such as caffeine and theobromine are involved in cocoa flavour development. Measurement of these compounds is used to determine the percentage of cocoa mass in chocolate. Additionally the relative level of these two compounds may be associated with the genetic and/or geographical origin of the cocoa. The present work was undertaken to assess the potential of NIRS, as a rapid and non-destructive method for measuring these compounds. The study was carried out on 284 cocoa samples over 5 production years (1999 to 2003). The samples came from Ivory Coast, Venezuela and Trinidad, and belonged to genotypes of the Forastero, Criollo and Trinitario genetic groups. The sampling protocol ensured good representation of the spectral and chemical variability of fermented and dried cocoa. Spectra acquisition was perform on 3 g of shelled ground and sieved beans in diffuse reflectance using a FOSS Nirsystem 6500. Partial Least Square models (PLS) were used to establish quantitative relations between NIR spectral data and caffeine and theobromine contents. Models were developed which fitted the data with coefficients of determination of 0.95 for caffeine content and 0.89 for theobromine content. The SECV errors were 0.04% for caffeine and 0.07% for theobromine. These values are close to the repeatability of the reference method. The ratios of SD to SECV were 3.5 and 2.6 for caffeine and theobromine respectively. The theobromine to caffeine ratio separated Ivorian Forastero cocoa from the Trinitario cocoa from Trinidad and the Criollo cocoa from Venezuela. Given the performance of the equations developed to represent the range of samples over spectral variability, NIRS can be considered to provide a routine analysis method for purine compounds. Whilst this study did not lead to definitive conclusions on discrimination between genetic groups according to their purine compositions, it did confirm earlier observations. With a tool such as NIRS that enables determination of purine composition, it shall be possible to make progress in that direction in the foreseeable future. [Résumé d'auteur

SiF4 anomalous behaviour reassessed

The Si 1s−1, Si 2s−1, and Si 2p−1 photoelectron spectra of the SiX4 molecules with X = F, Cl, Br, CH3 were measured. From these spectra the Si 1s−1 and Si 2s−1 lifetime broadenings were determined, revealing a significantly larger value for the Si 2s−1 core hole of SiF4 than for the same core hole of the other molecules of the sequence. This finding is in line with the results of the Si 2p−1 core holes of a number of SiX4 molecules, with an exceptionally large broadening for SiF4. For the Si 2s−1 core hole of SiF4 the difference to the other SiX4 molecules can be explained in terms of Interatomic Coulomb Decay (ICD)-like processes. For the Si 2p−1 core hole of SiF4 the estimated values for the sum of the Intraatomic Auger Electron Decay (IAED) and ICD-like processes are too small to explain the observed linewidth. However, the results of the given discussion render for SiF4 significant contributions from Electron Transfer Mediated Decay (ETMD)-like processes at least plausible. On the grounds of our results, some more molecular systems in which similar processes can be observed are identified