Showing papers by "École normale supérieure de Cachan published in 1999"

The molecular and supramolecular engineering of polymeric electro-optic materials

Abstract: A new class of electro-optic chromophores, of which 2-dicyanomethylen-3-cyano-4-{2-[ E -(4- N , N -di(2-acetoxyethyl)-amino)-phenylene-(3,4-dibutyl)thien-5]- E -vinyl}-5,5-dimethyl-2,5-dihydrofuran (denoted FTC) is the prototype, has been prepared, characterized, and used to fabricate electro-optic devices. The molecular hyperpolarizability and thermal stability of these chromophore molecules are exceptional. Strong intermolecular electrostatic interactions inhibit the efficient poling of these molecules. A statistical mechanical theoretical treatment is used to quantitatively predict the competition of poling, intermolecular electrostatic interactions, and thermal effects in defining achievable acentric order and hence macroscopic optical nonlinearity. Theory is used to predict the optimum chromophore structure and material composition (chromophore loading in a polymer matrix) for maximum electro-optic activity and minimum optical loss. Problems associated with lattice hardening to lock-in poling-induced order are discussed briefly.

Topographic Maps and Local Contrast Changes in Natural Images

Abstract: We call “natural” image any photograph of an outdoor or indoor scene taken by a standard camera. We discuss the physical generation process of natural images as a combination of occlusions, transparencies and contrast changes. This description fits to the phenomenological description of Gaetano Kanizsa according to which visual perception tends to remain stable with respect to these basic operations. We define a contrast invariant presentation of the digital image, the topographic map, where the subjacent occlusion-transparency structure is put into evidence by the interplay of level lines. We prove that each topographic map represents a class of images invariant with respect to local contrast changes. Several visualization strategies of the topographic map are proposed and implemented and mathematical arguments are developed to establish stability properties of the topographic map under digitization.

Wavelets, Generalized White Noise and Fractional Integration: The Synthesis of Fractional Brownian Motion.

Abstract: We provide an almost sure convergent expansion of fractional Brownian motion in wavelets which decorrelates the high frequencies. Our approach generalizes Levy's midpoint displacement technique which is used to generate Brownian motion. The low-frequency terms in the expansion involve an independent fractional Brownian motion evaluated at discrete times or, alternatively, partial sums of a stationary fractional ARIMA time series. The wavelets fill in the gaps and provide the necessary high frequency corrections. We also obtain a way of constructing an arbitrary number of non-Gaussian continuous time processes whose second order properties are the same as those of fractional Brownian motion.

On the convergence of markovian stochastic algorithms with rapidly decreasing ergodicity rates

Abstract: We analyse the convergence of stochastic algorithms with Markovian noise when the ergodicity of the Markov chain governing the noise rapidly decreases as the control parameter tends to infinity. In such a case, there may be a positive probability of divergence of the algorithm in the classic Robbins-Monro form. We provide sufficient condition which ensure convergence. Moreover, we analyse the asymptotic behaviour of these algorithms and state a diffusion approximation theorem

Durability of reactive powder composites: influence of silica fume on the leaching properties of very low water/binder pastes

Abstract: Reactive Powder Composites are new cement-based materials which could be used for the storage of nuclear wastes thanks to their excellent microstructural properties. This paper studies their durability when submitted in the laboratory to a water leaching attack. In order to understand the behaviour of the hydrates (CSH), the study was carried out on a pure cement paste and on a cement + silica fume paste. The beneficial effect of silica fume is demonstrated from considerations on the calcium leaching, from XRD analysis, from SEM observations and from the tritium diffusion and pore distribution analysis. It was found that the leaching greatly affects the microstructure, especially that of the anhydrous cement grains remaining in the paste.

Isotropic and anisotropic descriptions of damage in concrete structures

Abstract: Scalar damage models are very often implemented in computational analyses in order to predict the response and failure modes of concrete and reinforced concrete structures. In most situations, however, damage is not isotropic but has preferential directions. Therefore, there have been many questions about the pertinence and range of applicability of isotropic, scalar, damage models for describing a degradation process which is strongly geometrically oriented. In order to assess what are the limitations of such a simplifying assumption, a comparative study is presented. The constitutive relations used for this purpose derive from the same class of models with a gradual enhancement of the description of damage. The scalar damage model is compared to another model where damage-induced orthotropy is described, with the possibility of rotation of the principle axes of orthotropy. Both models incorporate crack closure effects and a plasticity damage coupling. Structural analyses on bending beams, compression-shear and tension-shear concrete panels are presented. Although it may appear to be simplistic, the scalar damage model provides accurate predictions when failure is mainly due to uniaxial extension. Crack closure introduces an additional anisotropy which is important in compression-shear problems. Finally, damage-induced anisotropy seems important when failure is due to multiaxial extensions, such as in shear-tension problems. Copyright © 1999 John Wiley & Sons, Ltd.

Tracking Control and π-Freeness of Infinite Dimensional Linear Systems

Abstract: This is a report on recent works [1, 17, 19, 20, 21, 39, 40, 41, 43] on the control tracking of some infinite dimensional linear systems. It is based on a new property, called π-freeness, which allows the tracking of a reference trajectory in a way which bears some analogy with flat finite dimensional nonlinear systems (see [15, 16] and the references therein). Several examples are examined and simulations are provided.

Local minimizers for the ginzburg–landau energy near critical magnetic field: part ii

Abstract: We find local minimizers of the two-dimensional Ginzburg–Landau functionals depending on a large parameter κ, which describe the behavior of a superconductor in a prescribed exterior magnetic field hex. We prove an estimate on the critical value Hc1 of hex(κ), corresponding to the first phase-transition in which vortices appear in the superconductor; and we locate these vortices.

New octupolar star-shaped strucures for quadratic nonlinear optics

Abstract: Two new classes of octupolar nonlinear chromophores, deriving from 1,3,5-triazines and oligothienylic crystal violet analogues, have been synthesized and their nonlinear properties investigated using the harmonic light scattering experiment. The easy preparation of these molecules makes them attractive candidates for further applications. Off-resonant measurements show high nonlinearities together with an excellent transparency, which make these compounds of promising interest for the emerging schemes of nonlinear optical applications whereby the multipolar character of the chromophores is of particular importance.

Complex Ginzburg-Landau equations in high dimensions and codimension two area minimizing currents

Abstract: There is an obvious topological obstruction for a finite energy unimodular harmonic extension of a S 1-valued function defined on the boundary of a bounded regular domain of R n When such extensions do not exist, we use the Ginzburg-Landau relaxation procedure We prove that, up to a subsequence, a sequence of Ginzburg-Landau minimizers, as the coupling parameter tends to infinity, converges to a unimodular harmonic map away from a codimension-2 minimal current minimizing the area within the homology class induced from the S 1-valued boundary data The union of this harmonic map and the minimal current is the natural generalization of the harmonic extension

Mauvaise réduction des variétés de Drinfeld et correspondance de Langlands locale

Abstract: Pour F d’egale caracteristique positive p differente de l, Laumon, Rapoport et Stuhler ([17]) ont prouve la correspondance de Langlands locale, c’est-a-dire l’existence d’une famille de bijections LF,d : AG Ld ∼ −→ G(d), d ≥ 1, possedant certaines proprietes de conservation de facteurs L et e (cf. le paragraphe 1.2 pour un enonce precis). En particulier pour d = 1, L1,F est induit par l’isomorphisme W ab F ' F× de la theorie du corps de classe abelien. Dans [12], Henniart a demontre la correspondance de Jacquet-Langlands locale, c’est-a-dire l’existence, pour tout d ≥ 1, d’une

Stable Configurations in Superconductivity: Uniqueness, Multiplicity, and Vortex‐Nucleation

Abstract: . We find new stable solutions of the Ginzburg‐Landau equation for high κ superconductors with exterior magnetic field h ex. First, we prove the uniqueness of the Meissner‐type solution. Then, we prove, in the case of a disc domain, the coexistence of branches of solutions with n vortices of degree one, for any n not too high and for a certain range of h ex; and describe these branches. Finally, we give an estimate on the nucleation energy barrier, to pass continuously from a vortexless configuration to a configuration with a centered vortex.

The linear boltzmann equation for long-range forces: a derivation from particle systems

Abstract: In this paper we consider a particle moving in a random distribution of obstacles. Each obstacle generates an inverse power law potential es/|x|s, where e is a small parameter and s>2. Such a rescaled potential is truncated at distance eγ-1, where γ∈] 0, 1[ is suitably large. We also assume that the scatterer density is diverging as e-d+1, where d is the dimension of the physical space. We prove that, as e→0 (the Boltzmann–Grad limit), the probability density of a test particle converges to a solution of the linear (uncutoff) Boltzmann equation with the cross-section relative to the potential V(x)=|x|-s.

Digital laser range finder: phase-shift estimation by undersampling technique

Abstract: The purpose of this paper is to present the conception and the first realization of a new laser range finder which is a digital system. Based on the flight time measurement principle, it uses a phase-shift conversion method which is achieved by the undersampling technique. Firstly, the authors remind readers of the synchronous demodulation principle and present the undersampling technique. Then the corresponding phase-shift measurement unit working at a 10 MHz modulation frequency is described. The choice of the sampling frequency, related to the modulation frequency, is important and leads to a very interesting simplification of the digital process. Performance evaluation of this unit shows a good accuracy in phase-shift estimation. The system is designed to be integrated in a complete autonomous device. An important evolution of the system is introduced in order to feature a high second modulation frequency at 240 MHz, which is required for high accuracy measurement. Nevertheless the new system keeps a rather simple architecture.

The Size of Objects in Natural and Artificial Images

Abstract: This paper is partly written as a review, in which we discuss old and new image analysis and synthesis methods. We also introduce a new method for analyzing scaling phenomena in natural images and draw some consequences as to whether natural images belong to the space of functions with bounded variation. In some sense, our analysis computes the size distribution of objects in an image. By using the dead leaves model, we study the influence of occlusion on size distribution, and prove compatibility with our experimental results.

Uniting local and global controllers

Abstract: We consider control systems for which we know two stabilizing controllers The former is "optimal" but local, the latter is global We look for a uniting control law providing a globally stabilizing locally optimal controller We study several solutions based on continuous, discontinuous, hybrid, time varying controllers One criterion of selection of a controller is the robustness of the global asymptotic stability to vanishing measurement noise This leads us in particular to consider a kind of generalization of Krasovskii solutions for hybrid systems

Nonlinear modeling of scattered multivariate data and its application to shape change

Abstract: We are given a set of points in a space of high dimension. For instance, this set may represent many visual appearances of an object, a face, or a hand. We address the problem of approximating this set by a manifold in order to have a compact representation of the object appearance. When the scattering of this set is approximately an ellipsoid, then the problem has a well-known solution given by principal components analysis (PCA). However, in some situations like object displacement learning or face learning, this linear technique may be ill-adapted and nonlinear approximation has to be introduced. The method we propose can be seen as a nonlinear PCA (NLPCA), the main difficulty being that the data are not ordered. We propose an index which favors the choice of axes preserving the closest point neighborhoods. These axes determine an order for visiting all the points when smoothing. Finally, a new criterion, called "generalization error", is introduced to determine the smoothing rate, that is, the knot number for the spline fitting. Experimental results conclude this paper: The method is tested on artificial data and on two databases used in visual learning.

Photoinduced orientation of azo dye in various polymer matrices

Abstract: Photoinduced dichroism in various polymers containing the same azo dye has been studied. “Angular hole burning” and molecular reorientation have been identified by analysis of dichroism dynamics at various probe wavelengths. The glass-transition temperature Tg is of major relevance for the reorientation of optically active molecules. For low Tg (below ambient temperature) thermal diffusion impedes anisotropy buildup. For higher Tg the angular distribution induced by the polarized pump beam is frozen. We propose a simple model based on diffusion rates in trans and cis molecules.

Effects of the airgap eccentricity on the SRM vibrations

Abstract: This paper describes an investigation of the effects of static eccentricity on SRM vibration. First we have to calculate the electromagnetic forces distribution based on the virtual work principle associated with the finite element method in order to determine the global forces generated by rotor eccentricity, which increase the radial vibration. Then we study the influence of saturation phenomena. In addition, we investigate the SRM vibratory behaviour by experimental measurement of acceleration.

Formal validation of PLC programs: A survey

Abstract: The aim of this paper is to give an overview of the validation methods for IEC 61131-3 programming languages for programmable logic controllers. More precisely we are focussing on the SFC structuring language and on the LD programming language. The SFC and LD validation methods are detailed after a short presentation of the usual formal methods and of the IEC 61131-3 standard.

Organic sulfides photooxidation using sensitizers covalently grafted on silica: towards a more efficient and selective solar photochemistry

Abstract: A visible light sensitized oxidation of organic sulfides using a derivative of 9,10 dicyanonoanthracene (DCA) covalently grafted on silica (DCAC-Si) is studied. The reaction is more efficient than the corresponding homogeneous reaction and some improvement in the rate of product formation is noticed. These observations are discussed. In the case of di n-butylsulfide, the sulfoxide/(sulfone + disulfide) selectivity is increased. Diphenyl sulfide leads selectively and efficiently to diphenylsulfoxide. Advantages of the process, using reusable sensitizer and solar light are emphasized. (C) 1999 Elsevier Science S.A. All rights reserved.