Oxydation de la face (111) d'un monocristal de chrome

Interaction of oxygen with (111) oriented chromium single crystal surface was studied by electron diffraction (LEED and RHEED). From the clean surface, oxygen adsorption induces a $\text{Cr}\text{(111)(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}$ -O structure. No change in the geometry of the LEED pattern occurs with additional oxygen exposure or heating, although the RHEED study denotes the nucleation of rhombohedral oxide on the surface. The orientation relationships with the substrate are determined and compared to those found in the case of (110) chromium surface.     

Procede experimental de restitution de la partie imaginaire de l'amplitude complexe d'un objet de phase

An experimental process is described, capable of restoring the imaginary part of the complex amplitude of a phase object. It is based on a combination of phase-modulation interferometry and scanning the object by a flying-spot method. This experiment proves that it is possible to obtain the sign of the phase, and so the direction of surface relief.     

Sur la contribution des differents porteurs à la zone de charge d'espace d'un semiconducteur multivallée

We study, in the case of the parabolic approximation, the space charge of a two-valley conduction band semiconductor; the electrons associated to these valleys are supposed to have different effective masses. By using a non-degenerate distribution we analyse the contribution of the different carriers to the space charge and the effect of surface band bending, temperature, effective masses, dope and type of material. We point out that the space charge can be governed by either type of carrier or both. By comparing the results with those given by a single valley model, we show the incidence of the initial hypothesis on the value and evolution of the space charge with temperature and surface band bending. As an exemple, we study briefly n-type GaSb.     

Tenseur de mobilite d'un super-reseau en presence de desordre d'interface

Mobility tensor, limited by island-like interface disorder, in type-I superlattices, is studied within a simple model (Kronig-Penney and plane waves). Computed results are presented, for GaAs-Ga_1?xAl_xAs superlattices, versus variations of temperature, superlattice period and defect parameters such as localisation, width and thickness. This interface scattering process has a significant influence on near equilibrium transport at low temperature.     

Calcul de la fonction correlation de phonon pour la diffraction d'une particule par un solide

A method of calculating expressions of the type $\text{<T exp[?i}\text{k}{}_1\text{u}{}_{\ln }\text{(t}{}_1\text{)] ... exp [?i}\text{k}{}_{\mathrm{n}}\text{u}{}_{\ln }\text{(t}{}_{\mathrm{n}}\text{>}$ which is simpler than that based on Wick's theorem is proposed. Expressions of the above type are widely used in studying the effects of temperature in electron and neutron diffraction.     

Etude spectroscopique d'un jet de plasma d'argon hors d'equilibre thermodynamique local

We have studied the properties of an argon plasmajet seeded with a few percent of hydrogen. At atmospheric pressure, significant deviations from local thermodynamic equilibrium (L.T.E.) are shown. The plasma parameters (electrondensity N_e, temperature T) have been determined from the broadening and the absolute intensity of hydrogen lines. The transition probabilities and the broadening parameters (width and shift) of some AI lines between 4000 and 6000 Å have been measured.     

Accroissement d'energie interne et de densite d'energie interne d'un milieu dielectrique polarise

For an isotropic dielectric medium, having an arbitrary shape and a stationary polarization, the polarization-induced increase of internal energy of the dielectric only is reexamined from Güntelberg's thermodynamic point of view. The value obtained for this increase is quite different from what was previously admitted, even in the case of dielectrics with simple shapes. The new value is in better agreement with earlier results. It is furthermore well in accord with the exact macroscopic relation between the internal energy and a density of internal energy, a relation which is more intricate than the simple relation of integration largely accepted up to now.     

Reconstitution d'un profil d'eclairement echantillonne par une ligne de micro-photodiodes

We have studied experimentally the spatial photosensitivity distribution of a self-scanned photodiode array. Using this data, we have calculated a reconstruction function which restores exactly the profile of a light distribution sampled by an array infinite extent under the Nyquist conditions. We briefly discuss the noise limitations and the truncation effects introduced by the finite number of diodes of a real array.     

Influence de dipoles magnetiques sur le parametre d'ordre d'un supraconducteur

It is shown that the order parameter of a superconductor containing insulated magnetic dipoles depends on the mean magnetisation of the sample. The model agrees with the results observed experimentally.     

Modification de l'absorption hertzenne d'un gaz sous l'action d'un champ infrarouge non resonant

With non-resonant infrared laser radiation, a frequency shift of the microwave absorption line of ammonia is observed. The experimental data are in good agreement with the theory of the Stark effect in a rapidly-varying field.     

Extension des methodes de moires a l'etude de la microstructure d'un objet diffusant: Moires aleatoires

The statistical analysis of a diffusing object (i.e. with random microstructure) by optical means is necessarily performed by studying the wavefront radiated under suitable illumination. Two approaches exist: either “in situ” analysis of this wavefront (i.e. at the object itself), or farfield analysis. We briefly recall the drawbacks and limitations of those approaches. We then demonstrate the possibility of a third one, based upon a generalization of moirés to random structures. This last approach has fewer theoretical and practical limitations than the two first ones. A particular case is treated theoretically and preliminary experimental results are given.     

Etude d'un nouveau type de fluorure antiferromagnetique a caractere bidimensionnel: NaFeF4

A new type of two-dimensional antiferromagnetic structure has been investigated by magnetic susceptibility measurements, neutron diffraction and Mössbauer resonance. The magnetic cell of NaFeF₄ is doubled along the a axis and the spins lie along the b axis. The κ^-1 vs T curve shows a sharp minimum at about 105 K close to the three-dimensional transition temperature determined by Mössbauer spectrometry (111.5 K). A calculation of z. snfc;J/kz. snfc; has been performed using a high temperature series expansions technique (J/k = -23 K). The variation of the hyperfine field in the range 0.6 ?T/T_N? 1 gave the value of the critical exponant β = 0.25.     

Fonction de transfert de modulation d'un systeme de gammagraphie holographique

In this paper, gamma ray imaging using Fresnel zone plates is studied. We show that an M.T.F. can be associated with the coding—decoding process giving an evaluation of the final image quality. An experimental result corroborates the theoretical study.     

Evolution de la caracteristique courant-tension des varistances a base d'oxyde de zinc avec la pression partielle d'oxygene de l'atmosphere de frittage

In order to decrease the resistivity of zinc oxide grains which is responsible for the intensity limitation observed at high current densities, the dependence of current-voltage characteristics of zinc oxide based varistors on oxygen partial pressure has been investigated. From these studies it appears that the conductivity increases with decreasing oxygen partial pressure, this phenomenon being more significant at low voltages than at higher ones. These results can be related to a slight increase of the donor density, while the superficial trap density decreases strongly, involving a collapse of the barrier height and of the non-linearity exponent.     

Mesure des sections efficaces de diffusion de la lumiere a l'aide d'un laser He/Ne de faible puissance

New measurements of light scattering cross sections by gases were made using a lower power (1 mW) He/Ne laser. They concern not only isotropic molecules but also non-isotropic ones. The results are in excellent agreement wih calculated values.     

Examen de la loi d'arrhenius de la diffusion dans les solides

Validity of the Arrhenius law has been investigated by measuring the influence of temperature on the transport processes in very pure single crystals of sodium iodide. For the first time, due to the high experimental accuracy and the precision of the temperature control, diffusion coefficients could be measured in a solid in steps of a few degrees up to a few tenths of a degree of the temperature of fusion. We have studied: (a) ionic conductivity, (b) self-diffusion of the cation Na⁺ and (c) self-diffusion of the anion I^?. The three transport processes follow the same temperature dependence: (a) strict verification of the Arrhenius law over several orders of magnitude up to about thirty degrees below the melting point and (b) in the last thirty degrees a positive departure from the simple exponential law is observed which increases rapidly to reach 50% at fusion. The different possible reasons of this departure are discussed. The phenomenon seems to be connected with the lattice dynamics before melting.     

Transmission d'images et d'hologrammes par une nappe de fibres optiques au moyen d'un codage chromatique

A new method of addition of 2-D complex optical signals is proposed. The addition is performed by suitably matched spatial filtering. For that purpose the effect of compacting of periodic objects images is employed.     

Utilisation des franges d'interference en lumiere diffuse pour l'etude de l'etat de surface d'un diffuseur

A diffuser, illuminated successively by light of two neighbouring wavelengths, produces on a photographic plate placed in a parallel plane two intensity distributions which are homothetic except for certain decorrelation terms. This homothecy (scale variation) can be compensated by a longitudinal displacement of the photographic plate between the two exposures and consequently, the decorrelation depends only on the structure of the diffuser. Under these conditions, if the photographic plate is given an additional displacement in its own plane between the two exposures, the contrast of Young's fringes obtained in the Fourier plane characterises the roughness of the diffuser.