Étude de la variation du travail de sortie lors de l'interaction de l'oxygène avec le niobium (110)

Variations of the work function of niobium (110) in the presence of oxygen at low pressure have been studied at 300 K by retarding field method and between 1290 and 1540 K by thermoionic emission measurements. In both cases the work function decreases at the beginning of the adsorption. In this initial stage, at high temperatures and at constant coverage, the variation of the work function depends linearly on the temperature (the decrease of work function being greater at lower temperatures): we estimate that there is a progressive incorporation of chemisorbed elements in the lattice during this temperature decrease.     

Etude par resonance paramagnetique electronique de sulfures de molybdene et de tungstene

Chalcogenides MeS_3−x (Me = Mo, W; 0 ⩽ x ⩽ 1) have been prepared by the thermal decomposition of tetrathiometallates. These compounds are largely amorphous; desulfurization as well as crystalline organization occur when the heat treatment temperature (TT) increases. At 1000°C a perfect hexagonal arrangement can be obtained (x = 1). ESR spectra of amorphous species having x in the range 0–1 have been recorded. A part of the ESR signals is ascribed to sulfur chains whilst the other part is attributed to Me (V) species. The intensity of both series of ESR lines is a decreasing function of TT. The spectra are characteristic of Me (V) in an axial symmetry environment. At TT ≅ 1000°C a narrow ESR signal remains, which is assigned to electron spin centers of impurities (g-value close to that of the free electron). The results are explained either in terms of continuous structural evolution of the solid during heat treatment or in terms of a biphasic mixture of amorphous MoS₃ and MoS₂. The variation of the spin concentration as a function of TT is reported and compared with the results previously obtained from a study of the transport properties of these solids. In all cases the values obtained for Me = W are smaller than those observed on Me = Mo.     

Etude de la relaxation dielectrique de l'hexachlorodiphenyl en inclusion dans une matrice de ceramique poreuse

The dielectric behaviour of a polar solute dispersed in a porous ceramic matrix is studied. Once principal components are characterized, a systematic study of heterogeneous substance as a function of frequency, temperature and various percentages of polymer solute is given.The data are analysed using a Cole and Davidson plot allowing the determination of the overall response. This paper is a contribution to laboratory studies concerning new materials with highly specific electromagnetic characteristics.     

Ondes de Stoneley à l'interface de deux cristaux hexagonaux

We examine the existence of localized waves at a (0001) interface between two hexagonal crystals. These waves correspond to the poles of the Green function for two elastic media with hexagonal symmetry bounded by an interface.     

Thermoluminescence de la glace

Different samples of normal and heavy water are irradiated with gamma rays at Liquid Nitrogen temperature and, then, progressively warmed up. During this period the thermoluminescent glow is recorded and its intensity and emission spectrum analyzed in function of temperature and irradiation dose. One of the two emission peaks appear to be directly linked to the network structure within the ice.     

A propos de l'interprétation des variations avec la température du travail de sortie des surfaces de GaSb

We expose the problems raised by the interpretation of the measurement of semiconductor work function variations obtained at different temperatures by the Kelvin condensator method. The variations of the electronic affinity versus temperature are calculated from band theory results.     

Comportement asymptotique de la fonction de distribution radiale dans les liquides

We study the influence of pair potential on the asymptotic behaviour of the radial distribution function in liquids. We use a perturbation expansion from a model of rigid spherical core inKirkwood's integral equation. It is pointed out that in metals as well as in insulators the asymptotic behavior is connected to that of the pair potential. However, the results are numerically different from those obtained from thePercus-Yevick approximation.     

Perspectives sur la théorie des pseudogroupes de transformations analytiques de type infini

This paper deals with the local and abstract structure of Lie pseudogroups of infinite type. The formal aspect is considered in several cannonical ways, one of which is compatible with the known pathological behaviour of the exponential function. This defines a very precise program for the development of a convergent abstract theory of analytic Lie pseudogroups of infinite type.     

Problèmes mathématiques de l'équation de Boltzmann relativiste

This work deals with relativistic Boltzmann equation and more particulary with integral operator of complete equation and integral operator of linearized equation. These operators depend on the differential cross sectionh(〈p, q〉, cos θ) which is a fonction of energy 〈p, q〉 and of the deviation angle θ. The only hypothesis is thath is a symetric function of cosθ. The second part deals essentially with linearized equation in Special Relativity. We take for the distribution function: $$F\left( {x,p} \right) = a e^{ - \frac{{\lambda p}}{2}} \left( {e^{ - \frac{{\lambda p}}{2}} + \varepsilon f\left( {x,p} \right)} \right)$$ wherea is a constant, λ a constant vector and ? a small constant so that ?² can be neglected. We obtain the equation: $$\frac{{p^\alpha }}{{p^0 }}\frac{{\partial f}}{{\partial x^\alpha }} = - K\left( p \right) \cdot f + G\left( f \right)$$ whereK(p) is a positive function andG an Hilbert-Schmidt operator. Then we resolve the Cauchy's problem by taking the Fourier's transformation off, and in the last part by investigating properties of the resolvent of ?K+G we establish that asx ⁰→+∞ the solution of this problem has for limit the equilibrium distributiona e ^?λp .     

Influence de la temperature sur les intensites des raies Raman de resonance

Using an analytical expression for the resonance Raman scattering tensor, the influence of the temperature on the scattered intensities is studied.The variation of the intensities as a function of the temperature is different from the result given by the polarisability theory. However, it is shown that the usual ratio $\text{I}{}_{\mathrm{<mtext>Stokes</mtext>}}\text{I}{}_{\mathrm{<mtext>antiStokes</mtext>}}$ is still valid with a good approximation.     

Decanalisation de particules α par les amas de Guinier-Preston dans un alliage AlCu 3.3% p

Dechanneling of α particles due to Guinier-Preston zones has been measured as a function of ageing time in an Al-3.3% wt Cu alloy. Dechanneling is extremely sensitive to the precipitation of Guinier-Preston zones.     

Étude de l'adsorption de l'oxygène sur ruban de rhénium par mesure des variations du travail d'extraction électronique

Adsorption and work function changes have been followed on a recrystallized rhenium ribbon [preferentially oriented with (0001) plane parallel to the surface] in an ultrahigh vacuum apparatus. Gas pressure was monitored by mass spectrometry and work function variations measured with the diode method in the retarding potential range, using an electron gun. Despite changes in saturation coverages, work function differences at saturation, and sticking coefficient with the adsorption temperature of the sample, curves giving the surface potential with coverage are characterized by a common part till a critical coverage ($\text{5 × 10}{}^{14}\text{atoms}\text{cm}{}^2$), in the 300–850 K range. Comparison with LEED, thermal desorption and electron induced desorption results of the literature is in agreement with an adsorption induced surface rearrangement, above this critical coverage.     

Auxiliary Conformally Invariant Higher-Spin Field in de Sitter Space

We employ de Sitter isometry to study a mixed symmetric rank-3 tensor field in de Sitter space by finding the field equation, solution and two-point function which are conformally invariant. It is proved that such a tensor field plays a key role in conformal theory of linear gravity (Binegar et al., Phys. Rev. D 27, 2249, 1983) . In de Sitter space from the group theoretical point of view this kind of tensor could associate with two unitary irreducible representations (UIR) of the de Sitter group (Takook et al., J.Math. Phys. 51, 032503, 2010), which one representation has a flat limit, namely, in zero curvature coincides to the UIR of Poincaré group, however, the second one which is named as auxiliary field, becomes significant in the study of conformal gravity in de Sitter background. We show that the rank-3 tensor solution can be written in terms of a massless minimally coupled scalar field and also the related two-point function is a function of a massless minimally coupled scalar two-point function.     

Variation du travail de sortie du W (100) par adsorption d'oxyge`ne,de ce´sium et co-adsorption d'oxyge`ne et de ce´sium

The work-function determination by the Kelvin method is used to study the adsorption of oxygen, cesium and co-adsorption of oxygen and cesium on a (100) tungsten surface, at room temperature. The work-function change of the clean surface with the oxygen exposure is used to estimate the sticking coefficient and the dipole moment of adsorbed oxygen. During cesium deposition on the clean surface, a minimum at 1.58 eV and a plateau at 1.80 eV are obtained. Starting from the minimum obtained with cesium, oxygen adsorption leads to a decrease of the work function down to 1.17 eV, when cesium adsorption on a previously oxygenated surface gives a 1.12 eV minimum and an increase of the plateau up to 2.20 eV. This last variation is shown to be consistent with the observed increase of the dipole moment of cesium adsorbed on a partially oxygenated surface, which accounts also for the lowering of the work function minimum.     

Conformal linear gravity in de Sitter space II

From the group theoretical point of view, it is proved that the theory of linear conformal gravity should be written in terms of a tensor field of rank-3 and mixed symmetry (Binegar et al. in Phys. Rev. D 27:2249, 1983). We obtained such a field equation in de Sitter space (Takook et al. in J. Math. Phys. 51:032503, 2010). In this paper, a proper solution to this equation is obtained as a product of a generalized polarization tensor and a massless scalar field and then the conformally invariant two-point function is calculated. This two-point function is de Sitter invariant and free of any pathological large-distance behavior.     

The de Haas-van Alphen effect in quasi-two-dimensional materials

We calculate the amplitude of magnetization oscillations for a quasi-two-dimensional electron system. In the two-dimensional case the behavior of this amplitude as a function of magnetic field and temperature differ completely from the conventional Lifshitz-Kosevich formula valid for three-dimensional metals. Previously only the ideal two-dimensional case has been considered, and the difference of the shape of the Fermi surface from cylindrical has not been taken into account. We obtain the general formula for the envelope of magnetization oscillations as a function of magnetic field, temperature, and the strength of the warping of the Fermi surface. This problem is important because of the great amount of interest in heterostructures and quasi-two-dimensional organic metals which has arisen in recent years. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 2, 139–144 (25 January 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Spacelike Fluctuations of the Stress Tensor for de Sitter Vacuum

The two-point function characterizing the stresstensor fluctuations of a massless, minimally coupledfield for an invariant vacuum state in de Sitterspacetime is discussed. This two-point function is explicitly computed for spacelike-separatedpoints which are geodesically connected. We show thatthese fluctuations are as important as the expectationvalue of the stress tensor itself. These quantum field fluctuations will induce fluctuations inthe geometry of de Sitter spacetime. This paper is afirst step toward the computation of such metricfluctuations, which may be of interest for large-scale structure formation in cosmology. The relevanceof our results in this context is brieflydiscussed.     

Infrared behavior and gauge artifacts in de Sitter spacetime: the photon field

We study the infrared (long-distance) behavior of the free photon field in de Sitter spacetime. Using a two-parameter family of gauge-fixing terms, we show that the infrared (IR) behavior of the two-point function is highly gauge-dependent and ranges from vanishing to growing. This situation is in disagreement with its counterpart in flat spacetime, where the two-point function vanishes in the IR region for any choice of the gauge-fixing parameters. A criterion to isolate the physical part of the two-point function is given and is shown to lead to a well-behaved two-point function in the IR region.     

Proprietes optiques de l'arsenic cristallise

The reflectivity of arsenic single crystals is measured between 0.08 and 70 eV; the optical and dielectric constants are obtained by Kramers-Kronig analysis of experimental data. The reflectivity singularities are interpreted in term of interband transitions from valence band states and from core levels. The maximum of the energy loss function is attributed to the plasma excitation of valence electrons.     

Synthese et etude des transitions de phases de ceramiques et de cristaux de composition Li0, 02Na0, 98NbO3

Ceramics and single crystals of composition Li_{0. 02}Na_{0. 98}NbO₃ have been synthetised and studied as a function of temperature by X-ray diffraction, microcalorimetry, dielectric and pyroelectric measurements. Optical observation of domains and birefringence properties have been also carried out. The material shows five transitions at T₁ = 163 ± 10K, T₂ = 540 ± 20K, T₃ = 640 ± 20K, T₄ = 825 ± 10K and T₅ = 940 ± 15K. This sequence is quite different from that previously observed for pure NaNbO₃, the Li doped material being ferroelectric below T₃ and ferroelastic below T₅.