Showing papers in "Journal De Physique in 1976"

Diagonalisation d'une classe d'hamiltoniens de spin

Abstract: On diagonalise une classe a 2 N parametres d'hamiltoniens quadratiques dependant de N spins de grandeur arbitraire. Diverses extensions et applications sont donnees, en particulier celle de l'oscillateur couple a une collection de systemes independants a deux niveaux.

Remarks on polyelectrolyte conformation

Abstract: We discuss the conformations of linear polyions assuming that a) the corresponding uncharged chain is flexible ; b) electrostatic forces dominate the monomer-monomer interactions; c) no salt is added. 1) For the dilute case (non overlapping chains) correcting a recent self-consistent calculation by Richmond [1a], we find an overall polyion size R = Nd which is a linear function of the polymerization index N in agreement with the early work of Hermans and Overbeek, [1b], Kuhn, Kunzle, and Katchalsky [1c]. 2) There is a range of very low concentration c (c** c**) : here we expect that the polyions build up a 3-dimensional periodic lattice ; however, the detection of such an extremely dilute lattice appears difficult. 3) Practically all experiments on salt-free polyelectrolytes have been performed at concentrations c > c* where different chains overlap each other. To discuss this regime we restrict our attention to cases where the charge per unit length is near (or above) the condensation threshold : then a single length ξ( c) characterizes the correlation; in 3 dimensions ξ scales like the Debye radius associated with the counter ions. We consider several possible conformations : a) hexagonal lattice of rigid rods ; b) cubic lattice of rigid rods ; c) isotropic phase of partially flexible chains. The various rigid rod structures appear to have very similar electrostatic energies. This suggests that the isotropic phase might possibly be the most favorable. We analyse this latter phase using the same scaling methods which have recently been helpful for neutral polymer solutions (2). In the isotropic model each chain behaves like a succession of segments of size. Inside one segment electrostatic effects are important and similar to case (1) above. Between segments the interactions are screened out, and tach chain is ideal on a large scale, with radius R (c) ∼ c-1/4 N1/2 . If we (tentatively) assume that the dynamical effects of entanglements are weak, we are than led to a viscosity ηsp/c ∼ Nc-1/2.

The curvature elasticity of fluid membranes : A catalogue of vesicle shapes

Abstract: 2014 Shapes of closed fluid membranes such as those formed by lecithin in water were calculated as a function of enclosed volume, membrane area and spontaneous curvature. As the area can be taken to be constant, the only elasticity controlling the shapes of these vesicles is that of curvature. A large variety of rotationally symmetric shapes are presented, allowing for indentations, cavities and contact of the membrane with itself. LE JOURNAL DE PHYSIQUE TOME 37, NOVEMBRE 1976, Classification .Physics Abstracts 7.130

Scaling theory of polymer adsorption

Abstract: We discuss the equilibrium properties of flexible chains adsorbed on a flat surface, assuming that a) adsorption is weak, resulting in large thicknesses D for the adsorption layer, b) the chains are very long, c) the solvent is good, so that excluded volume effects become important (contrary to an early opinion of Hoeve). The repulsion between monomers are incorporated through a scaling theory which goes beyond the usual mean field (Flory-Huggins) approximations. The central assumption is that polymer concentrations in the first layer and in the next layers scale in the same way, (although they do differ by a constant factor). We can then predict the surface exponents of interest in terms of standard exponents for self avoiding walks; for the latter we use the Flory values. One then finds three regimes of adsorption : (i) In the dilute limit (separate coils) D is independent of the polymerization index N, and varies with the strength of the effective monomer surface attraction (measured by a small parameter δ) according to D ∼ δ -3/2 instead of D ∼ δ-1 in mean field. (ii) There is a semi dilute regime where the coils begin to overlap, but where D remains essentially the same. (iii) The most important case corresponds to the plateau in the adsorption isotherm : here we show that the correlations inside the layer are similar to those in a three dimensional system. Then D ∼ Nδ(ln cB) -1 where cB is the bulk concentration. The dependence on N is stronger than in mean field (D ∼ N1/2) but not incompatible with existant data.

Localized time-dependent perturbations in metals : formalism and simple examples

Abstract: Methods introduced by Keldysh to treat non-equilibrium problems are applied to strong, localized time-dependent perturbations in metals. After having introduced the formalism, we treat simple examples which are linked to the dynamics of atoms near surfaces : ionization probabilities of atoms leaving a metallic surface and friction coefficients of atoms near a metallic surface.

Temperature-concentration diagram of polymer solutions

Abstract: Starting from a tricritical approach to the polymer theta point, we deduce a (T, C) diagram for polymer solutions where T is the temperature, C the monomer concentration. We define four different regions in this diagram. For each of these regions we calculate the T, N, C, dependence of the mean square end to end distance of a chain, the screening length, the osmotic pressure, and the second virial coefficient. We also give a scaling form for the equation of state.

Surface tension and deformations of membrane structures : relation to two-dimensional phase transitions

Abstract: We analyse the thermal fluctuations of two types of flexible membranes : 1) Closed systems (such as the membrane of a biconcave red blood cell) where the chemical constituents of the membrane are fixed in number, but the membrane surface S is adjustable. 2) Open systems (such as bilayer films) where constituent molecules can be exchanged with a reservoir (at the outer surface of the rim). For the first case, the surface tension is zero; fluctuations are then very large, and limited only by certain anharmonic terms. We show that these terms can be analysed exactly - the problem being related to the spherical model of phase transitions. Although the anharmonic terms are strong, they do not modify the laws derived for the light scattering in the simpler harmonic model constructed earlier by one of us [1]. For the second case, there is a finite surface tension, and the fluctuations are much less pathological. This seems to agree with recent measurements by inelastic light scattering on black films by Grabowski and Cowen.

Shear flow instability in nematic liquids : theory steady simple shear flows

Abstract: 2014 We study shear flow instabilities in nematic liquids of planar geometry (molecules parallel to the plates limiting the sample) and where molecules are aligned (along Ox) perpendicular to the shear flow direction (along Oy). An external magnetic field reinforces the alignement imposed by surface effects. We develop a bidimensional model which takes into account exactly boundary conditions on the director and the velocity fields. The two types of instability which have been observed by Guyon and Pieranski (homogeneous distortion and convective rolls parallel to Oy) are interpreted within the framework of this model. We obtain instability thresholds together with the value of the critical wave vector qcx at threshold. Under weak fields the homogeneous distortion (qcx = 0) is achieved, whereas in high fields a roll instability develops (qcx ~ 0). The crossover from one regime to the other takes place at H ~ 1 kG. Moreover, we show that in either case, as far as the director is concerned, the aspect of the distortion along the z-direction is dominated by only one wave vector qz. Contrary to the case of the homogeneous distortion where qz varies strongly with H, for the roll instability qz remains nearly constant and close to 03C0/2a (2 a is the thickness of the sample). This allows us to develop an approximation which leads to results in good agreement with those obtained exactly. LE JOURNAL DE PHYSIQUE TOME 37, AVRIL 1976, Classification Physics Abstracts 6 . 300 7 . 130

Spectral narrowing of selective reflection

Abstract: 2014 A theory for the spectral narrowing of selective reflection near a resonance line of an atomic vapor is developed. The narrowing is ascribed to transient polarization behaviour of vapor atoms associated with wall collisions. For normally incident light the reflectivity of a glass-vapor interface is calculated both for diffuse and for specular wall collisions and for a symmetric distribution of atomic velocities. The theoretical results are compared with recent experimental results. LE JOURNAL DE PHYSIQUE TOME 37, MAI 1976,

Recoil-distance lifetime measurement in 37Cl

Abstract: Using the recoil-distance technique in the 27Al(12C, 2p)37Cl reaction, mean lives of (19.6 ± 3.0) ps for the 7/2 -, 3 103-keV state and (32.8 ± 2.0) ps for the 9/2-, 4 011-keV state in 37Cl have been measured. The strength of the M2 decay from the 3 103 keV level appears to be the strongest among known M2 transitions in 2s-ld nuclei. An upper limit of 0.8 ps for the feeding time in the fusion-evaporation reaction 27Al(12C, pn)37Ar was estimated from an attenuated Doppler-shift measurement.

A simple stochastic description of desorption rates

Abstract: 2014 The kinetic theory of desorption rates for an adatom on a solid surface is reconsidered from a stochastic point of view. The results of Kramers, obtained within the framework of a FokkerPlanck equation, are thereby generalized. For a deep potential well, the desorption rate is related to the average sticking probability of an incoming thermal particle. The latter in turn depends only on the average energy loss 03B4 of the particle to the substrate during a single oscillation. It is shown that in most cases the loss to lattice vibrations is so strong as to justify a posteriori the absolute rate theory of Eyring. The theory is extended to more complicated situations (adparticle with internal degrees of freedom, activated barrier, ...). Finally, a fully time-dependent formulation, valid for shallow wells, is sketched; it is shown how, for deep wells, it reduces to the above simple formulation, based on detailed balance only. LE JOURNAL DE PHYSIQUE TOME 37, NOVEMBRE 1976, Classification Physics Abstracts 7.854

Effect of high frequency irradiation on the dynamical properties of weakly bound electrons

Abstract: On etudie comment le comportement d’un electron dans des champs statiques electrique ou magnetique est modifie lorsque cet electron interagit egalement avec une onde electromagnetique haute frequence. Les nouvelles proprietes dynamiques de l’electron sont decrites par un hamiltonien effectif dont l’expression est etablie en utilisant une description quantique du champ electromagnetique et en tenant compte des corrections relativistes jusqu’a l’ordre 1 /c2. Outre une correction de masse bien connue, on trouve que les nouvelles proprietes dynamiques de l’electron peuvent etre comprises en termes de facteurs de forme electrique et magnetique qui ont une interpretation physique simple. Ces resultats generaux sont enfin appliques a 2 cas simples : perturbation des niveaux de Rydberg d’un atome et modification des frequences cyclotron et de precession de spin d’un electron dans un champ magnetique uniforme.

L'effet néphélauxétique pour les configurations 4fN en phase solide

Abstract: 2014 The change in energy of the levels 2P1/2(4f3), 5D0(4f6) and 6P7/2(4f7) was recorded after doping of the corresponding elements in isomorphous, or structurally related matrices across the lanthanide series. It is shown, that when a 4fN ion is introduced into a solid, the Racah parameters E1, E2, E3 are reduced as the lanthanide to ligand distance is made smaller, for the same crystalline geometry. The respective percentages of reduction for each of the three parameters are unequal and depend on the nature of the crystal structure. LE JOURNAL DE PHYSIQUE TOME 37, JUIN 1976, PAGE

Analysis of the electron excitation spectra in heavy rare earth metals, hydrides and oxides

Abstract: 2014 The energy loss spectra of 75 keV electrons transmitted through thin evaporated foils of heavy rare earths (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in three different chemical states (metal, hydride and oxide) exhibit two main peaks due to collective « plasmon » excitations in the 10-17 eV energy range and to inner 5p excitations between 30 and 40 eV. A quantitative analysis of the intensity spectral distribution allows one to calculate the energy loss function and, through a KramersKrönig inversion, the dielectric constant and the dipole oscillator strength. Various results are then explained in terms of band structure, so that we are lead to propose a simple model for the interband transitions occuring in oxides. LE JOURNAL DE PHYSIQUE TOME 37, AVRIL 1976, Classification Physics Abstracts 2.820 8.800 8.120

A molecular theory of surface tension in nematic liquid crystals

Abstract: The surface tension at the free surface of a nematic liquid crystal is computed using the Fowler-Kirkwood-Buff approximation to the interface, and a mean field approximation to the molecular pair distribution function. It is found for a Van der Waals interaction between molecules that : a) the molecules always prefer an orientation in the plane of the surface; b) there is a gap at the nematic-isotropic phase transition, but the surface tension is less in the nematic phase; c) there may be observable excess surface order which persists into the bulk isotropic phase. General expressions are given for the surface order parameter using free energy arguments. The effect of a permanent dipole interaction on the surface tension is considered, and it is found for uniaxial molecules, that this interaction also leads to the condition that the molecules be parallel to the surface. LE JOURNAL DE PHYSIQUE TOME 37, OCTOBRE 1976,

Sign change of the c.e.f. parameters in light rare earth compounds in relation with the delocalization of the 4f shell

Abstract: 2014 The crystal field level scheme of RX3 (R = Ce, Pr, Nd ; X = In, Pb) intermetallic compounds has been determined by fitting susceptibility curves and specific heat Schottky anomalies ; an inelastic neutron diffraction experiment has been performed for NdIn3. The fourth order C. E. F. parameter A04 r4 > of the cérium compounds is positive ; it decreases for the praseodymium compounds and becomes negative for the neodymium compounds. The sixth order C. E. F. parameter A06 r6 > exhibits the same trend to negative values from praseodymium to neodymium compounds. This sign change of the C. E. F. parameters appears to be related to the small energy difference between the conduction band and the 4f levels at the beginning of the first rare earth series. LE JOURNAL DE PHYSIQUE TOME 37, FÉVRIER 1976, Classification Physics Abstracts 8.512

(3He, xn), (3He, pxn) and (3He, fission) reactions on 206Pb between 80 and 200 MeV

Abstract: The reactions induced in 206Pb by 3He particles having energies between 80 and 200 MeV have been studied. Excitation functions for (3He, xn) with x = 3 to 14 and for (3He, p xn) with x = 2 to 5 have been obtained. Angular distributions of fission fragments were measured at 100, 125, 150 and 175 MeV and total fission cross-sections were deduced from the data. On the basis of these results, analysis is attempted to examine the characteristics of reaction mechanisms. From these results we get the conclusion that non-compound processes play an important role in the reactions. Two features are characteristic of these processes : large cross-sections for charged particle emission and angular distribution of fission fragments closed to isotropy in the laboratory system. In the energy range 25 to 45 MeV/nucleon, a comparison was made between the present results and those from an experimental study of α-particle induced reactions on 206Pb. Also a comparison was made with an α-nucleus collision model applied to 206Pb. All the observations strongly suggest a breakup of the projectile 3He followed by the interactions of the fragments with the target nucleus.

Static strain waves in cholesteric liquid crystals. - I. homeotropic boundary conditions

Abstract: 2014 Periodic strain waves have been studied using dilute solutions of cholesteric liquid crystals in nematic solvents. Experimental observations are in qualitative agreement with computer solutions of one constant elasticity theory. For the boundary conditions of parallel plates with homeotropic pinning of the director field a periodic solution is found to have slightly lower energy than the solution which is translationally invariant in the plane of the plates when a critical thickness is exceeded. LE JOURNAL DE PHYSIQUE TOME 37, AVRIL 1976, Classification Physcis Abstracts 7.130

Poiseuille flow in nematics : experimental study of the instabilities

Abstract: This work discusses experimentally the hydrodynamic instabilities of an oriented nematic film contained in a plane Poiseuille flow cell such that the director is initially perpendicular to the velocity and velocity gradient. Homogeneous instabilities having a uniform distortion in the plane of the boundary plates are obtained with or without distortion of the flow profile. The thresholds are analysed and compared with the theoretical analysis given in the following paper. The effect of alternating flow and of the application of external fields on the nature of the mode and on the threshold is analyzed with attention to the transition from the homogeneous to roll instabilities. No detailed theory exists for these problems and the analysis is carried with reference to the simple shear flow case although marked differences are observed. LE JOURNAL DE PHYSIQUE TOME 37, OCTOBRE 1976,

Sur l'origine des alphas de scission

Abstract: On presente un mecanisme d'emission pour les particules α de scission a partir duquel le spectre energetique, la frequence d'apparition et leur variation en fonction de divers parametres recoivent une explication unitaire. On discute, en outre, de nouvelles possibilites d'etudes de la structure et de la dynamique du systeme fissionnant, au-dela du dernier point-seuil.

Ultrasonic investigation of nematic liquid crystals in the isotropic and nematic phases

Abstract: We report measurements of the frequency dispersion of ultrasonic attenuation for a nematic liquid crystal, the n-pentyl cyanobiphenyl in the vicinity of the nematic-isotropic transition. In the isotropic phase, the experimental results are well described by the theory recently proposed by Imura and Okano. The frequency ωmax at which the maximum of attenuation per wave length αλ occurs, as well as the peak value (α λ) max, diverge at the transition with the critical exponents predicted by the theory. We have extended the Imura-Okano theory to the nematic phase on the basis of a coupling between sinusoidal temperature variation induced by sound wave and the elastic Frank constants. This model accounts semi-quantitatively for the experimental results. We have also investigated the dependence of the low frequency attenuation with respect to the angle between the director and the acoustical wave-vector for a nematic liquid crystal (Merck V) which exhibits a broad mesomorphic range. Far below the transition temperature the experimental data are well described by the theory of Forster et al., provided that the frequencies are far below the intramolecular relaxational frequency.

Electronic properties of mixed uranium-thorium monosulphides (U xTh1-x)S

Abstract: 2014 We have measured bulk magnetization, neutron magnetic scattering and specific heat between 1.5 and 300 K of mixed uranium-thorium monosulphides (UxTh1-x)S with large uranium concentrations (x ~ 0.20). Long-range ferromagnetic order is observed at low temperature for 0.43 x ~ 1, but no long-range magnetic order occurs for x 0.43. Magnetization and Curie temperature decrease linearly with uranium content. The electronic specific heat is maximum at the critical concentration xc = 0.43. Our experiments failed to reveal Schottky anomalies in specific heat or crystal-field levels by inelastic neutron scattering. Our results favour a band description rather than a localised description for uranium 5f electrons. LE JOURNAL DE PHYSIQUE TOME 37, OCTOBRE 1976, 1

Neutron scattering study of methyl group rotation in solid para-azoxyanisole (PAA)

Abstract: 2014 Using incoherent neutron scattering, we show that the methyl groups in solid PAA undergo random rotational motion around their three-fold axis, and that rotations about other molecular axes on the same time scale can be excluded. A model for uniaxial rotational jumps between three equidistant sites on a circle of radius a = 1.032 Å is fitted to the quasielastic spectra. The mean jump time 03C4 between two consecutive jumps is determined and found to vary from 2.93 x 10-11 to 0.95 x 10-11 s between 17 and 117 °C, yielding an activation energy of Ea = 2.54 ± 0.13 kcal/mole. The intense peak observed in the inelastic region at 31 meV energy transfer is assigned to the 03BD = 1 ~ 03BD = 0 torsional frequency of the methyl group in a three-fold potential. For a cosine potential one obtains a barrier V3 = 4.21 ± 0.37 kcal/mole. Attempts to relate Ea and V3 values lead to the conclusion that the potential shape differs appreciably from the cosine form. Our results are then compared with other experimental data. In particular, it seems that the barrier to rotation of the methyl groups arises mainly from intramolecular interactions. LE JOl RNAL DE PHYSIQUE TOME 37, MAI 1976, Classification Physics Abstracts 7.130 7.114

On the origin of a strong central peak in the local density of d states at the surface of transition metals

Abstract: 2014 The central peak observed on the (100) surfaces of Mo and W cannot be thought of as due to a van Hove anomaly of the d band of the surface atomic plane, broadened by interaction with the volume of the metal. It is argued that it might be considered as due to virtual d states on the surface atoms, broadened by interaction with the volume of the metal. A basic difference between closepacked and not so closepacked surfaces in stressed in this connection. Other cases where surface peaks have been computed in the LCAO approximation are also considered. LE JOURNAL DE PHYSIQUE TOME 37, JUILLET-AOiJT 1976, Classification Physics Abstracts 8.322

Driving force for the electromigration of an impurity in a homogeneous metal

Abstract: Through a microscopic analysis the thermodynamic definition of the effective valence as the flux of electric charge associated with a unit flux of the solute is shown to be equivalent to the usual definition in terms of the force acting on the impurity in an electric field. Using the thermodynamics of irreversible processes, we show that the force acting on an impurity is proportional to its specific resistivity in the host metal, whatever the strength of the impurity potential.