Showing papers in "Journal De Physique Lettres in 1984"

On the stability of the quasi-onedimensional metallic phase in magnetic fields against the spin density wave formation

Abstract: For a simple anisotropic metal model with two open nested Fermi surfaces, it is shown that the tendency to spin density wave formation increases when a magnetic field is applied, which could lead to a series of phase transitions as functions of the field value. The results are suggested as an explanation of the phase diagram of the (TMTSF) 2 X compounds La stabilite d'un etat a ondes de densite de spin d'un «metal quasi-1D», a surface de Fermi ouverte, augmente sous champ magnetique ce qui peut conduire a une serie de transitions sous champ. Ce modele pourrait interpreter le diagramme de phases des composes (TMTSF) 2 X sous champ magnetique

Stability of the spin density wave phases in (TMTSF)2ClO4 : quantized nesting effect

Abstract: The stability of the SDW phases in (TMTSF) 2 ClO 4 can be interpreted by simple arguments of anisotropic Fermi surfaces nesting: first, we show that anion ordering, by introducing new Bragg's reflexions, perturbs the nesting and lowers the SDW stability. Then, we improve Gor'kov and Lebed's argument on the effect of a magnetic field on nesting by allowing the wave vector Q(H) to adapt to the field so that nesting is quantized. We predict that H induces a succession of SDW phases with Q(H) varying linearly, separated by first order transitions with Q(H) discontinuities. At low T, the Hall voltage is constant within in SDW phase, as in the Quantized Hall Effect. This is in agreement with the Hall effect data Interpretation de la stabilite des phases a onde de densite de spin (SDW) par de simples arguments d'ajustement de surfaces de Fermi anisotropes. Perturbation de l'ajustement et abaissement de la stabilite de l'onde de densite de spin par l'ordre des anions (nouvelles reflexions de Bragg). Amelioration de l'argument de Gor'kov et Lebed, sur l'effet d'un champ magnetique sur l'ajustement, en permettant au vecteur d'onde Q(H) de s'adapter a H, de sorte que l'ajustement est quantifie. Prevision d'une succession de phases magnetiques, induite par H, ou Q(H) varie lineairement. Constance de la tension de Hall a basse temperature a l'interieur d'une phase

The Benard-Von Karman instability : an experimental study near the threshold

Abstract: The evolution of the velocity components of the wake behind a cylinder has been investigated in the range 40 L'evolution des composantes de la vitesse dans le sillage d'un cylindre est etudiee dans la gamme 40

Chemically limited versus diffusion limited aggregation

Abstract: In realistic growth processes, both kinetic and chemical factors determine the structure of the aggregates. Here, a model is proposed which interpolates between a purely diffusive and a purely chemical situation. There is crossover from a new, chemically limited growth model, with fractal exponents D=1.55±0.05 (2.00±0.08) in two (three) dimensions, to diffusive cluster aggregation Proposition d'un modele qui tient compte des facteurs cinetiques et chimiques pour la determination de la structure des agregats formes par croissance. Calcul de l'exposant fractal de l'agregat chimique a 2 et 3 dimensions. Comportement de crossover entre les regimes d'agregation chimique et diffusive

Cylindrical microemulsions: a polymer-like phase ?

Abstract: The regions of stability of spherical, cylindrical, and lamellar phases of microemulsions are calculated within mean-field theory. In the cylindrical phase, thermal fluctuations determine a temperature dependent persistence length below which the cylinders are rigid (rod-like) and above which the cylinders are flexible (polymer-like). The length of the polymer-like chains depends on concentration and temperature. The radii of gyration of these flexible microemulsions are also calculated.

Two-dimensional grid turbulence in a thin liquid film

Abstract: We present the first observation of grid-generated two-dimensional turbulence in a thin fluid film. The evolution of the vortex sizes, the processes of vortex merging and the general aspect of the velocity field correspond qualitatively to theoretical predictions and to recent numerical simulations On observe la turbulence derriere un peigne tire dans un film liquide mince. L'evolution de la taille des tourbillons, les processus de fusion des tourbillons et l'aspect general du champ de vitesse correspondent qualitativement aux prevision theoriques et aux simulations recentes concernant la turbulence bidimensionnelle

Two-dimensional aggregation and crystallization of a colloidal suspension of latex spheres

Abstract: Etude experimentale de cristallisation bidimensionnelle de spheres de latex calibrees, Browniennes en suspension dans l'eau. L'interaction, controlee par un champ electrique alternatif, s'avere attractive dans une large gamme de frequence. Elle conduit a un phenomene d'agregation de cristallites fractals. La dimension fractale mesuree D=1,74±0,05 est en bon accord avec la theorie d'agregation dynamique des amas. La fusion continue des cristallites intervient lorsque la frequence augmente. La diffusion de la lumiere montre alors l'existence d'une phase intermediaire de symetrie hexatique

Aggregation by kinetic clustering of clusters in dimensions d > 2

Abstract: The hierarchical model recently introduced to describe the mechanism of kinetic aggregation by clustering of clusters is investigated numerically in dimensions d>2. The fractal dimension of the clusters, D, is evaluated from their radius of gyration and estimated to be 1.42±0.03, 1.78±0,05, 2.04±0.08, 2.30±0.20 for d=2,3,4,5 respectively. These results are compared with those of diffusion limited aggregation and with those of linear and branched polymers at equilibrium Le modele hierarchique recemment introduit pour decrire le mecanisme d'aggregation cinetique par collage d'amas est etudie numeriquement pour des dimensions d superieures a d=2. La dimension fractale des amas, D, estimee a partir de leur rayon de giration, vaut 1,42±0,03; 1,78±0,05; 2,02±0,06; 2,25±0,15 pour d=2,3,4,5. On compare ces resultats avec ceux du modele d'aggregation par diffusion limitee et avec ceux des polymeres, lineaires et branches, a l'equilibre

Spectral properties of the Laplacian and random matrix theories

Abstract: We investigate the fluctuation properties of the eigenvalues of the Laplacian in two dimensions with Dirichlet boundary conditions on a stadium. They are found to be consistent with the fluctuations of eigenvalues of random matrices (GOE). It is conjectured that this is true for any boundary such that the motion of a free particle elastically reflected by the boundary is a strongly chaotic motion On etudie les proprietes de fluctuation des valeurs propres du laplacien a deux dimensions avec des conditions aux limites de Dirichlet sur un stade. Elles sont consistantes avec les fluctuations des valeurs propres de matrices aleatoires (GOE). On fait la conjecture que ceci est vrai en general pourvu que la frontiere soit telle que le mouvement d'une particule libre reflechie elastiquement par la frontiere (billard) soit un mouvement tres chaotique

On the nature of the lipid monolayer phase transition

Abstract: Nous presentons une technique amelioree pour l'etude par microscopie electronique et diffraction d'electrons de films monomoleculaires de lipide transferes d'une interface air/eau. Cette methode, combinee avec une technique de microfluorescence, fournit un outil puissant pour etudier la microstructure des monocouches. Nous montrons que la pente non nulle qui apparait dans les isothermes a la transition de phase fluide-cristalline provient de la coexistence de phases fluide et cristalline jusqu'a la transition de type second ordre vers l'etat condense completement cristallin. Nous presentons un modele electrostatique qui rend compte (1) de la stabilite thermodynamique dans la region de coexistence de l'etat fluide et de l'etat solide, (2) de la pente non nulle dans la region de transition, (3) de l'existence de plaquettes cristallines de taille uniforme a pression donnee au debut de la region de transition. Cas de couches minces d'acide dimyristoylphosphatidique

Structural and electrical properties of (TMTTF)2MF6 salts (M = P, As, Sb). Rôle of the anions

Abstract: The structural and electrical properties of different (TMTTF) 2 MF 6 compounds are described and the role of the anion size is investigated through the behaviour of the SbF 6 salt for which a critical increase of the resistivity occurs below 154 K. The effects of alloying (TMTTF) 2 SbF 6 with AsF 6 or PF 6 is also studied and we show that this electrical anomaly still exists for small amount of doping. Finally the existence of structural instabilities involving centrosymmetrical anions is briefly discussed Etude des proprietes structurales et electriques de ces sels ou l'on etudie le role de la taille de l'anion, a partir du comportement du sel de SbF 6 pour lequel un accroissement critique de resistivite est observe a 154 K. Effet du dopage de (TMTTF) 2 SbF 6 par AsF 6 ou PF 6 : l'anomalie de resistivite persiste aux faibles taux de dopage. Discussion de l'existence d'instabilites structurales impliquant des anions centrosymetriques

On the statistical mechanics of optimization problems of the travelling salesman type

Abstract: We show that two very different temperatures regimes exist for problems of the travelling salesman type, and that the annealed approximation is valid for the high-temperature regime Random-link models are introduced, for which upper and lower bounds on the free energy are obtained in the low-temperature regime A soluble model is presented, which possesses a phase transition strongly reminiscent of the spin-glass transition Il existe deux regimes tres differents en temperature pour les problemes du type voyageur de commerce et l'approximation recuite est correcte dans le regime de haute temperature Interdiction des modeles de liaisons aleatoires et determination des limites inferieure et superieure pour leur energie libre, dans le regime basse temperature Presentation d'un modele soluble qui possede une transformation de phase rappelant fortement la transition verre de spin

Cooperative phenomena in (TMTSF)2ClO4 : an NMR evidence

Abstract: We present 1 H and 77 Se NMR relaxation data on oriented (TMTSF) 2 ClO 4 single crystals in the relaxed state. The data show the existence of three distinct regimes for the nuclear relaxation rate T 1 −1 . In the high temperature part (T>25 K) the Korringa law seems to be well satisfied whereas in the lower temperature part we have strong deviations down to 8 K or so. At T≤6 K, a different and enhanced Korringa regime appears to be restored. Preliminary results on (TMTSF) 2 FSO 3 under pressure also show similar features. We propose an interpretation of the T 1 −1 behaviour based on the existence of one-dimensional 2k F spin correlations for repulsive short range intrachain electron-electron interactions. The coherent interchain hopping acts as the main mechanism for the restored but renormalized Korringa behaviour. Moreover, from the low-field data, the proposed scheme of interpretation is not incompatible with the presence of superconducting precursor effects Analyse des donnees de relaxation spin-reseau pour 1 H et 77 Se dans des monocristaux de ce compose. Observation de trois regimes distincts; loi de Korringa suivie a T>25 K, ecart important de T=25 a 8 K environ, nouveau regime de Korringa renormalise et augmente a T≤6 K. Interpretation du comportement de T 1 −1 par l'existence d'importantes correlations 1D de spins a 2k F pour une interaction repulsive intrachaine a courte portee

Electron stimulated desorption of solid argon via exciton creation

Abstract: The correlation between the intrinsic luminescence of solid argon excited by slow electrons (a few eV to 100 eV) and desorption was studied. Related to exciton creation, a sharp threshold of desorption and of luminescence is observed at the same energy of the impinging electrons. The desorption rate is proportional to the exciton density at the surface. The surface nature of different luminescence bands is established. The mechanism of desorption corresponds to the mechanism of cavity formation around self-trapped excitons in solid argon Etude comparee de la luminescence intrinseque de Ar solide et de la desorption du cristal excite par electrons lents (quelques eV a 100 eV). Pour une meme energie des electrons incidents, observation d'un seuil tres marque du a la creation d'excitons. Proportionnalite du taux de desorption a la densite d'excitons a la surface. Preuve du caractere superficiel de plusieurs raies de luminescence. Mecanisme de desorption, par formation d'une cavite entourant l'exciton autopiege dans Ar solide

a.c. response of fractal networks

Abstract: We calculate the a.c. frequency response of Sierpinski-gasket networks, in which the bonds consist of resistors R (or of impedances Z h ) and all nodes are connected to the circuit ground by identical capacitors C (or by impedances Z v ). The resulting complex, size-dependent admittance between any of the «principal» nodes and the circuit ground can be accurately described at all frequencies less than 1/RC by a finite-size scaling function whose exponents are combinations of the fractal dimension d f and the spectral or «fracton» dimension d s of the Sierpinski gasket. The response function also bears a striking similarity to experimental observations of the a.c. response of a random mixture of conducting and insulating particles Nous avons calcule la reponse en frequence en courant alternatif pour un reseau du type Sierpinski dans lequel les liens sont soit des resistances R (ou des impedances Z h ) et ou tous les nœuds sont relies a la terre par l'intermediaire de capacites C identiques (ou d'impedances Z v ). Pour toutes les frequences plus petites que 1/RC, l'admittance complexe resultante entre chacun des nœuds «principaux» et la terre peut etre exprimee avec precision au moyen d'une fonction d'echelle avec effet de taille finie, tous les exposants de cette fonction etant des combinaisons des dimensions fractale d f et spectrale d s du tamis de Sierpinski. La dependance en frequence de la fonction de reponse presente une tres forte ressemblance avec celle d'un melange aleatoire de particules conductrices et isolantes

Mechanical stability of tenuous objects

Abstract: Certaines structures de basse densite, telles que des amas colloidaux, se deforment plus facilement lorsque leur taille augmente. Nous utilisons des proprietes d'homogeneite qui ont ete recemment proposees afin d'etudier les effets dus aux fluctuations thermiques ainsi qu'aux forces de gravitation sur ces amas. Pour des amas colloidaux typiques, formes de particules de taille 100 A, le diametre ne doit pas depasser 10 3 a 10 4 fois celui des particules sans que se produisent des deformations importantes dues a ces deux effets. Les particules composantes de cette taille sont optimales: un important changement de taille quelconque rendrait l'amas moins stable

Long-time properties of trapping on fractals

Abstract: We investigate the long-time decay behaviour of a nearest-neighbour random walker which gets trapped at the first encounter of a sink. We consider both regular and fractal lattices and establish for compact exploration the asymptotic decay Φ∼exp[−Ctα] with α≡d~/(d~+2) where d~ is the spectral dimension. The numerical simulations support the Φ structure, but with a larger α for both the square lattice and the d~=1.365 Sierpinski gasket On etudie la probabilite de survie aux temps longs d'un marcheur aleatoire qui est capture par le 1er piege qu'il rencontre. On considere aussi bien des structures regulieres que fractales et on trouve dans le cas d'une exploration compacte que la loi asymptotique de survie est donnee par Φ∼exp[−Ct * ] avec α≡d~/(d~+2) ou d~ indique la dimension spectrale. Les simulations numeriques donnent des resultats compatibles avec cette forme de Φ, mais indiquent aussi des valeurs de α plus grandes que predites dans les cas d'un reseau carre et du tamis de Sierpinski avec d~=1,365

Dilution and polydispersity in branched polymers

Abstract: Des proprietes experimentalement interessantes d'une sol. diluee par un bon solvant sont etudiees. La dilution laisse la distribution en masse inchangee mais conduit a un gonflement de chaque polymere ramifie, et donc a un changement de proprietes mesurees telles que le rayon moyen 2 , le coefficient de diffusion 2 et la viscosite intrinseque

Spreading laws for liquid polymer droplets : interpretation of the « foot »

Abstract: Depending upon the nature of the polymer-substrate system, it has been observed that the shape of a spreading polymer droplet on a flat surface could be either a spherical cap ― as for low viscosity liquids ― or a spherical cap with a projecting macroscopic «foot». We interpret this «foot» by an effect of finite slip of the polymer liquid at the surface: the existence of a large extrapolation length (b) for the velocity field of polymeric liquids near a smooth surface was predicted long ago by one of us. We calculate the detailed shape of the foot, and show that the foot thickness is comparable to b; this length may reach very high values (>100 microns) for entangled polymers. On the other hand, if the surface is rough, or if some chains are strongly bound to it, b is drastically reduced: this explains why the existence of the «foot» depends on the precise conditions at the surface Une goutte de polymere fondu s'etale sur certaines surfaces en montrant un «pied»» macroscopique qui precede le corps de la goutte. On explique ce «pied» par un effet de glissement du polymere sur la surface solide. On calcule la forme detaillee du pied et on montre que sa hauteur est comparable a la longueur d'extrapolation. Cette longueur est tres sensible a l'etat de surface du solide

Finite size effects in cluster-cluster aggregation

Abstract: A finite size estimate is proposed for the fractal dimension of clusters obtained through hierarchical kinetic cluster-cluster aggregation. This estimate is shown to be exactly size-independent for d = 1 and above the upper critical dimension dc. For 1 < d < dc, it varies only weakly with size, leading to better values for the extrapolated fractal dimension, as shown by numerical results on Brownian and linear trajectories for 2 ≤ d ≤ 5.

Discotic mesophase of copper(II) laurate

Abstract: Copper(II) laurate (copper dodecanoate) shows a discotic mesophase at temperatures above 107°C. This mesophase is characterized by a hexagonal columnar lattice with a repeat distance of about 20 A. Each column is made of stacked units with a period of about 4.7 A. The repeating unit in a column is the binuclear complex Cu 2 (C 11 H 23 CO 2 ) 4 which is the first example of a discotic mesogen with only four peripheral chains showing a thermotropic hexagonal mesophase Formation d'une phase discotique aux temperatures superieures a 107°C, caracterisee par un reseau hexagonal de colonnes distantes d'environ 20 A. Ces colonnes resultent de l'empilement de motifs elementaires distants d'environ 4,7 A. Le motif elementaire est le complexe binucleaire Cu 2 (C 11 H 23 CO 2 ) 4 qui constitue le premier mesogene discotique a quatre chaines peripheriques donnant une mesophase hexagonale thermotrope

Weak anchoring energy and pretilt of a nematic liquid crystal (

Abstract: We extend the wedge technique of Riviere et al. to measure the pretilt angle Φ and the (weak) anchoring energy of a nematic liquid crystal oriented on a suitably treated glass surface. The liquid crystal is placed in a wedge shaped space formed by the plate to be studied and a «standard» plate giving a strong homeotropic anchoring. Instead of using a total reflection technique, we measure the optical path difference (Δl) between the ordinary and extraordinary waves, as a function of the sample thickness (d). From an analysis of this variation, we obtain the pretilt angle from the region of large thickness, and the anchoring energy from the region of small thickness, a domain which could not be studied with the previous technique. The transition thickness d~ is comparable with the extrapolation length L associated with the surface energy. Typical values of Φ and L for rubbed silane coated plates are ∼0.8 radian and ∼1 μm respectively Extension de la technique de Riviere pour mesurer la preinclinaison et l'energie d'ancrage d'un cristal liquide nematique oriente sur une lame transparente

Spin rotation effects and spin waves in gaseous 3He

Abstract: Spin polarized gases such as H↑ or 3He↑ can propagate damped spin waves at low temperatures; their origin is the so-called « identical spin rotation effect » in collisions between identical particles. We report here the observation of such effects in 3He↑ in the temperature range 2 < T < 6 K, and preliminary measurements of the coefficient μ which characterizes the quality factor of the spin oscillations. The measurements are in agreement with the numerical calculations of C. Lhuillier.

Hierarchical structure of defects in non-crystalline sphere packings

Abstract: We describe the defect structure of a tetrahedral close packed model. Defects appear as a hierarchy of interlaced disclination networks which form the locus of sites where the local order deviates from a perfect icosahedral environment. This defect structure is generated by an iterative process which gradually decreases the curvature of an ideal polytope model.

Theory of the memory effect in thiourea. Defect density waves in modulated systems

Abstract: A simple Landau-Ginzburg theory is shown to account qualitatively and semi-quantitatively for the memory effect anomalies of the susceptibility and the birefringence in thiourea. The importance of the gradient amplitude coupling energy in thiourea is emphasized. The possibility of creating a defect-induced locked incommensurate phase around any arbitrary temperature within the modulated phase is demonstrated. Such a locked phase will appear as a consequence of the condensation of a periodic defect concentration if extrinsic mobile impurities are allowed to interact for a sufficient time with a static modulation. We briefly discuss some aspects of such defect density waves in the physics of modulated structures Emploi d'une approche de Ginzburg, Landau pour rendre compte qualitativement et semi-quantitativement des anomalies de susceptibilite magnetique et de birefringence, liees a l'effet memoire, le couplage gradient amplitude jouant un role determinant dans la thiouree. Demonstration de la possibilite de creer, autour d'un point arbitraire dans la phase modulee du diagramme de phase, une phase incommensurable bloquee induite par defauts extrinseques

Structural defects of a lamellar lyotropic mesophase : a neutron scattering study

Abstract: Contrarily to previous expectations, the flat geometry is not the configuration of lowest free energy for the bilayers of SDS in the lamellar phase of SDS/water. Indeed, neutron scattering reveals an unusual pattern of diffuse scattering at small angles, in addition to the regular diffraction spots of a layered mesophase. The diffuse scattering away from the beam is produced by an array of structural defects, with a pseudo-period of 60 A within one layer. The diffuse scattering around the beam is caused by textural defects, with dimensions larger than 200 A Mise en evidence de deux types de defauts sur le systeme SDS/eau selon les regions de l'espace reciproque considerees, des defauts de structures dont la pseudoperiode est de 60° a l'interieur d'une lamelle et des defauts de texture

Pole condensation and the Riemann surface associated with a shock in Burgers' equation

Abstract: Using a paradigmatic initial condition, we completely analyse the spatial analytic structure of Burgers velocity field and its dependence on time t and viscosity v. The viscous solution has an infinite number of complex poles appearing suddenly at t=0 + . When v↓0 these poles condense on Stokes lines giving rise to the inviscid singularities; the 3-sheeted Riemann surface associated with the latter provides an analytic path across the jump of the shock Pour une condition initiale paradigmatique, nous analysons completement la structure analytique du champ de vitesses, solution de l'equation de Burgers, en fonction du temps t, et de la viscosite v. La solution visqueuse a un nombre infini de poles complexes qui apparaissent soudainement a t=0 + ; quand v↓0, leur condensation sur les lignes de Stokes donne naissance aux singularites inviscides; la surface de Riemann a 3 feuillets associee a ces dernieres permet un franchissement analytique du saut du choc

A structural investigation of the liquid crystal phases of 4-(2'-methylbutyl)phenyl 4'-n-octylbiphenyl-4-carboxylate

Abstract: The smectic phases of the compound 4-(2'-methylbutyl)phenyl 4'-n-octylbiphenyl-4-carboxylate (2M4P8BC) were studied by means of polarized optical microscopy, miscibility, DSC, and X-ray diffraction techniques. The phase sequence Cr↔G↔J↔I↔C↔A has been determined, and consequently 2M4P8BC has been shown to be the first example of a pure material exhibiting a J to G phase transition. In the optically pure form, the I* phase is a chiral hexatic phase, whereas the J* and G* phases were found to be nonchiral crystal phases. The C-centred monoclinic lattice parameters have been determined Etude par microscopie optique polarisee, miscibilite, calorimetrie differentielle a balayage et diffraction RX. Mise en evidence de la sequence de phases Cr↔G↔J↔I↔C↔A. Dans sa forme optiquement pure, la phase I* est une phase chirale hexatique alors que les phases J* et G* sont des phases cristallines non chirales. Parametres cristallins de la phase monoclinique C-centree

Polymer melt : reptation of a chain and viscosity

Abstract: HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Polymer melt : reptation of a chain and viscosity J. Des Cloizeaux

Quantum Hall effect and Fermi surface instabilities in (TMTSF) 2ClO4

Abstract: Peaks in magnetoresistance at the same magnetic field as the steps in the Hall voltage are observed in (TMTSF) 2 ClO 4 . These measurements were performed between 0.1 and 1.2 K, in magnetic fields up to 8.3 T, on a sample having a resistivity ratio R(T=300 K, H=0)/R(T→0, H→0)>1600. These results are discussed within a model associating the quantum Hall effect with phase transitions Observation de pics de magnetoresistance au meme champ magnetique que les marches de l'effet Hall, lors de mesures entre 0,1 et 1,2 K dans des champs magnetiques allant jusqu'a 8,3 T sur un echantillon ayant un rapport de resistivite superieur a 1600. Discussion a l'aide d'un modele associant effet Hall quantique et transformations de phase