Publications (69)82.18 Total impact


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ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.  [Show abstract] [Hide abstract]
ABSTRACT: We report in this paper detailed measurements of the linear and nonlinear optical properties of thin films of P(VDFTrFE) copolymers (with composition 70/30 mol%). The polymer films are cast from solutions using the spin coating technique and poled at saturation using very lowfrequency a.c. fields. The refractive index (n = 1.42) is determined from measurements of the Brewster incidence, and the thickness from measurements of the optical path close to the normal incidence. Recording of the Maker fringes and calibration of the setup with a quartz plate allow us to determine the secondharmonic coefficients: d33 = 4.3, d31 = 0.22 10−12 m/V and the optical dispersion n(2ω) − n(ω) = 0.0077 (ω is the frequency of the incident laser beam at 1060 nm). The spincoated copolymer films are optically unaxial due to a preferential orientation of the chain axes parallel to the plane of the film, and measurement of their birefringence gives ne − no = −0.0083. Therefore, this feature allows us to find a direction of phase matching along which the first and secondharmonic waves have an infinite coherence length. It is observed using an indexmatching liquid.  [Show abstract] [Hide abstract]
ABSTRACT: Without application of an external electric field, the temperature dependence of the shear angle uxy, which is proportional to the spontaneous polarisation and therefore to the order parameter, has been measured by means of Bragg scattering of 412 keV gamma radiation in KH2PO4 and RbH2PO4. Simultaneously, optical observations of the domain structure were performed. The shape of the rocking curve of KDP and RbDP are compared and discussed. The ferroelectric transition in RbDP is of second order in contrast to KDP which exhibits a firstorder one. The modified Slater's theory provides for RbDP a good fit to the experimental data uxy(T) in the whole temperature range investigated. The fact that beta =0.5 near Tc can be understood as an effect of the dipolar interaction; so this system exhibits a cross over from a Slater behaviour to a dipolar behaviour.  [Show abstract] [Hide abstract]
ABSTRACT: A phenomenological theory of the incommensurate phase transition which occurs close to the alpha  beta transition in quartz is presented. This theory assumes that the coupling between the soft optic mode of the alpha  beta transition and the acoustic modes play an important role in the formation of the modulated structure. According to the values of the phenomenological parameters, different kinds of incommensurate structures can form, in particular singlek or triplek structures. The available experimental data concerning the quartz transition are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: Monolayers of short chiral alcohols at the water surface provide a simple model for chiral interactions between molecules. This paper is focused on alcohols with two particular chain lengths, 2tridecanol (2C13) and 2tetradecanol (2C14). Thermodynamic and structural parameters were measured, varying the ratio of left and right enantiomers within monolayers. The evolution of melting temperatures and entropies is not consistent with the formation of a racemate. Grazing Xray diffraction, revealing the molecular stacking, shows clearly that there is no chiral separation. In these two compounds molecules spontaneously selfassemble at the water surface as a solid solution. The chiral polar headgroup is partially screened by the chain thermal disorder of the rotator phase. 2tridecanol exhibits a hexagonal rotator phase stable in time and independent of temperature. Experiments performed close to the melting point show pretransitional effects for mixtures different from 50/50. This broadening of the peak can be related to defects in 2D crystals, phenomena already observed for the melting of some 3D systems.  [Show abstract] [Hide abstract]
ABSTRACT: We formulate a simple Landau type model describing macroscopic behavior recently discovered in new smectic phases composed of achiral bentshaped molecules. Films of such smectics exhibit three types of ordering related to dipole polarization, molecular tilt, and chirality. However, due to specific thirdorder coupling of the order parameters, these three symmetrybreaking types are not independent and this fact leads to specific domain structures that are observed in experiments. (C) 2000 MAIK "Nauka/Interperiodica".  [Show abstract] [Hide abstract]
ABSTRACT: We formulate a simple Landau type model describing macroscopic behaviour recently discovered new smectic phases composed of achiral bentshaped molecules. Films of such smectics exhibit three types of ordering related to dipole polarization, molecular tilt, and chirality. However due to specific third order coupling of the order parameters these three types of symmetrybreaking are not independent ones, and this fact leads to specific domain structures really observed in experiments.  [Show abstract] [Hide abstract]
ABSTRACT: Structural relaxations near surfaces and interfaces are analysed in a simple, generic model with first, second and thirdlayer interactions. The relaxations have exponential envelopes with three types of structural distortion: (i) ferrodistortive, (ii) antiferrodistortive, and (iii) modulated (incommensurable). Their stability conditions in the field of control parameters and their relationship with structural phase transitions is derived. A tricritical point is found for threelayer interactions and a transition to an incommensurate phase. All phase transitions to the ferrodistortive phase are firstorder in the model. The theory is applied to the analysis of lattice relations near internal interfaces in ionic polytypic materials (e.g. PbI2). Diffuse Xray scattering and the shift of diffraction angles are the typical fingerprints for such relaxations. The relevant structure factors are calculated. There is tentative agreement between the calculated and observed diffraction profiles.  [Show abstract] [Hide abstract]
ABSTRACT: Biquadratic coupling between two order parameters Q1 Q2 can lead to transitions between a phase with only one of the two order parameters active (phase I or II) and a mixed phase III in which both order parameters are symmetry breaking. Domain walls for positive coupling energies are chiral under thermodynamic conditions close to the transition points IIII and IIIII. Annihilation of chirality in the wall leads to wall widening and wall bifurcations. The widths of the walls are different for the two order parameters leading to greater lattice distortions on either side of the wall than in the centre of the wall.  [Show abstract] [Hide abstract]
ABSTRACT: The electrostatic quadrupolar interaction between spherical nematic droplets in an isotropic (and nonconducting) liquid is calculated. It is found to have an anisotropic form U q ∝1/R 5, where R is the distance between droplets, with repulsion for droplets having parallel orientation of the quadrupole moments and attraction at oblique angles around the orthogonal orientation. In an external magnetic field aligning the orientations of the quadrupole moments, a competition of the quadrupolar repulsion and van der Waals attraction (U vdW∝ 1/R 6) leads to a specific spatial organization of droplets which is in fact often reported in experimental observations (see the monograph by P. Drzaic, Liquid Crystal Dispersions, World Scientific, Singapore (1995) and references cited therein).  [Show abstract] [Hide abstract]
ABSTRACT: In the lactam structure, C6H7NO, molecules are linked by hydrogen bonds between NH and C=O [N ... O 2.885(2)Angstrom] to form ribbons parallel to the [100] 2(1) axis. The shortest intermolecular distances outside the ribbons are relatively large [C ... O 3.535 (2) and C ... C 3.600 (2) Angstrom].  [Show abstract] [Hide abstract]
ABSTRACT: It has been shown previously [J. Lajzerowicz and A. P. Levanyuk, Phys. Rev. B 49, 15 475 (1994)] that in a perfect threedimensional crystal the commensurateincommensurate transition is always discontinuous due to a fluctuationinduced longrange attraction between domain walls. At the same time it has been concluded long ago that in real crystals a strong repulsion between the domain walls exists due to impurityinduced roughening of the domain walls. This poses the question whether the commensurateincommensurate transition becomes continuous in real crystals. We argue that the most important point to answer this question is that a very slow activation motion of domain walls over disorderinduced potential barriers is necessary for making the repulsion operative, so that over a reasonable time of experiment this interaction ‘‘propagates’’ to distances that are no more than one order of magnitude bigger than the domainwall width. Meanwhile the attractive interactions between the domain walls ‘‘propagate’’ almost instantaneously, thus leading to the conclusion that experimentally observed commensurateincommensurate transition will always be discontinuous. Our estimates show that for structural incommensurate systems the ‘‘equilibrium’’ distance between the domain walls near transition point has the same order of magnitude as the domainwall width. © 1996 The American Physical Society.  [Show abstract] [Hide abstract]
ABSTRACT: The structure factor of a crystalline monolayer on the surface of a liquid is calculated. It is shown that in the first approximation the finite rigidity of the monolayer maintains the powerlaw profile of quasiBragg peaks typical for twodimensional systems but leads to a reduction of the Xray scattering intensity. It is found that this effective DebyeWaller factor has very strong dependence on the length h of the chains composing the monolayer. Namely, it is proportional to exp(h3/ξa2), where ξ is a chain persistence length and a is a characteristic atomic scale. We have also studied the spectrum of modes where the motion of the liquid is localised near the film. It turns out that for thick films there are three propagating soundlike modes: the transverse sound with a velocity which becomes zero if the wave vector q is directed along the normal to the film (zaxis); the conventional longitudinal sound with the almost isotropic velocity (determined mainly by the compressibility of the film); and the mode analogous to the second sound in smectics. For thin films the last mode transforms into the fast pure relaxational one. For tilted or hexatic layers there is also an orientational diffusion mode. This mode strongly influences the viscosity of the system. The fluctuation contribution in the viscosity coefficients is proportional to (T/Γ)In(qa), where Γ is the orientational diffusion coefficient. Transverse and longitudinal sound are transformed into overdamped modes with the following dispersion law ω ∝ (√3  i)q4/3. 

Article: Melting of Short 1Alcohol Monolayers on Water: Thermodynamics and XRay Scattering Studies
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ABSTRACT: From surface tension measurements we extract the melting entropy Deltas2D of fattyalcohol monolayers on water. Deltas2D is found to be 4kB/mol lower than in the bulk. Because of the role of the conformational entropy, the melting transition is discontinuous for long chains, but tends to be continuous for molecules shorter than 1nonanol. For 1decanol Deltas2D is sufficiently small to allow observation of critical fluctuations; the diffraction peak, measured with a high resolution synchrotron experiment, is described by a powerlaw singularity which broadens as the temperature approaches the melting point.  [Show abstract] [Hide abstract]
ABSTRACT: Owing to macroscopic electric fields arising from thermal fluctuations of a ferroelectric domain wall, the attraction energy of two parallel walls decays at large distances between the walls (h) as h2 with a prefactor that is proportional to T and to a ratio of dielectric constants that is equal to unity for the isotropic medium. For proper ferroelectrics, and improper ones with a quadratic dependence of the spontaneous polarization on the order parameter, this law is valid for all h greater than the domain wall thickness (which is the minimum length of the problem). For improper ferroelectrics with cubic (or higherorder) dependence of the polarization on the order parameter, there exists a region with a logarithmic dependence of the interaction energy on h. This attraction, which proves to prevail over the Van der Waals one, leads to a discontinuity of the ferroelectricincommensurate transition. Estimation of the period of the incommensurate phase at the transition provides a reasonable order of magnitude and shows that this period does not depend much on the crystal parameters other than the width of the domain wall. The interaction of the ferromagnetic and ferroelastic domain walls is qualitatively analogous to that of the ferroelectric ones. The role of quantum fluctuations becomes important at not very low temperatures. For nonferroelectric, nonferroelastic, nonferromagnetic domain walls the powerlaw fluctuationinduced attraction is shown to arise as well.  [Show abstract] [Hide abstract]
ABSTRACT: We argue that any commensurateincommensurate phase transition in a crystalline media is discontinuous due to powerlaw attraction of the domain walls. The main origin of this attraction are the thermal fluctuations of the walls which are always accompanied, in a crystal, by at least one of the longrange fields: electric, magnetic, elastic.  [Show abstract] [Hide abstract]
ABSTRACT: Some analogies exist between parametrically driven dynamical system and anisotropic XY model. We discuss one aspect of this analogy : the existence of domain walls, their different structures and the existence of “wetting” like transitions. The work is done in a LandauGinzburg framework.
Publication Stats
931  Citations  
82.18  Total Impact Points  
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Institutions

19892010

University Joseph Fourier  Grenoble 1
Grenoble, RhôneAlpes, France


1988

University of Grenoble
Grenoble, RhôneAlpes, France


1978

French National Centre for Scientific Research
Lutetia Parisorum, ÎledeFrance, France
