Ordered interfaces for dual easy axes in liquid crystals

International audienceUsing nCB films adsorbed on MoS 2 substrates studied by x-ray diffraction, optical microscopy and Scanning Tunneling Microscopy, we demonstrate that ordered interfaces with well-defined orientations of adsorbed dipoles induce planar anchoring locked along the adsorbed dipoles or the alkyl chains, which play the role of easy axes. For two alternating orientations of the adsorbed dipoles or dipoles and alkyl chains, bi-stability of anchoring can be obtained. The results are explained using the introduction of fourth order terms in the phenomenological anchoring potential, leading to the demonstration of first order anchoring transition in these systems. Using this phenomenological anchoring potential, we finally show how the nature of anchoring in presence of dual easy axes (inducing bi-stability or average orientation between the two easy axes) can be related to the microscopical nature of the interface. Introduction Understanding the interactions between liquid crystal (LC) and a solid substrate is of clear applied interest, the vast majority of LC displays relying on control of interfaces. However this concerns also fundamental problems like wetting phenomena and all phenomena of orientation of soft matter bulk induced by the presence of an interface. In LCs at interfaces, the so-called easy axes correspond to the favoured orientations of the LC director close to the interface. If one easy axis only is defined for one given interface, the bulk director orients along or close to this axis [1]. It is well known that, in anchoring phenomena, two major effects compete to impose the anchoring directions of a liquid crystal, first, the interactions between molecules and the interface, second, the substrate roughness whose role has been analyzed by Berreman [2]. The influence of adsorbed molecular functional groups at the interface is most often dominant with, for example in carbon substrates, a main influence of unsaturated carbon bonds orientation at the interface [3]. In common LC displays, there is one unique easy axis, but modifications of surfaces have allowed for the discovery of promising new anchoring-related properties. For instance, the first anchoring bi-stability has been established on rough surfaces, associated with electric ordo-polarization [4] and the competition between a stabilizing short-range term and a destabilizing long-range term induced by an external field, can induce a continuous variation of anchoring orientation [5]. More recently, surfaces with several easy axes have been studied extensively. It has been shown that control of a continuous variation of director pretilt, obtained in several systems [6, 7], is associated with the presence of two different easy axes, one perpendicular to the substrate (homeotropic) and one planar [7, 8]. Similar models can explain the continuous evolution of anchoring between two planar orientations observed on some crystalline substrates [9]. However, in the same time, two easy axes can also lead to anchoring bi-stability [10, 11] or discontinuous transitions of anchoring [9], which is not compatible with the model established to interpret observed control of pretilt. In order to be able to predict if bi-stability or continuous combination of the two easy axes occurs for one given system, it becomes necessary to understand the microscopic origin of the easy axes

Modeling the optical properties of self-organized arrays of liquid crystal defects

International audienceLocal full Mueller matrix measurements in the Fourier plane of a microscope lens were used to determine the internal anisotropic ordering in periodic linear arrays of smectic liquid crystal defects, known as 'oily streaks'. We propose a single microstructure-dependent model taking into account the anisotropic dielectric function of the liquid crystal that reproduces the smectic layers orientation and organization in the oily streaks. The calculated Mueller matrix elements are compared to the measured data to reveal the anchoring mechanism of the smectic oily streaks on the substrate and evidence the presence of new type of defect arrangement. Beyond the scientific inquiry, the understanding and control of the internal structure of such arrays offer technological opportunities for developing liquid-crystal based sensors and self-assembled nanostructures

Suppression of the thermal hysteresis in magnetocaloric MnAs thin film by highly charged ion bombardment

We present the investigation on the modifications of structural and magnetic properties of MnAs thin film epitaxially grown on GaAs induced by slow highly charged ions bombardment under well-controlled conditions. The ion-induced defects facilitate the nucleation of one phase with respect to the other in the first-order magneto-structural MnAs transition with a consequent suppression of thermal hysteresis without any significant perturbation on the other structural and magnetic properties. In particular, the irradiated film keeps the giant magnetocaloric effect at room temperature opening new perspective on magnetic refrigeration technology for everyday use

Structure of smectic defect cores: an X-ray study of 8CB liquid crystal ultra-thin films

We study the structure of very thin liquid crystal films frustrated by antagonistic anchorings in the smectic phase. In a cylindrical geometry, the structure is dominated by the defects for film thicknesses smaller than 150 nm and the detailed topology of the defects cores can be revealed by x-ray diffraction. They appear to be split in half tube-shaped Rotating Grain Boundaries (RGB). We determine the RGB spatial extension and evaluate its energy per unit line. Both are significantly larger than the ones usually proposed in the literatureComment: 4 page

Self-organized arrays of dislocations in thin smectic liquid crystal films

International audienceCombining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of smectic liquid crystal 8CB with thickness 100-300 nm confined between air and a rubbed PVA polymer substrate which impose hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations control the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring condition over the entire polymer substrate and whose thickness does not depend on the one of the film. The strength of the planar anchoring appears to be high, larger than 10 −2 J/m 2 , compensating for the high energy cost of creating an additional 2D defect between an horizontal smectic layer and perpendicular ones. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated

Magnetic properties changes of MnAs thin films irradiated with highly charged ions

We present the first investigation on the effect of highly charged ion bombardment on a manganese arsenide thin film. The MnAs films, 150 nm thick, are irradiated with 90 keV Ne$^{9+}$ ions with a dose varying from $1.6\times10^{12}$ to $1.6\times10^{15}$ ions/cm$^2$. The structural and magnetic properties of the film after irradiation are investigated using different techniques, namely, X-ray diffraction, magneto-optic Kerr effect and magnetic force microscope. Preliminary results are presented. From the study of the lattice spacing, we measure a change on the film structure that depends on the received dose, similarly to previous studies with other materials. Investigations on the surface show a strong modification of its magnetic properties

Etude par Microscopie a Effet Tunnel de polymeres conjugues: adsorbats sur graphite, films electropolymerises sur or et sur graphite, defauts de surface du graphite

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 84253 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Etude des capacités d'intégration sensorielle toucher-vision chez des enfants présentant des troubles des apprentissages

The thesis aims to assess whether the transfer of information on the form between touch and vision are properly among children with oral language (dysphasia) or written (dyslexia) or unrest praxia (dyspraxia). In the first study, we conducted a neuropsychological evaluation in order to clarify what level of information processing are possible difficulties. This study shows auditivo-verbal working memory disorder within the three pathologies. More specifically, sequential processing disorders in children with dyslexia, oral language disorders expressive, and unrest visuo-constructifs among dysphasia children, and unrest motor, visuo-perception, visuo-construct and a low spatial memory among dyspraxia children. In a second study, we evaluated a group of child aged 4 to 8 years. The results show an evolving capacities of intramodal T-T and crossmodal TV and V-T of 4 to 8 years with performances almost perfect dice 5 years for simple geometric shapes and performance clearly moving towards 6-7 years for forms oriented.The latest study assesses the capacity for transfer into learning disorders. It showed sensory integration capabilities between touch and vision and efficiency but limited by the slowing cognitive disorders each pathology.Cette thèse a pour objectif d'évaluer si le transfert des informations sur la forme entre le toucher et la vision se fait adéquatement chez des enfants présentant des troubles du langage oral (dysphasiques) ou écrit (dyslexiques) ou des troubles praxiques (dyspraxiques). Dans une première étude, nous avons effectué une évaluation neuropsychologique afin de préciser à quel niveau de traitement des informations se situent les éventuelles difficultés. Cette étude montre des troubles de mémoire de travail auditivoverbale dans les trois pathologies. Plus spécifiquement, des troubles de traitement séquentiel chez les enfants dyslexiques, des troubles du langage oral expressif, et des troubles visuo-constructifs chez les enfants dysphasiques, et des troubles praxiques gestuels, visuo-perceptifs, visuo-constructifs, ainsi qu'une faible mémoire spatiale chez les enfants dyspraxiques. Dans une deuxième étude, nous avons évalué un groupe d'enfant tout-venant âgé de 4 à 8 ans. Les résultats montrent une évolution des capacités de transfert intramodal T-T et intermodal T-V et V-T de 4 à 8 ans avec des performances presque parfaites dés 5 ans pour des formes géométriques simples et des performances évoluant nettement vers 6-7 ans pour des formes composites orientées. La dernière étude évalue les capacités de transfert dans les troubles des apprentissages. Elle montre des capacités d'intégration sensorielle entre le toucher et la vision efficience mais limitée et ralentit par les troubles cognitifs de chaque pathologie