Quantitative imaging of anisotropic material properties with vectorial ptychography

Following the recent establishment of the formalism of vectorial ptychography [Ferrand et al., Opt. Lett. 40, 5144 (2015)], first measurements are reported in the optical range, demonstrating the capability of the proposed method to map the four parameters of the Jones matrix of an anisotropic specimen, and therefore to quantify a wide range of optical material properties, including power transmittance, optical path difference, diattenuation, retardance, and fast-axis orientation.Comment: 5 figures, accepted for publication in Optics Letter

Forêt d’Orléans

Les prospections ont permis d’identifier deux sites. Le premier est un édifice romain sur butte de Foulaubin à Chambon-la-forêt. Il est situé à proximité de la voie romaine. Il s’agit d’une vaste structure peu visible dans sa partie sud, mais bien marquée dans sa partie nord par un dénivelé important de 1 m environ sur moins de 1 m au sol avec angle droit à son extrémité (pente à plus de 45° sur 1 m). Le second site est situé sur la commune de Nancray-sur-Rimarde, aux lieux-dits la Grosse Bor..

Identification of a low-frequency elastic behaviour in liquid water

International audienceThis article deals with the identification of solid-like properties measured at room temperature at a sub-millimetre length scale in liquid water. At a macroscopic scale, normal liquids (i.e. above melting temperature), and in particular water, are typically and empirically defined by the absence of shear elasticity, in contrast to solids or plastic fluids that require a stress threshold for flowing. A novel method optimizing the transmission of the shear stress to the sample enables a more complete probe of the mechanical response of liquids. It reveals that glass formers and viscous alkanes actually exhibit finite macroscopic shear elasticity away from any phase transition. This protocol is here applied for the first time to liquid water at room temperature, revealing, at the sub-millimetre scale, a low-frequency solid-like property

Bringing to light hidden elasticity in the liquid state using in-situ pretransitional liquid crystal swarms

International audienceThe present work reveals that at the sub-millimeter length-scale, molecules in the liquid state are not dynamically free but elastically correlated. It is possible to " visualize " these hidden elastic correlations by using the birefringent properties of pretransitional swarms persistent in liquids presenting a weak first order transition. The strategy consists in observing the optical response of the isotropic phase of mesogenic fluids to a weak (low energy) mechanical excitation. We show that a synchronized optical response is observable at frequencies as low as 0.01Hz and at temperatures far away from any phase transition (up to at least 15°C above the transition). The observation of a synchronized optical signal at very low frequencies points out a collective response and supports the existence of long-range elastic (solid-like) correlations existing at the sub-millimeter length-scale in agreement to weak solid-like responses already identified in various liquids including liquid water. This concept of elastically linked molecules differs deeply with the academic view of molecules moving freely in the liquid state and has profound consequences on the mechanisms governing collective effects as glass formation, gelation and transport, or synchronized processes in physiological media

Identification of finite shear-elasticity at low thickness in the liquid state of molecular (OTP) and polymeric glass formers (PBuA)

International audienceFinite shear elasticity has been identified at low gap in the liquid state of various viscous fluids. The present study is expanded to a van der Waals glass former, the o-Terphenyl and to an ordinary polymer melt, the polybutylacrylate. Elasticity is also identified in these materials at the sub-millimeter scale and far above any phase transition. This macroscopic property is discussed in the frame of the terminal relaxation modes (-process or reptation times) and of their molecular interpretation

Two-dimensional neutron scattering in a floating heavy water bridge

International audienceWhen high voltage is applied to pure water filled into two beakers close to each other, a connection forms spontaneously, giving the impression of a floating water bridge. This phenomenon is of special interest, since it comprises a number of phenomena currently tackled in modern water science. In this work, the first two dimensional structural study of a floating heavy water bridge is presented as a function of the azimuthal angle. A small anisotropy in the angular distribution of the intensity of the first structural peak was observed, indicating a preferred orientation of a part of the D 2 O molecules along the electric field lines without breaking of the local tetrahedral symmetry. The experiment is carried out by neutron scattering on a D 2 O bridge

Richness of side-chain liquid-crystal polymers: From isotropic phase towards the identification of neglected solid-like properties in liquids

International audienceVery few studies concern the isotropic phase of Side-Chain Liquid-Crystalline Polymers (SCLCPs). However, the interest for the isotropic phase appears particularly obvious in flow experiments. Unforeseen shear-induced nematic phases are revealed away from the N-I transition temperature. The non-equilibrium nematic phase in the isotropic phase of SCLCP melts challenges the conventional timescales described in theoretical approaches and reveal very long timescales, neglected until now. This spectacular behavior is the starter of the present survey that reveals long range solid-like interactions up to the sub-millimetre scale. We address the question of the origin of this solid-like property by probing more particularly the non-equilibrium behavior of a polyacrylate substituted by a nitrobiphenyl group (PANO2). The comparison with a polybutylacrylate chain of the same degree of polymerization evidences that the solid-like response is exacerbated in SCLCPs. We conclude that the liquid crystal moieties interplay as efficient elastic connectors. Finally, we show that the " solid " character can be evidenced away from the glass transition temperature in glass formers and for the first time, in purely alkane chains above their crystallization temperature. We thus have probed collective elastic effects contained not only in the isotropic phase of SCLCPs, but also more generically in the liquid state of ordinary melts and of ordinary liquids

Dielectric anisotropy in the GW space-time method

Excited-state calculations, notably for quasiparticle band structures, are nowadays routinely performed within the GW approximation for the electronic self-energy. Nevertheless, certain numerical approximations and simplifications are still employed in practice to make the computations feasible. An important aspect for periodic systems is the proper treatment of the singularity of the screened Coulomb interaction in reciprocal space, which results from the slow 1/r decay in real space. This must be done without introducing artificial interactions between the quasiparticles and their periodic images in repeated cells, which occur when integrals of the screened Coulomb interaction are discretised in reciprocal space. An adequate treatment of both aspects is crucial for a numerically stable computation of the self-energy. In this article we build on existing schemes for isotropic screening and present an extension for anisotropic systems. We also show how the contributions to the dielectric function arising from the non-local part of the pseudopotentials can be computed efficiently. These improvements are crucial for obtaining a fast convergence with respect to the number of points used for the Brillouin zone integration and prove to be essential to make GW calculations for strongly anisotropic systems, such as slabs or multilayers, efficient. (C) 2006 Elsevier B.V. All rights reserved

Random-phase approximation and its applications in computational chemistry and materials science

The random-phase approximation (RPA) as an approach for computing the electronic correlation energy is reviewed. After a brief account of its basic concept and historical development, the paper is devoted to the theoretical formulations of RPA, and its applications to realistic systems. With several illustrating applications, we discuss the implications of RPA for computational chemistry and materials science. The computational cost of RPA is also addressed which is critical for its widespread use in future applications. In addition, current correction schemes going beyond RPA and directions of further development will be discussed.Comment: 25 pages, 11 figures, published online in J. Mater. Sci. (2012

H. N. Werkman

A piece commissioned for COLDFRONT – Singular Vispo :: First Encounters. Artists were asked to talk about their introduction to Visual Poetry and the piece of work that changed the way they saw language