[Show abstract][Hide abstract] ABSTRACT: The second-order ordering transition of the AuAgZn2 alloy has been studied by coherent x-ray scattering. Within a few degrees above the critical temperature Tc, equilibrium critical fluctuations are observed together with some pretransitional local ordering connected to sample defects. The speckles observed correspond to heterodyne interference between local ordering and fluctuations and show a mixed static and dynamical behavior in a narrow domain of a few tenths of degree above Tc. The dynamical behavior is shown to correspond to the critical slowing down of the fluctuations in the vicinity of the transition (model "A" of Hohenberg and Halperin [Rev. Mod. Phys. 49, 435 (1977)]10.1103/RevModPhys.49.435). A rough comparison can be carried out with the classical diffusion models. Some improvements of the method are discussed.
[Show abstract][Hide abstract] ABSTRACT: The slow magnetic dynamics, from seconds to kiloseconds, of the canonical 3-k antiferromagnet USb have been probed, using X-ray photon correlation spectroscopy (XPCS). In this work, XPCS is combined with resonant X-ray diffraction to focus on scattering at an antiferromagnetic Bragg peak. High quality coherent magnetic diffraction patterns were recorded (speckle contrast of ~ 88%) and magnetic domains were observed; the number of domains increases on warming to T* ~ 160 K, where the spin waves soften to zero frequency, and again on warming to TN = 218 K. The intensity auto-correlation, g2(t), is primarily static over 1000 s, with a small dynamical process (change of ~ 0.4%) that increases in rate close to the transitions.
Full-text · Article · May 2014 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: Coherent x-ray micro-diffraction and local mechanical loading can be combined
to investigate the mechanical deformation in crystalline nanostructures. Here
we present measurements of plastic deformation in a copper crystal of
sub-micron size obtained by loading the sample with an Atomic Force Microscopy
tip. The appearance of sharp features in the diffraction pattern, while
conserving its global shape, is attributed to crystal defects induced by the
Full-text · Article · May 2013 · Journal of Physics Conference Series
[Show abstract][Hide abstract] ABSTRACT: The strain field of individual epitaxial sub-micrometric copper islands is studied using coherent X-ray diffraction and finite element modelling. The strain inhomogeneity in each island is so large that the characteristic features of the island shape tend to disappear in the diffraction pattern, which is dominated by strain effects. The model confirms the tensile strain imposed to the island by the thermal mismatch occurring during the preparation of the samples. An evaluation of the residual strain is obtained by qualitatively fitting the diffraction data.
[Show abstract][Hide abstract] ABSTRACT: The complexity of the mechanical behavior of filled elastomers can be partly attributed to the fact that the duration of an applied strain plays a crucial role. In order to bring new insights into this still incompletely solved problem, we look for relationships between the macroscopic mechanical relaxation and the relaxation of the filler particles at the nano- to mesoscale. To this end, X-ray photon correlation spectroscopy (XPCS) in homodyne and heterodyne configurations combined with tensile stress relaxation is employed. The paper is devoted to the study of the role of the filler–filler and the filler–matrix interactions in a cross-linked elastomer on the aging mechanisms under strain. The fillers investigated are carbon black, as an example of strong filler–matrix interactions, and hydroxylated silica for which the filler–filler interaction is strong (H-bonds). Homodyne XPCS correlation reveals features of jammed systems (compressed exponential and ballistic motion) for both systems. The exponents characterizing the aging of the homodyne relaxation times are not the same in the carbon black and in the silica filled samples. For both systems, the decrease of the particle velocity determined by heterodyne detection with aging time follows a power law. The silica sample is characterized by a slow decrease of the velocity during aging. For the carbon black sample, the velocity remains small and decreases faster than for the silica sample. The reverse is observed for the behavior of the tensile force.
[Show abstract][Hide abstract] ABSTRACT: In a coherent x-ray scattering experiment, interference of the waves diffracted across the sample is observed. This gives a speckle pattern in the observed scattering intensity, whereas a standard experiment leads to diffuse intensity or to broadening of a Bragg peak. Speckles correspond to the disorder configuration of the irradiated volume and their dynamics provide microscopic information of the time evolution of the sample microstructure.After a brief description of the techniques used for the observation of speckles in an x-ray experiment, some examples of measured time dynamics are given. These concern reversible fluctuations and irreversible decomposition of unmixing or ordering alloys. A few experiments in defect imaging are described and the studies opened by free electron lasers are briefly discussed.RésuméEn conditions de diffraction cohérente des rayons X, on observe les interférences entre les ondes diffractées par lʼéchantillon. Cela conduit à lʼobservation de tavelures là où une expérience classique donne une intensité diffuse ou une raie de Bragg élargie. Les tavelures correspondent à la configuration locale du désordre dans le volume irradié.Dans cet article, après une brève description des techniques mises en oeuvre pour observer des tavelures dans une expérience de rayons X, quelques exemples dʼobservation de leur dynamique temporelle sont donnés. On discutera dʼabord le cas de fluctuations réversibles et ensuite de lʼutilisation de cette technique pour étudier la dynamique irréversible de la décomposition dʼalliages à démixtion ou à mise en ordre. On décrira quelque sepériences dʼimagerie des défauts et on discutera des voies ouvertes par lʼapparition des lasers à électrons libres.
[Show abstract][Hide abstract] ABSTRACT: Using coherent x-ray scattering, we evidenced atomic step roughness at the  vicinal surface of a silicon monocrystal of 0.05 degree miscut. Close to the (1/2 1/2 1/2) anti-Bragg position of the reciprocal space which is particularly sensitive to the  surface, the truncation rod exhibits a contrasted speckle pattern that merges into a single peak closer to the (111) Bragg peak of the bulk. The elongated shape of the speckles along the direction confirms the monoatomic step sensibility of the technique. This experiment opens the way towards studies of step dynamics on crystalline surfaces.
[Show abstract][Hide abstract] ABSTRACT: We report on a new approach to probe bulk dislocations by using coherent x-ray diffraction. Coherent x rays are particularly suited for bulk dislocation studies because lattice phase shifts in condensed matter induce typical diffraction patterns which strongly depend on the fine structure of the dislocation cores. The strength of the method is demonstrated by performing coherent diffraction of a single dislocation loop in silicon. A dissociation of a bulk dislocation is measured and proves to be unusually large compared to surface dislocation dissociations. This work opens a route for the study of dislocation cores in the bulk in a static or dynamical regime, and under various external constraints.
Preview · Article · Feb 2011 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: Carbon black is widely used as a filler in order to modify the mechanical or the electrical properties of polymers. Such composites display significant non-linear effects. Moreover, examination of the large number of papers devoted to the physical properties of carbon black filled polymers indicates that each composite, even composites apparently consisting of similar matrixes and similar carbon blacks, may behave differently when prepared by different mixing methods. The present work aims to show that these particular behaviours can be related to the fact that carbon blacks used for composites are mass fractals of low dimensionality (Df <2) that are able to interpenetrate each other to an extent that depends on the filler-matrix surface interaction and on the volume fraction of filler. Small-angle X-ray scattering (SAXS) is a convenient method for studying disordered systems at length scales ranging between a few tenths and a few hundred nm. SAXS is therefore particularly advantageous for exploring the morphology of carbon black aggregates and their degree of interpenetration when dispersed in a matrix. Furthermore, the use of an area detector yields two-dimensional images and hence information about anisotropy of the arrangement of scatterers. It is shown that this arrangement profoundly influences the physical properties of the composites. Analysis of SAXS curves obtained for a rubber grade carbon black (N330) and for composites prepared by dispersing it into polyethylene or EPR will be presented. As an example, the temperature and frequency dependence of the electrical conductivity will be discussed and compared to theoretical models. Finally, the mutual consistency of the electrical and mechanical behaviour, theoretical models and information deduced from the scattering curves will be shown.
No preview · Article · Jan 2011 · MRS Online Proceeding Library
[Show abstract][Hide abstract] ABSTRACT: Random cross-linking in rubbers produces local variations in the elasticity of the network. These variations, whose characteristic size lies in the range 1-100 nm, are revealed when the rubber is swollen in a low molecular weight solvent, owing to the competition between the osmotic pressure of the solvent and the local elastic constraints, which affects the local polymer concentration. Such concentration fluctuations can be measured by small angle X-ray or neutron scattering (SAXS or SANS) as well as by dynamic light scattering. In filled elastomers, the filler modifies the distribution of the polymer and of the elastic constraints. Swelling these systems in a solvent in which the deuteron/proton ratio can be varied permits the different components in the scattering function of the polymer and of the filler to be separated. Observations on silica particles in a poly(dimethyl siloxane) (PDMS) rubber yield measurements not only of the surface area of the particles but also of the fraction of the surface a rea occupied by the polymer. Analysis of the dynamic light scattering response of these systems gives confirmation of the validity of the procedure. Coherent X-ray scattering measurements, combined with dynamic light scattering measurements of the filled uncross-linked polymer melts in the absence of solvent reveal that the structural relaxation process that follows an external mechanical perturbation is a diffusion-controlled process.
No preview · Article · Jan 2011 · MRS Online Proceeding Library
[Show abstract][Hide abstract] ABSTRACT: The use of coherent x-ray beams has been greatly developing for the past decades. They are now used by a wide scientific community to study biological materials, phase transitions in crystalline materials, soft matter, magnetism, strained structures, or nano-objects. Different kinds of measurements can be carried out: x-ray photon correlation spectroscopy allowing studying dynamics in soft and hard matter, and coherent diffraction imaging enabling to reconstruct the shape and strain of some objects by using methods such as holography or ptychography. In this article, we show that coherent x-ray diffraction (CXRD) brings a new insight in another scientific field: the detection of single phase defects in bulk materials. Extended phase objects such as dislocations embedded in the bulk are usually probed by electron microscopy or X-ray topography. However, electron microscopy is restricted to thin samples, and x-ray topography is resolution-limited. We show here that CXRD brings much more accurate information about dislocation lines (DLs) in bulk samples and opens a route for a better understanding of the fine structure of the core of bulk dislocations. Comment: Article is 8 pages, with 4 figures and 29 references; 6 pages of additional information together with 4 figures
[Show abstract][Hide abstract] ABSTRACT: Bragg coherent X-ray diffraction imaging is demonstrated with a micro-focused illumination. The 2D projected density of the 3D nano-crystal is successfully retrieved from the inversion of its diffraction intensity pattern. The analysis of the phase field at the sample position, which holds in principle the strain information, emphasizes the high sensitivity of the technique with regard to the wavefront structure. The ptychography approach is a proposed solution to discriminate the wavefront function from the sample electron density distribution. It is based on a redundancy of the collected information obtained by measuring a series of diffraction patterns for different but overlapping beam positions onto the sample. Applicability to the Bragg geometry still needs to be demonstrated.
Full-text · Article · May 2010 · Journal of Modern Optics
[Show abstract][Hide abstract] ABSTRACT: Using soft x-ray resonant magnetic scattering with a coherent beam, we find evidence for memory effects in the magnetic configuration of a periodic array of submicron ferromagnetic lines under external magnetic field. The memory effect is explained by the dipolar coupling between the lines which is lost when the sample is saturated by the external magnetic field.
Full-text · Article · Nov 2009 · New Journal of Physics