No abstract available

[en] We report on the study of a magnetic dislocation in pure chromium. Coherent X-ray diffraction profiles obtained on the incommensurate Spin Density Wave (SDW) reflection are consistent with the presence of a dislocation of the magnetic order, embedded at a few micrometers from the surface of the sample. Beyond the specific case of magnetic dislocations in chromium, this work may open up a new method for the study of magnetic defects embedded in the bulk.

[en] The exceptional properties of synchrotron radiation: high flux, possibility of varying the wave-length, coherence, polarization, have allowed spectacular advances in the structural analysis of condensed matter. This document gathers 5 short articles that illustrate the great diversity of the results: the first article deals with the crystallography of biological molecules that opens the way to studying the relationship between the shape of the protein and its functioning, the second article presents the study of magnetism of multi-ferroic materials with the help of X-rays, the third article describes how X-ray coherent diffraction can reveal deformations and topological defects of crystals, the fourth article shows how grazing incidence techniques shed light on the organisation of surfaces, interfaces or nano-structures, the last article presents the potentiality of the synchrotron radiation for studying the behaviour of matter under extreme conditions of pressure and temperature. (A.C.)

[en] Single crystal X-ray diffuse scattering photography has been used to provide insights into the orientational ordering of the C₆₀ molecules. The observation of anisotropic modulations of the diffuse scattering intensity in reciprocal space demonstrates the existence of orientational correlations at room temperature. A new structural transition is observed at T₂ = 225 ± 2 K, below the face-centered cubic (f.c.c.) to simple cubic orientational ordering phase transition at T₁ = 259 ± 1 K. The transition at T₂ leads to a low temperature superstructure, presumably f.c.c., with a doubling of the cell constants. Fluctuations associated to both transitions are seen at room temperature while some anisotropic diffuse scattering is still present at 20 K. (orig.)

[en] The exceptional properties of synchrotron radiation: high flux, possibility of varying the wave-length, coherence, polarization, have allowed spectacular advances in the structural analysis of condensed matter. This document gathers 5 short articles that illustrate the great diversity of the results: the first article deals with the crystallography of biological molecules that opens the way to studying the relationship between the shape of the protein and its functioning, the second article presents the study of magnetism of multi-ferroic materials with the help of X-rays, the third article describes how X-ray coherent diffraction can reveal deformations and topological defects of crystals, the fourth article shows how grazing incidence techniques shed light on the organisation of surfaces, interfaces or nano-structures, the last article presents the potentiality of the synchrotron radiation for studying the behaviour of matter under extreme conditions of pressure and temperature. (A.C.)

[en] We present an x-ray diffuse scattering study of the anion ordering (AO) transition in the (TMTSF)₂(ReO₄)_1-x(ClO₄)_x solid solution of quasi-one-dimensional organic conductors and superconductors. As (TMTSF)₂ReO₄ and (TMTSF)₂ClO₄ present AO instabilities at different critical wave vectors [q₁=( (1) /(2) , (1) /(2) , (1) /(2) ) and q₂=(0, (1) /(2) ,0), respectively] this solid solution allows for the study of the influence of competing interactions on a structural transition. The order parameter of this transition can be characterized by an Ising variable representing the two possible orientations of the ReO₄^- or ClO₄^- anion in cavities delimited by organic TMTSF molecules. Furthermore, the random substitution of the anions shows that these interactions are randomly distributed. The phase diagram of the AO transition has been determined as a function of x. At low temperatures (T) a q₁ long-range order (LRO) occurs for x≤0.5 and a q₂ LRO occurs for 1≤x≤0.97, while a short-range order (SRO) appears in the intermediate concentration range. For 0.7< x<0.94 one observes coexistence of q₁ and q₂ SRO. The basic features of the (T,x) phase diagram can be accounted for by a mean-field treatment of the Ising model with random interactions. This model, however, fails to reproduce all the details of the phase diagram, such as the x dependence of the critical wave vector. Finally, we discuss the implications of this structural phase diagram on the electronic properties of the solid solution. copyright 1997 The American Physical Society

[en] Time resolved experiments are one of the major services that synchrotrons can provide to scientists. The short, high frequency and regular flashes of synchrotron light are a fantastic tool to study the evolution of phenomena over time. To carry out time resolved experiments, beamlines need to synchronize their devices with these flashes of light with a jitter shorter than the pulse duration. For that purpose, Synchrotron SOLEIL has developed the TimBeL (Timing Beamlines) board fully interfaced to TANGO framework. The TimBeL system is a compact PCI board. It is made of a mother with one daughter board. All functions are performed inside a FPGA (Field Programmable Gate Array) implemented on the mother board. A PLX Technology chip is used to communicate with the compact PCI crate. To enable experiments to remain always synchronous with the same bunch of electrons, the storage ring clock (CLK-SR) and the radio frequency clock (CLK-RF) are provided by the machine to beamlines. These clocks are used inside the FPGA as main clocks for state machines. Because the jitter is too large on the FPGA outputs, a daughter board with a jitter cleaner has been added to the system. This board also provides delay lines for compensating time offsets by 10 ps steps. This paper presents the main features required by time resolved experiments and how we achieved our goals with the TimBeL board

[en] We present an X-ray diffuse scattering investigation of the structural instabilities exhibited by a new series of 2:1 organic conductors based on the TMDTDSF molecule which is a hybrid between the TMTTF and TMTSF molecules. The results are discussed in relationship with the orientational disorder of the TMDTDSF molecule characterized in part I. (TMDTDSF)₂PF₆ shows a 2 k_F structural instability having a mixed spin-Peierls (SP)-charge density wave (CDW) character which exhibits a thermal behavior intermediate between that exhibited by the (TMTTF)₂PF₆ SP instability and the (TMTSF)₂PF₆ CDW one. At low temperatures this instability does not condense into a tri-dimensional long range order SP ground state. Probably because of the disorder, one-dimensional local SP distortions coexist with a magnetic order. The influence of the orientational disorder of the TMDTDSF molecule on the anion ordering (AO) transitions is discussed in terms of random interactions and random fields. Disorder is found to have a weak effect in (TMDTDSF)₂ReO₄ which undergoes a long range q₁ = (1/2, 1/2, 1/2) AO phase transition at a critical temperature intermediate between those of (TMTSF)₂ReO₄ and (TMTTF)₂ReO₄. Random field effects are found to be important in (TMDTDSF)₂BF₄ which exhibits a q₁ quasi AO phase transition at a ''critical'' temperature sizeably depressed with respect to those of (TMTTF)₂BF₄ and (TMTSF)₂BF₄. Random fields as well as random interactions are believed to be responsible for the absence of a q₁ AO phase transition in (TMDTDSF)₂ClO₄. (orig.)

[en] We present a study of the (TMDTDSF)₂X materials, where TMDTDSF (Tetramethyldithiadiselenafulvalene) is a hybrid of the TMTSF and TMTTF molecules and X (PF₆, AsF₆, SbF₆, ReO₄, BF₄ and ClO₄) a monovalent anion. This paper deals with an X-ray diffuse scattering study of the static disorder of the orientation of non-centrosymmetrical TMDTDSF molecules. We present a calculation of the diffuse scattering by a model of disorder which takes into account the intrachain correlations between first neighbor molecules and the associated displacements (size effect). Computer simulations of the diffuse scattering intensity allows us to account for the experimental results. We find that in all the compounds investigated the two orientations of the molecules are not correlated and have equal probability in agreement with structural refinements. The interpretation of the diffuse scattering of (TMDTDSF)₂PF₆ gives a size effect between first- and second-neighbor molecules of about 0.1 A and 0.03 A respectively. As the orientational disorder is random, these systems are good candidates for the study of the influence of a random disorder on the structural transitions of these materials such as the anion ordering and the Spin-Peierls instability. (orig.)

[en] The SOLEIL synchrotron recently launched an ambitious project which aims at producing synchrotron radiation pulses of 100 fs width: the femto-slicing project. It is based on the interaction between a 30 fs infrared laser pulse and one of the 70 ps electron bunches that rotate in the storage ring. When it takes place under suitable conditions, this interaction leads to a change in energy of the electrons in a fs-slice of the bunch, which in turn causes a slight change in their trajectory. The 'sliced' electrons then radiate fs x-ray pulses which can be spatially separated from the regular ps-pulses in a beamline. Obtaining short (ps) or ultra-short (fs) pulses enables to probe the dynamics of solids as they occur after a fs laser excitation, in the so-called 'pump-probe' experiment scheme. In a diffraction pump-probe experiment, one has to detect the signal that originates from the sliced bunch only. Indeed, the other bunches give rise to signals which correspond to different time-delays after excitation: detecting those would smear the time-resolution. In collaboration with the CCPM (Marseille, France) and the CRG D2AM beamline at ESRF, the Detectors group at SOLEIL currently develops a hybrid pixel 2D detector, whose 3.2 version has been optimised to be able to count during a period specified by the width of a logic gate, at any frequency. A chip from this detector was tested on the CRISTAL beamline in SOLEIL's 8-bunch mode