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The emerging CdTe-BeTe semiconductor alloy that exhibits a dramatic mismatch in bond covalency / stiffness clarifying its vibrational-mechanical properties is used as a benchmark to test the limits of the percolation model (PM) worked out to explain the complex Raman spectra of the related but less contrasted Zn1‑xBex‑chalcogenides. The test is done by way of experiment (x< 0.11) – combining Raman scattering with X-ray diffraction at high pressure – and ab initio calculations (x~0 – 0.5, x~1). The (macroscopic) bulk modulus B0 drops below the CdTe value on minor Be incorporation, at variance with a linear B0 vs. x increase predicted ab initio, thus hinting at large anharmonic effects in the real crystal. Yet, no anomaly occurs at the microscopic (bond) scale as the regular bimodal PM-type Raman signal predicted ab initio for Be-Te in minority (x~0, 0.5) is (barely) detected experimentally. At large Be content (x~1) the same bimodal signal “relaxes” down to inversion – an unprecedented case. However, specific pressure dependencies of the regular (x~0, 0.5) and inverted (x~1) Be-Te Raman doublets are in line with PM-predictions. Hence, the PM applies as such to Cd1‑xBexTe – albeit in a “relaxed” form, without further refinement. This enhances the model’s validity as a generic descriptor of phonons in alloys.

International audience; Matter is dynamic and changes under the effect of various external constraints, in particular light. The development of ultra-fast techniques now makes it possible to study the elementary mechanisms of ultrafast photoinduced phenomena, in particular by following the dynamics of electronic and structural degrees of freedom of matter. These studies provide a better understanding of these non-equilibrium processes in order to control the physical properties of molecules and materials by light. In this introductory article we discuss different aspects of studying ultra-fast condensedmatter phenomena.

Recent experiments have shown that the high-temperature incommensurate (I) charge density wave (CDW) phase of 1T-TaS2 can be photoinduced from the lower-temperature, nearly commensurate CDW state. In a first step, several independent regions exhibiting I-CDW phase modulations nucleate and grow. After coalescence, these regions form a multidomain I-CDW phase that undergoes coarsening dynamics, i.e. a progressive increase of the domain size or I-CDW correlation length. Using time-resolved X-ray diffraction, we show that the wave vector of the photoinduced I-CDW phase is shorter than in the I-CDW phase at equilibrium, and progressively increases towards its equilibrium value as the correlation length increases. We interpret this behaviour as a consequence of a self-doping of the photoinduced I-CDW, following the presence of trapped electrons in the vicinity of CDW dislocation sites. Putting together results of the present and past experiments, we develop a scenario in which the I-CDW dislocations are created during the coalescence of the I-CDW phase regions., Plusieurs expériences récentes ont montré que les impulsions laser dans les domaines optique ou proche infrarouge permettent de déclencher des transitions entre états à onde de densité de charge (ODC) dans 1T-TaS2. Nous nous intéressons ici à la transition entre l’état à ODC presque commensurable (NC) et l’état à ODC incommensurable (I), habituellement observé au-dessus de 350 K. Lors de cette transition, plusieurs régions présentant les modulations de l’état I se forment et se développent. Lorsque la coalescence a lieu, ces régions se muent en domaines de la phase I photoinduite de 1T-TaS2, caractérisés chacun par un phasage particulier de l’ODC I. La phase I ainsi fragmentée en domaines subit alors une dynamique de mûrissement, c’est-à-dire une augmentation progressive de la taille de domaine ou encore de la longueur de corrélation de l’ODC I. En utilisant la diffraction des rayons X résolue en temps, nous montrons que le vecteur d’onde de l’ODC I photoinduite est plus court que dans l’ODC I observée à l’équilibre thermodynamique. Celui-ci s’allonge progressivement vers sa valeur d’équilibre, en même temps que la longueur de corrélation de l’ODC I augmente. Nous attribuons ce comportement à un autodopage de l’ODC I photoinduite, dû à la présence d’électrons piégés au voisinage de dislocations de l’ODC I. En réalisant une synthèse des résultats des différentes expériences menées jusqu’à présent, nous développons un scénario dans lequel les dislocations de l’ODC I sont créées au moment de la coalescence., Comptes Rendus. Physique, 22 (S 2), ISSN:1631-0705, ISSN:1878-1535

Coherent x-ray diffractive imaging is a nondestructive technique that extracts three-dimensional electron density and strain maps from materials with nanometer resolution. It has been utilized for materials in a range of applications, and has significant potential for imaging buried nanostructures in functional devices. Here, we show that coherent x-ray diffractive imaging is able to bring new understanding to a lithography-based nanofabrication process for engineering the optical properties of semiconducting GaAs1-yNy on a GaAs substrate. This technique allows us to test the process reliability and the manufactured patterns quality. We demonstrate that regular and sharp geometrical structures can be produced on a few-micron scale, and that the strain distribution is uniform even for highly strained sub-microscopic objects. This nondestructive study would not be possible using conventional microscopy techniques. Our results pave the way for tailoring the optical properties of emitters with nanometric precision for nanophotonics and quantum technology applications. Coherent x-ray diffractive imaging is a powerful technique for determining strain on the nanometer scale. Here, it is used to image semiconducting GaAs1-yNy structures on a GaAs substrate and to measure strain, demonstrating its potential for studying highly strained interfaces in devices.

New insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20 GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10 GPa that gradually passes into an insulating state, typical of charge ordering, which totally suppresses superconductivity above 13 GPa. The changes in resistivity are accompanied by the apparition of sharp Bragg peaks in the X-ray diffraction patterns, indicating that the charge ordering is accompanied by a 3D oxygen ordering. Considering that pressure induces a charge transfer of about 0.02 at 10 GPa, our results are the first observation of charge order competing with superconductivity developed in the overdoped region of the phase diagram of a Hg-based cuprate.

The investigation of ultrafast dynamics, taking place on the few to sub-picosecond time scale, is today a very active research area pursued in a variety of scientific domains. With the recent advent of X-ray free-electron lasers (XFELs), providing very intense X-ray pulses of duration as short as a few femtoseconds, this research field has gained further momentum. As a consequence, the demand for access strongly exceeds the capacity of the very few XFEL facilities existing worldwide. This situation motivates the development of alternative sub-picosecond pulsed X-ray sources among which femtoslicing facilities at synchrotron radiation storage rings are standing out due to their tunability over an extended photon energy range and their high stability. Following the success of the femtoslicing installations at ALS, BESSY-II, SLS and UVSOR, SOLEIL decided to implement a femtoslicing facility. Several challenges were faced, including operation at the highest electron beam energy ever, and achievement of slice separation exclusively with the natural dispersion function of the storage ring. SOLEIL's setup also enables, for the first time, delivering sub-picosecond pulses simultaneously to several beamlines. This last feature enlarges the experimental capabilities of the facility, which covers the soft and hard X-ray photon energy range. In this paper, the commissioning of this original femtoslicing facility is reported. Furthermore, it is shown that the slicing-induced THz signal can be used to derive a quantitative estimate for the degree of energy exchange between the femtosecond infrared laser pulse and the circulating electron bunch.

1T-TaS2 is a 2D metallic compound which undergoes a series of electronically driven phase transitions toward charge density wave and Mott phases. Its intricate electron–phonon interactions and electron–electron correlations have been promising peculiar out-of-equilibrium dynamics. In this paper, we provide the first direct information on the atomic structure response to an ultra-fast infrared laser pulse in the commensurate phase of 1T-TaS2, by using femtosecond time-resolved X-ray diffraction. We show that ultra-fast excitation with near-infrared photons drives a displacive excitation of the amplitude mode of the commensurate charge density wave. About 3 ps after laser excitation, the system reaches a new, photo-induced state that is maintained for at least 10 ps. We give evidence that this long-lived state exhibits the same structural modulation as in the thermodynamically stable commensurate phase, with a large correlation length. Only the average amplitude of the modulation is found to decrease. We propose that the long-lived state is formed from the commensurate phase by reducing the modulation amplitude on few superlattice nodes. The underlying mechanism proposed is the annihilation of self-trapped polarons.

Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS$_2$, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.

Grâce a la brillance des sources synchrotron de troisieme generation, on obtient des faisceaux de rayons X coherents d’intensite superieure a 109 photons par seconde. Ces faisceaux permettent des experiences de diffraction qui revelent les franges d’interference ou les tavelures, non visibles dans des experiences classiques.Cet article donne quelques exemples d’etudes en diffraction coherente : reconstruction de deformations dans des nanocristaux, mise en evidence de defauts topologiques, et ptychographie. Des experiences de diffraction coherente sont notamment realisees sur la ligne CRISTAL a SOLEIL, ainsi qu’a l’ESRF.

Anomalous coherent X-ray diffraction in Bragg geometry is a promising technique to study three-dimensional composition and strain fields in nanostructures. This paper presents results of coherent X-ray diffraction experiments on single Ag/Au core–shell nanowires with a typical diameter of 100 to 300 nm in the as-prepared state and during in-situ heating. Experimental coherent diffraction patterns are compared with finite-element simulations. Moreover, experimental constraints owing to radiation damage are discussed.

Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the phonon-polariton transverse optical modes (with mixed electrical and mechanical character) of the II-VI ZnSeS mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly resolved percolation-type Zn-S Raman doublet of the three oscillator [1(Zn-Se),2(Zn-S)] ZnSe68S32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende-to-rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe68S32, due to Zn-S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn- S sub-mode, due to Zn-S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar phonon freezing was earlier evidenced with the well-resolved percolation-type Be-Se doublet of ZnBeSe [Pradhan et al. Phys. Rev. B 81, 115207 (2010)], that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse and convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition., 20 pages, 7 figures

International audience; We study the complex spin-state switching mechanism in a bistable molecular crystal undergoing a stepwise conversion from high-spin to low-spin states at thermal equilibrium as well as during the relaxation process from the photoinduced metastable state. We experimentally evidence that such steps are associated with complex types of long-range and short-range ordering phenomena, resulting from the occupational modulation of bistable molecular magnetic states. The conversion is then described by using two order parameters: the totally symmetric average high spin fraction and the symmetry breaking ordering parameter. The use of the anisotropic next-nearest-neighbor Ising (ANNNI) model allows us to describe the microscopic origin of the ordering, and Monte Carlo simulations reproduce the observed stepwise thermal phase transition as well as the stepwise relaxation from the photoinduced high-spin state to the low-spin state.

The intergrowth crystal of n-tetracosane/urea presents a misfit parameter, defined by the ratio γ = ch/cg (chost/cguest), that is very close to a commensurate value (γ ≅ 1/3). High-resolution diffraction studies presented here reveal an aperiodic misfit parameter of γ = 0.3369, which is found to be constant at all temperatures studied. A complex sequence of structural phases is reported. The high temperature phase (phase I) exists in the four-dimensional superspace group P6122(00γ). At Tc1 = 179(1) K, a ferroelastic phase transition increases the dimension of the crystallographic superspace. This orthorhombic phase (phase II) is characterized by the five-dimensional (5D) superspace group C2221(00γ)(10δ) with a modulation vector ao* + cm* = ao* + δ · ch*, in which the supplementary misfit parameter is δ = 0.025(1) in host reciprocal units. This corresponds to the appearance of a modulation of very long period (about 440 ± 16 Å). At Tc2 = 163.0(5) K, a 5D to 5D phase transition leads to the crystallographic superspace group P212121(00γ)(00δ) with a very similar value of δ. This phase transition reveals a significant hysteresis effect.

(TMTTF)2X, with X=PF6, exhibits, at TSP, a spin-Peierls transition. Below TSP elastic neutron scattering measurements performed on large protonated and deuterated samples reveal the formation of very weak superstructure reflections at the (1/2,1/2,1/2) reduced wave vector. Upon deuteration TSP decreases by 30% while the charge ordering (CO) transition increases by 25%. In this framework we stress the importance of the anions, X, and of their organic cavities to stabilize the CO transition of the (TMTTF)2X salts.

In this review we present three representative examples of molecular disorder: substitution disorder for carbon nanotubes in zeolite, coupling between substitution disorder and displacements through the pinning of charge-density waves in one-dimensional molecular conductors, and the combination of random packing with orientational disorder in the C60—C60 dimers systems.

(EDT-TTF-CONH2)6[Re6Se8(CN)6], space group R, was prepared by electrocrystallization from the primary amide-functionalized ethylenedithiotetrathiafulvalene, EDT-TTF-CONH2 (E(1/2)1 = 0.49 V vs SCE in CH3CN), and the molecular cluster tetraanion, [Re6Se8(CN)6]4- (E(1/2) = 0.33 V vs SCE in CH3CN), equipped with hydrogen bond donor and hydrogen bond acceptor functionalities, respectively. Its Kagome topology is unprecedented for any TTF-based materials. The metallic state observed at room temperature has a strong two-dimensional character, in coherence with the Kagome lattice symmetry, and the presence of minute amounts of [Re6Se8(CN)6](3-)* identified by electron spin spectroscopy. A structural instability toward a distorted form of the Kagome topology of lesser symmetry is observed at ca. 180 K. The low-temperature structure is associated with a localized, electrically insulating electronic ground state and its magnetic susceptibility accounted for by a model of uniform chains of localized S = 1/2 spins in agreement with the 100 K triclinic crystal structure and band structure calculations. A sliding motion, within one out of the three (EDT-TTF-CONH2)2 dimers coupled to the [Re6Se8(CN6)(3-)*]/[Re6Se8(CN6)4-] proportion at any temperature, and the electronic ground state of the organic-inorganic hybrid material are analyzed on the basis of ESR, dc conductivity, 1H spin-lattice relaxation, and static susceptibility data which qualify a Mott localization in [EDT-TTF-CONH2]6[Re6Se8(CN)6]. The coupling between the metal-insulator transition and a structural transition allows for the lifting of a degeneracy due to the ternary axis in the high temperature, strongly correlated metallic phase which, in turn, leads to Heisenberg chains at low temperature.

The one-dimensional (1D) 3d1 system BaVS3 undergoes a metal–insulator (MI) transition at T MI = 70 K , whose origin is still a matter of controversy. We show that the MI transition is accompanied by the formation of a superstructure which doubles the chain periodicity. The superstructure has been refined in the monoclinic Im space group from high resolution X-ray synchrotron powder diffraction. We show that the MI transition sets a bond order wave on the V chains. At 5 K, we present structural evidence for a charge disproportionation on the V sites, accompanied by a distortion wave of the sulphur octahedra.

The charge ordering observed in the (TMTTF) 2 X family has been studied by X-ray absorption spectroscopy. XANES measurements at the Sulfur K-edge show no evidence of charge disproportionation larger than 0.5 e, and EXAFS at the Phosphorus K-edge indicate no displacements of the PF 6 anion larger than 0.05 A. The difficulty to observe a structural signature of this charge ordering is due to the triclinic symmetry of these salts. By contrast, in the monoclinic charge transfer salt (DMtTTF) 2 ClO 4 , a screw axis symmetry constrains the molecular stacks to be uniform. In this real 1/4-filled system, charge localization is observed at about 150 K. We show that around this temperature an incommensurate modulation of reduced wave vector (0.58,0,-0.275) is stabilized. The transition is strongly hysteretic, but no long range order is established. We suggest that this modulation, which has the 4kF periodicity in the chain direction, stabilizes a local antiferroelectric state similar to the one previously observed in (TMTTF) 2 SCN.

The inorganic quasi-one dimensional (1D) S = 1/2 antiferromagnetic (AF) system CuGeO3 undergoes a 2nd order spin-Peierls (SP) phase transition at T SP = 14.2 K. In this study we present an X-ray synchrotron radiation investigation which confirms that the SP instability is announced by an important regime of pretransitional structural fluctuations which have been detected until 36 K. Furthermore we show that these fluctuations are 1D above 24 K, a feature expected for a structural instability triggered by the Cu2 + chains of spin 1/2. By extrapolating the thermal dependence of the correlation length in the chain direction, we show that formation of singlet dimers begins at about 50 K, a temperature that we identify as the mean field temperature of the SP chain. The critical nature of the pretransitional fluctuations does not change when low amounts (

We present a room temperature high-pressure X-ray diffraction study of the layered compound 1T-TaS2 up to 20 GPa. This material is known to exhibit a variety of structural phase transitions that are ascribed to the stabilization of charge density wave states. It has been recently shown that at pressures larger than 3 GPa and up to 25 GPa, 1T-TaS2 becomes superconductor below about 5 K. It was suggested that this superconductivity coexists with different CDW states, an hypothesis that can be tested by X-ray diffraction. Our first results at room temperature show that at around 1.9 GPa, the nearly-commensurate (NCCDW) phase transforms into a phase similar to the high temperature incommensurate phase (ICCDW). Above 9 GPa, we show the existence of another IC phase, still discernable up to 20 GPa despite the pressure-induced crystal damage above 13 GPa. These results are consistent with resistivity measurements, but call for a complete exploration of the P–T phase diagram of 1T-TaS2.

Mixed-stack charge-transfer (CT) complexes undergoing the neutral-ionic (N-I) phase transition are molecular materials formed of stacks where electron donor (D) and acceptor (A) molecules regularly alternate. In the N phase, the CT is low and molecules are situated on inversion centers, while in the I phase, the increase of CT is accompanied by a lattice distortion (dimerization process and symmetry breaking). The one-dimensional (1D) architecture triggers the chain multistability by stabilizing lattice-relaxed (LR)-CT excitations ...D° A° D° A° $(D^+A^-)(D^+A^-)(D^+A^-)$ Do A" D° A° D°... These 1D nano-scale objects are at the heart of the equilibrium N-I phase transition and govern the fascinating physical properties such as giant dielectric response or photo-induced phase transformations. In this contribution, the 1D character of these critical excitations will be demonstrated by direct observation using high resolution X-Ray diffraction.

Dans cet article, nous presentons des effets de diffraction originaux dus a la presence d'impuretes dans les metaux. A partir du cas connu des zones de Guinier-Preston, ou la premiere observation des effets d'asymetrie d'intensite a ete realisee, nous montrons que celle-ci peut fournir des informations importantes sur l'accrochage des ondes de densite de charge dans les systemes de basse dimension. Selon la premiere interpretation de Guinier, une telle asymetrie est caracteristique de l'existence de correlations entre le desordre de substitution et de position. Nous montrerons que dans la limite des faibles concentrations d'impuretes, cette asymetrie d'intensite peut etre interpretee comme un effet de diffraction holographique . Nous discutons ensuite un autre type d'asymetrie, relie a la presence d'oscillation de Friedel autour d'impuretes chargees.

Correlated quasi-1D electron gas exhibit 2k F and 4k F CDW/BOW instabilities widely observed among the organic conductors. The Bechgaard and Fabre salts exhibit mixed SDW-CDW, Anti-ferromagnetism (AF) and spin peierls (SP) ground states achieving a subtle coupling between the spin and lattice degrees of freedom. The SP and AF nature of the Fabre salts’ ground state is analyzed in relation with the presence of a high temperature 4k F CDW-ferroelectric phase transition.

We report first direct evidence, by X-ray diffuse scattering, of lattice-relaxed charge transfer exciton strings, which are one-dimensional self-trapped multiadjacent excitons. These non-linear excitations are key objects in quasi–one-dimensional organic compounds that undergo the neutral-ionic transition and are responsible for their physical properties. The one-dimensional fluctuations are very well described within the framework of the transfer integral method and physical parameters are deduced from both X-ray and dielectric measurements.

International audience; The study, tailoring, and control of the coupling between magnetic layers triggered the development of spintronics and has been up to now focused on systems comprising at least one ferromagnetic layer. Here we present a system where two antiferromagnetic layers separated by an ultrathin tunnel barrier interact with each other. We have studied with neutron and x-ray diffraction two sets of Cr/MgO/Cr trilayers, with either thin (five monolayers or less) or thick (eight monolayers or more) MgO layers, along with isolated Cr layers used as references. While the behavior of the trilayers with thick MgO layers can be explained by the strain state of each layer, that of trilayers with thin barriers cannot be related to the well known behavior of isolated chromium layers. We indeed report in these samples the presence of anomalous magnetic phases (modified propagation vector, direction of spins and propagation vectors inconsistent with the strain state of the sample) that we ascribe to the existence of a tunnel magnetic coupling between the AF layers through the insulating barrier. This unexpected finding opens opportunities for spintronics using solely antiferromagnets.

Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K(0.3)MoO(3) over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.

International audience; We study the basic mechanisms allowing light to photoswitch at molecular scale a spin-crossover material from low-to high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced HS state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.

Spatial uctuations of spin density wave (SDW) and charge density wave (CDW) in chromium have been compared by combining coherent and simultaneous X-ray diffraction experiments. Despite their close relationship, spatial fluctuations of the spin and of the charge density waves display a very different behavior: the satellite reflection associated to the charge density displays speckles while the spin one displays an impressive long-range order. This observation is hardly compatible with the commonly accepted magneto-elastic origin of CDW in chromium and is more consistent with a purely electronic scenario where CDW is the second harmonic of SDW. A BCS model taking into account a second order nesting predicts correctly the existence of a CDW and explains why the CDW is more sensitive to punctual defects., 4 pages, 4 figures; Accepted in Phys. Rev. B

We use time-resolved hard X-ray diffraction to directly follow the dynamics of structural symmetry change during the ultrafast melting of a charge density wave. We observe a transient recovery of the periodic lattice distortion on a sub-picosecond timescale.

We present an x-ray diffraction study of the vanadium-doped blue bronze ${\mathrm{K}}_{0.3}({\mathrm{Mo}}_{0.972}{\mathrm{V}}_{0.028}){\mathrm{O}}_{3}.$ At low temperature, we have observed both an intensity asymmetry of the $\ifmmode\pm\else\textpm\fi{}{2k}_{F}$ satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that the profile asymmetry is due to Friedel oscillation around the V substituant and that the intensity asymmetry is related to the charge-density-wave (CDW) pinning. These two effects, intensity, and profile asymmetries, give access to the local properties of CDW in disordered systems, including the pinning and even the phase shift of Friedel oscillations.

The quasi-one-dimensional organic conductors di-perylene hexafluorophosphate and -antimonate undergo three and two structural phase transitions below room temperature, respectively. The unequivocal structural proof of the Peierls transition at T p in both compounds is provided by X-ray diffuse scattering experiments. The characterization of the structural phase transitions above T p is done using the analysis of X-ray patterns together with previously known ESR results. The combined techniques offer new insight into the relative displacements and rotations of the perylene stacks. The relationship between the Peierls transition and the precursor transitions is discussed.

We report on the study of density wave defects in pure chromium. The incommensurate satellite reflections associated with the spin density wave (SDW) and with the charge density wave (CDW) have been explored in condition of non-resonant coherent X-ray diffraction. First, a line shapes analysis of the SDW and CDW reveals that the correlation length of the CDW is smaller than the SDW one. This observation shows that the CDW in chromium is not stabilized by a pure magnetostrictive coupling with the SDW. Then, coherent X-ray diffraction profiles obtained on the SDW reflection are consistent with the presence of an edge dislocation of the magnetic order, embedded at a few micrometers from the surface of the sample. Beyond the specific case of dislocations in chromium, this work may open up the way in studying magnetic defects embedded in the bulk.

We present an x-ray diffuse scattering study of the perturbation of the charge-density-wave structure by impurities in doped ${\mathrm{NbSe}}_{3}$. Substitution of the transition-metal element niobium is performed either by an isoelectronic impurity, tantalum, or by a nonisoelectronic impurity, titanium, with the concentration equal to $x=2%$ and $x=1%$, respectively. The nature of the pinning is determined for each type of doping, and we evaluate the size of the domains of coherence of the charge-density waves.

In spite of recent progress, our present understanding of the intermolecular interactions in solid C 60 is still imperfect and no single model is able to describe correctly its whole physical properties. These interactions induce elaborate orientational properties which have motivated thorough investigations. In this review paper we concentrate on the orientational properties in the high temperature phase. It is shown how the preferred orientations and the orientational correlations could be accessed through the analysis of the Bragg and diffuse scattering intensity data. The results obtained using either mean-field treatments or molecular dynamics calculations have brought information on the existence of competing local orientational orders. The scattering data have also been exploited to discriminate between different models of intermolecular potential. Within the mean-field treatment, necessary conditions for any model to satisfy scattering results are put forward.

We present an X-ray diffuse scattering study of the pretransitional structural fluctuations of the magneto-elastic transition in alpha'-NaV2O5. This transition is characterized by the appearance below Tsp~35K of satellite reflections at the reduced wave vector (1/2,1/2,1/4). A large regime of structural fluctuations is measured up to 90 K. These fluctuations are three dimensional between Tsp and ~50K and quasi-one dimensional above ~60K. At 40 K the anisotropy ratio is found to be (xib :xia :xic)= (3.8 : 1.8 : 1), which reveals the importance of transverse interactions in the stabilization of the low temperature phase. We discuss our results within the framework of recent theories dealing with the simultaneous occurrence of a charge ordering, a spin gap and a lattice distortion in this intriguing compound.

We present an x-ray diffuse scattering study of the anion ordering (AO) transition in the (TMTSF){sub 2}(ReO{sub 4}){sub 1{minus}x}(ClO{sub 4}){sub x} solid solution of quasi-one-dimensional organic conductors and superconductors. As (TMTSF){sub 2}ReO{sub 4} and (TMTSF){sub 2}ClO{sub 4} present AO instabilities at different critical wave vectors [{bold q}{sub 1}=( (1) /(2) , (1) /(2) , (1) /(2) ) and {bold q}{sub 2}=(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 ReO{sub 4}{sup {minus}} or ClO{sub 4}{sup {minus}} 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 {bold q}{sub 1} long-range order (LRO) occurs for x{le}0.5 and a {bold q}{sub 2} LRO occurs for 1{le}x{le}0.97, while a short-range order (SRO) appears in the intermediate concentration range. For 0.7{lt}x{lt}0.94 one observes coexistence of {bold q}{sub 1} and {bold q}{sub 2} SRO. The basic features of the (T,x)more » 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} {ital 1997} {ital The American Physical Society}« less

We have performed X-ray scattering investigations of (TMTSF)2PF6 and (TMTTF)2 Br in their magnetic ground states. Very weak 2kF and 4kF satellite reflections are observed in the spin density wave (SDW) ground state of (TMTSF)2PF6. They are probably the signature of a purely electronic charge density wave (CDW) modulation of about one tenth of electron per molecule. Our X-ray results together with magnetic measurements show that the ground state of (TMTSF)2PF6 will exhibit an unprecedented 2kF mixed CDW-SDW modulation. 4kF satellite reflections are observed in the antiferromagnetic (AF) ground state of (TMTTF)2Br. In this salt the 4kF CDW modulation is probably set by magnetoelastic coupling of the lattice degrees of freedom with the AF modulation.

Diffuse scattering, which contains two-body information, is a rich source of knowledge on the interactions between the ${\mathrm{C}}_{60}$ molecules. We present a mean-field theory for calculating the diffuse scattering in the high-temperature phase of ${\mathrm{C}}_{60}$ from microscopic models of intermolecular interactions. The diffuse scattering is calculated and discussed for most of the models available to date. It is compared with single-crystal x-ray diffuse scattering data. Overall, the positions of the diffuse scattering maxima, corresponding to competing instabilities, are found to be independent of the model of interactions. This suggests that these instabilities are related to the shape of the large ${\mathrm{C}}_{60}$ molecule. No model is fully satisfactory for fitting the relative intensities of the diffuse scattering features. The best fit is obtained with the Lamoen-Michel model, which accounts also correctly for the Bragg peak intensity in the high-temperature phase. The physics of the models is discussed and some improvements are proposed.

BaVS3 is an unusual 3d1 quasi-one dimensional (1D) correlated system presenting an interplay of charge, orbital and spin degrees of freedom. At T MI = 70 K , BaVS3 undergoes a metal–insulator transition whose origin is still a matter of controversy. We show that this transition is associated with a structural modulation preceded by an extended temperature range of 1D lattice fluctuations. We have refined the superstructure and showed that it corresponds to a complex modulation of the V bonds leading to the formation of an Unconventional Charge Density Wave (CDW) ground state.

Two systems are homometric if they are indistinguishable by diffraction. We first make a distinction between Bragg and diffuse scattering homometry, and show that in the last case, coherent diffraction can allow the diffraction diagrams to be differentiated. The study of the Rudin-Shapiro sequence, homometric to random sequences, allows one to manipulate independently two-point and four-point correlation functions, and to show their effect on the statistics of speckle patterns. Consequences for the study of real materials is discussed., 5 pages, 4 figures, article, rewritten

Structural aspects of the Bechgaard salts (TMTSF) 2 X and related 2:1 salts, mostly based on the sulfur donors TMTTF and BCPTTF as well as on the hybrid TMDTDSF donor, are considered in relationship with their physical properties. The basic structure of these salts is reviewed with a special emphasize on the dimerization of the organic stack and on the disorder of the anions (X). Then the density waves instabilities driven by the donor stacks are discussed. It is shown in particular that (TMTSF)2PF6 exhibits a mixed 2k F CDW-SDW ground state. The 4k F charge localization phenomena and the 2k F instability of the organic stacks are described in relationship with the structural dimerization and the importance of Coulomb interactions. In the strong charge localization case the spin Peierls instabilities of (TMTTF)2PF6 and of (BCPTTF)2PF6 are more particularly detailed. General aspects of the anion ordering transitions exhibited by salts with non centrosymmetrical anions are pointed out as well as the influence of such transitions on the electronic properties. Finally the relevance of these structural features is briefly discussed by comparing the Bechgaard salts series' of quasi-one dimensional conductors to the family of two dimensional (BEDTTTF) 2 X conductors.