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International audience; The evolution of the Au30Pd70(110) surface was studied by coupling grazing incidence X-ray diffraction and mass spectrometry under oxygen-rich conditions at moderate temperatures (300 to 470 K). This allows us to correlate the depth profile of its structure to its catalytic properties for carbon monoxide (CO) oxidation. Under increasing pressure from ultrahigh vacuum up to 100 mbar, both oxygen and CO induce Pd segregation, even at room temperature. However, in pure oxygen the surface is reorganized with a (1 × 2) missing row reconstruction, whereas in pure CO it is strongly roughened. When oxygen pressure is increased a phase corresponding to the initial step of the oxidation with oxygen dissolution in the subsurface region appears at first. Then, from about 400 K onward, an oxidized thin Pd layer (≤1 nm) is formed growing in the [100]PdO direction. This PdO phase is strained and does not coincide with the P42/mmc structure usually observed for this oxide under ambient conditions. It is more probably consistent with the high pressure I4/mmm PdO structure strained by epitaxy on the underneath alloy. For higher oxidizing conditions and layer thickness, the oxide will then relax to the usual PdO structure. This strained oxide is easily reduced by CO and exhibits a very high activity for CO oxidation. Its catalytic performance at 470 K is comparable to the one found on surfaces of pure palladium at higher temperatures. Furthermore, on the clean Au30Pd70(110) surface, surface oxidation is hindered up to 470 K if CO is introduced prior to oxygen. This indicates that when Pd is alloyed with gold, its binding with CO is stronger than with oxygen. The weakening of the Pd–O binding by surrounding gold atoms is the key of the formation of a well-ordered and very active thin PdO film on Au30Pd70(110).

International audience; Localized Surface Plasmon Resonance (LSPR) of noble metal nanoparticles has attracted a lot of attention in recent years as enhancer of the photocatalytic activity in the visible light domain. Rare are the experimental in situ studies, coupling structural and optical responses, but they are mandatory for a deep understanding of the mechanisms underlying LSPR. Herein we present an in situ investigation during the growth of gold nanoparticles (NPs) on TiO 2 (110) in the 2-6 nm size range. We probed the structural and morphological properties of the supported nanoparticles by performing GIXRD and GISAXS simultaneously with their optical response in p and s polarizations recorded by SDRS. The rutile surface state turns out to have a major effect on the Au NPs growth and on their plasmonic response, both in frequency and vibration modes. The roughening of the TiO 2 (110) surface weakens the interaction strength between the NPs and the substrate, favoring the growth of textured in-plane randomly orientated NPs. Compared to the epitaxial clusters growing on the flat TiO 2 surface, these textured NPs are characterized by a LSPR blue shift and by the presence of LSPR vibration modes perpendicular to the surface for sizes smaller than about 4 nm.

International audience; Au-Cu bimetallic nanoparticles (NPs) grown on TiO 2 (110) have been followed in-situ using grazing incidence x-ray diffraction and x-ray photoemission spectroscopy from their synthesis to their exposure to a CO/O 2 mixture at low pressure (P < 10-5 mbar) and at different temperatures (300 K-470 K). As-prepared samples are composed of two types of alloyed NPs: randomly oriented and

The structure and composition of the Au30Pd70(110) surface were followed by grazing incidence X-ray diffraction under increasing oxygen pressure from ultrahigh vacuum (UHV) to 500 mbar at moderate temperatures (300–420 K). These measurements were complemented by Auger electron spectroscopy and environmental scanning tunneling microscopy (STM) images with atomic resolution up to 500 mbar. After the cleaning and preparation procedures in UHV, the Au30Pd70(110) surface is (1 × 1) with a quasipure topmost layer of segregated gold. Under oxygen pressure, Pd segregation occurs and progressively enriches the near-surface region. The surface evolves through different states that slightly differ from those of pure Pd(110). First, the (1 × 2) missing-row reconstruction was induced by oxygen adsorption that is stable in a large range of pressures depending on the temperature. Actually, STM images show regular vacancies every two atoms along the dense [110] rows. Then, for higher oxygen pressures, a transition phase.

International audience; We present an ab initio numerical tool to simulate surface resonant X-ray diffraction experiments. The crystal truncation rods and the spectra around a given X-ray absorption edge are calculated at any position of the reciprocal space. Density functional theory is used to determine the resonant scattering factor of an atom within its local environment and to calculate the diffraction peak intensities for surfaces covered with a thin film or with one or several adsorbed layers. Besides the sample geometry, the collected data also depend on several parameters, such as beam polarization and incidence and exit angles. In order to account for these factors, a numerical diffractometer mimicking the experimental operation modes has been created. Finally two case studies are presented in order to compare our simulations with experimental spectra: (i) a magnetite thin film deposited on a silver substrate and (ii) an electrochemical interface consisting of bromine atoms adsorbed on copper.

Nanostructured substrates exhibiting a regular two-dimensional network of defects represent ideal systems for growing ordered nanoparticle arrays with a narrow size distribution. Here, the growth of Pd–Au nanoparticle arrays on a thin Al2O3 film obtained by oxidation of a Ni3Al(111) single crystal surface has been examined. The nanoparticle composition ranges from pure palladium to almost pure gold with a diameter of ∼2 nm. Small-angle X-ray scattering and X-ray diffraction in grazing incidence (GISAXS and GIXRD) were performed in situ during the sequential deposition of Pd and Au. GISAXS provided evidence of a diffuse scattering contribution whose appearance is linked to the progressive disordering of the arrays. It is correlated to the size of the initial Pd clusters and to the Au/Pd final ratio: the lower the ratio is, the better the organization of the nanoparticle array. More specifically, the arrays start to disorder as soon as one atomic-thick Au layer is deposited on the Pd seeds. The morphologica...

Pt(1 1 0) is one of the most closely investigated metal surface structures because it displays a variety of “missing-row” reconstructions, which are only marginally stable. The ground state is usually found to have 1 × 2 translational symmetry, but a 1 × 3 form has also been seen. Between 1 × 2 and 1 × 3, a series of disordered structures has been recorded, which shows a slight preference for 1 × 5 periodicity. Under the preparation conditions used in this study, a stable 1 × 5 structure was found for Pt(1 1 0). Investigation by surface X-ray diffraction has led to a complete three-dimensional structure, which closely resembles an alternation of 1 × 2 and 1 × 3 unit cells. Pt(1 1 0) shows an interesting example of two “homometric” structures that are indistinguishable by diffraction, but are distinguishable by virtue of their subsurface relaxation pattern.

Pd ultra thin films on Au(1 1 0)-(1×2) have been prepared at 300 K by vapour deposition and studied using medium energy electron diffraction (MEED), surface X-ray diffraction (SXRD), low energy electron diffraction, low energy ion scattering (LEIS) and Auger electron spectroscopy (AES). The (1×2) superstructure intensities are strongly attenuated during the deposition of the first 0.5 ML. Simultaneously, LEIS indicates that gold atoms cover a large fraction of the deposited Pd atoms. A quantitative SXRD analysis of the 0.5 ML Pd film reveals a significantly ordered interface alloy covered by two incomplete Au layers. When more Pd is deposited up to about three layers, the growth proceeds in a quasi layer by layer mode, while the surfactant effect of gold persists. In this range of thickness, the film, mainly composed of Pd atoms, is under tensile strain in coherent epitaxy with the Au(1 1 0) substrate.

Co ultra-thin films deposited on Pt(111) are analysed in detail, using LEED, Auger spectroscopy and x-ray diffraction. The growth is quasi layer-by-layer up to about 3 ML. Co grows as islands, in incoherent epitaxy with the Pt substrate; the corresponding satellites are observed even for coverages as low as 0.12 ML. The Co in-plane parameter is very close to its own bulk value. For thicker films, deposited at room temperature, face centred cubic (fcc), twin fcc and hexagonal close packed (hcp) fractions are present. Using x-ray diffraction, we followed in real-time the transformations of the deposited film upon annealing. The role of the Co film structure (which depends on the film thickness) is predominant. For `thick' films (6 ML) a sharp transition occurs around 670 K: below this temperature, the film is mainly hcp so that it allows little Pt incorporation, while above it turns to a very homogeneous fcc alloy of composition close to Pt60Co40. Segregation phenomena were analysed and found similar to those occurring at the surface of the corresponding bulk alloys.

X-ray-diffraction measurements have been performed on ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}(001)$ thin films strained in ${\mathrm{I}\mathrm{r}/\mathrm{F}\mathrm{e}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}/\mathrm{Ir}(001)$ sandwiches and in $[{\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}/\mathrm{Ir}{]}_{20}$ superlattices with $x=0.7$ and $x=0.9.$ These concentrations were chosen with regard to the magnetic properties of the alloys. Superconducting quantum interference device measurements showed a ferromagnetic state above $x\ensuremath{\approx}0.75$ and an antiferromagnetic or a nonmagnetic one below. The samples have been investigated by using the atomic contrast available through anomalous diffraction and by recording reciprocal space maps (RSM). The (111) RSM, sensitive to both in-plane and out-of-plane parameters, has been studied as a function of the grazing incidence angle. It has revealed that the $[{\mathrm{Fe}}_{0.9}{\mathrm{Mn}}_{0.1}/\mathrm{Ir}{]}_{20}$ superlattice has a complex structure with two phases stacked along the growth axis whose origin stems from the existence of the two cubic structures (bcc and fcc) of the bulk alloys. In each case, we have shown that the ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}$ alloys are strained in a bct structure. This tetragonalization allows us to investigate the magnetic properties throughout a continuous transformation from a bcc phase $(c/a=1)$ to a fcc one $(c/a=\sqrt{2}).$ Together with x-ray resonant magnetic reflectivity measurements, our results show that the tetragonalization plays a dominant role on the magnetic state observed for the investigated samples. For $c/a$ in the range 1.2--1.3, the Fe atoms are found to be in a ferromagnetic low-spin state.

Long range order (LRO) in ultrathin Pt–Co films was studied by surface X-ray diffraction (XRD). Several fcc alloys of nanometric thickness were grown in situ by annealing at 460°C Co layers deposited onto a Pt(111) single crystal. Superstructure reflections were observed, which agreed with the extinction rules of either the L1 2 or the L1 0 chemically ordered bulk phases. The relationship between their structure factor and the atomic scattering factors was found with anomalous scattering performed near both Pt L III and Co K edges. This method is very promising for studies of surface alloying. The films were also studied in real time during annealing. They evolved in a quite different way, depending on the initial Co thickness, but LRO always occurred by heating above 400°C. At this temperature the films became Pt-rich, with a stoichiometry close to Pt 60 Co 40. We did not succeed in obtaining long range chemical order in a Co-rich alloy by annealing a Co/Pt(111) deposit, contrary to what happens in films grown by codeposition.

We describe a new surface X-ray diffractometer, which is optimized to combine surface X-ray diffraction (SXRD), absorption spectroscopy (SEXAFS) and grazing incidence small angle scattering (GISAXS). This instrument is particularly well suited for real time studies of material elaborated in-situ with molecular beam deposition (MBD) techniques. The goniometer allows for large in-plane and out-of-plane momentum transfer with high accuracy. Owing to the flipping mechanism of the sample holder, it can perform absorption experiments with polarisation directions normal and parallel to the sample surface, while keeping a grazing incidence. Several MBD sources as well as complementary surface sensitive electron techniques (RHEED and Auger spectroscopy) can be used simultaneously with X-rays. Finally, the samples can easily be inserted in the X-ray chamber via an UHV transport system and a fast entry load-lock module.

We study the alloying of ultrathin Co deposits (3 and 10 monolayer) on a Pt(111) substrate. The surface film evolution is followed in real time during annealing by X ray diffraction, which provides the depth profile with atomic resolution. We fully characterize the formation of Pt–Co surface alloys before dissolution; the kinetics is shown to depend on the Co thickness. For the 10 monolayer film, the dominant hcp phase exhibits a strong resistance to interdiffusion while for the 3 monolayer deposit the surface film is progressively enriched in Pt. Finally, whatever the annealing treatment and the film thickness, the stabilized alloy is close to Pt60Co40 bulk-like, after heating around 450°C.

International audience; The relationship between the catalytic activity and the size was studied in operando in the case of gold nanoparticles on TiO2(110) model catalyst during carbon monoxide oxidation. The geometrical parameters, the shape and the dispersion of the particles on the oxide support were examined in detail. The catalytic activity was found optimum for a nanoparticle diameter of about 2 nm and a height of six atomic monolayers. Above the maximum, it fits a power law of the diameter D^-2.4 +/-0.3. This indicates that the low-coordinated sites play a major role in the catalytic activity, however such a model still fails to explain the activity maximum. The nanoparticle sintering was also investigated since it is suspected of being responsible for the decrease of the catalyst activity in the course of time. It was clearly observed for particles with a size around the maximum of activity and smaller. At the very beginning of the CO conversion into CO2, the sintering is strongly activated. The nanoparticles mobility is dependent upon the TiO2(110) surface direction under consideration: it is higher along the [001]TiO2 than along the [1-10]TiO2. Then, the sintering greatly slows down. This could be explained by a nanoparticles' pinning at the step edges. The thermal energy released by the exothermic CO oxidation reaction was evaluated and it suggests that the sintering results from a more complex process than from a reaction-induced local heating.

International audience; The origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of gold nanoparticles supported on TiO2(110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O2, Ar, or a mixture of O2 + CO and of Ar + CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle's diameter of 2.1 +/- 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both "model" and "real" catalysts.

Elevated pressure in situ STM studies were crucial to show that CO adsorption on gold surfaces strongly modifies their surface structures [1–3]. However oxygen dissociation does not occur spontaneously on such surfaces preventing efficient CO oxidation. The addition of an oxygen-dissociative metal (such as Pd) to gold may overcome this difficulty. In this work we present results concerning the behaviour of Pd70Au30(110) under elevated pressures of CO and O2 and compare them to the results obtained on Au(110) under similar conditions.

International audience; The catalytic activity of oxide-supported gold nanoparticles depends crucially on their size. The present work describes a dedicated set-up in which particle size is determined by grazing incidence small angle x-ray scattering (GISAXS) and reactivity is analysed via a mass spectrometer. Catalytically active gold nanoparticles supported on TiO2(110) of size ranging between 2.4 and 5 nm were characterized during the CO oxidation at pressures in the range 0.1-100 mbar. The growth was found 3D and the particles were best modelled by a truncated sphere. The reaction rate per Au atom measured at 470 K was seen to increase in a monotone manner as the cluster size decreases, without reaching any maximum. Particles of size lower than 3 nm were stable under oxygen but sintering occurs when CO is added at 470K. That dimension coincides with the switch which was previously observed from nucleation-growth, with particles pinned on defects, to coalescence where particles become independent of defects.

Non-linear transport properties have now been measured in numerous one-dimensional transition metal tri- and tetrachalcogenides. For all these compounds the threshold electric field above which the non-linear state appears goes through a minimum in the vicinity of the Peierls temperature transition. The value of this minimum is all the higher as the Peierls transition occurs at a higher temperature. The variation of the fundamental frequency of the time-dependent voltage is shown to follow a (E-Ec)y law (with y ∼ 1.5) near the threshold indicating a collective pinning. Finally non-linear properties of a NbSe3 sample have been studied when both contacts are above the Peierls temperature : in these conditions an a.c. voltage is still detected. This result is discussed in relation with theories on the origin of this voltage generation.

International audience; A new experimental setup has been developed to enable in situ studies of catalyst surfaces during chemical reactions by means of surface x-ray diffraction (SXRD) and grazing incidence small angle x-ray scattering. The x-ray reactor chamber was designed for both ultrahigh-vacuum (UHV) and reactive gas environments. A laser beam heating of the sample was implemented; the sample temperature reaches 1100 K in UHV and 600 K in the presence of reactive gases. The reactor equipment allows dynamical observations of the surface with various, perfectly mixed gases at controlled partial pressures. It can run in two modes: as a bath reactor in the pressure range of 1-1000 mbars and as a continuous flow cell for pressure lower than 10−3 mbar. The reactor is connected to an UHV preparation chamber also equipped with low energy electron diffraction and Auger spectroscopy. This setup is thus perfectly well suited to extend in situ studies to more complex surfaces, such as epitaxial films or supported nanoparticles. It offers the possibility to follow the chemically induced changes of the morphology, the structure, the composition, and growth processes of the model catalyst surface during exposure to reactive gases. As an example the Pd8Ni92(110) surface structure was followed by SXRD under a few millibars of hydrogen and during butadiene hydrogenation while the reaction was monitored by quadrupole mass spectrometry. This experiment evidenced the great sensitivity of the diffracted intensity to the subtle interaction between the surface atoms and the gas molecules.

International audience; The morphology and the atomic scale structure of thin gold films (up to 2.5 ML) on Pd(110) were studied by means of scanning tunneling microscopy and surface X-ray diffraction. At room temperature the films exhibit a multilayer growth mode accompanied by the formation of highly anisotropic islands. Annealing above 500 K significantly increases the smoothness of the gold films, which are in registry with the substrate. Above a critical threshold of two monolayers a (1x2) missing-row reconstructed film is found. This reconstructed surface is well ordered after annealing at temperatures above 580 K. The specific gold film morphology is envisaged as a way to relax the strain caused by the mismatch between gold and palladium.

Atomically thin Co/Pt(1 1 1) interfaces grown at different temperatures are characterized by very different values of perpendicular magnetic anisotropy as a consequence of the local structure and coordination. Here we present a study of the structural, magnetic and electronic properties for interfaces grown in UHV onto clean Pt(1 1 1) in different kinetic conditions. When one monolayer of Co is deposited at 540 K a thermally activated exchange reaction leads to a sharp Pt–Co double interface giving rise to a strong increase of the magneto-optical response with respect to the Co monolayer deposited at room temperature. The results are interpreted in terms of atomic hybridization as detected by valence band photoelectron spectroscopy.

International audience; The (Nx1) reconstruction (N = 5-6) of the clean Pd8Ni92(110) surface has been investigated by surface X-ray diffraction. A good fit with experimental data collected under UHV conditions is obtained when introducing undulations in the outer dense rows, constituted mainly of Pd atoms which strongly segregate to the surface. This reconstruction can be regarded as a way to relax the strains induced on the bigger Pd surface atoms, at least partially. The modifications of the surface were studied under butadiene then hydrogen as well as during butadiene hydrogenation at elevated pressure at 300 K and the (Nx1) reconstruction was found to be still present. The main changes are observed under pure hydrogen and during butadiene hydrogenation.

X-ray anomalous scattering has been used for the selective characterisation of the structure of each sublattice in Fe/Ir (001) superlattices. The data have been analysed by comparing the simulated and experimental spectra in q-space at various wavelengths near the absorption edges. This approach, applied to symmetric reflection scans, enables a model-independent determination of the perpendicular lattice spacings of the individual layers.

High intensity for diffraction experiments with high energy resolution, on an intense source like bending magnet at the European Synchrotron Radiation Facility, requires a strict control of the curvature of the optical elements placed in the beam for geometrical focusing and for wavelength monochromatization. Unwanted curvature of the first crystal of the monochromator can arise from the thermal load. Indeed, due to the size of the crystal, only cooling from the rear is conceivable. This induces a front‐to‐rear thermal gradient and, as a consequence, a strong spherical curvature. A new technique was developed for the CRG/D2AM beamline in order to control this effect. It was shown by calculation that this curvature can be exactly compensated, whatever the heat load power is, by the thermal expansion of a metallic layer coated at the rear of the crystal. First results confirm the predicted behavior and show how sensitive the technical problem of the fixation of this bilayer is on its cooling device.

The first stages of the growth of ${\mathrm{Pt}}_{C}{\mathrm{Co}}_{1\ensuremath{-}C}$ films on Pt(111) were investigated in situ by x-ray reflectometry and grazing incidence x-ray diffraction. Co-Pt ultrathin alloys are among the most interesting candidates for magneto-optic recording media. In this study we show that, at a given substrate temperature, a surface-induced equilibrium state is established since the very beginning, resulting in a bilayer where Pt segregates on top of a Co-rich buried layer. The sample obtained by codeposition of about 1 monolayer ${\mathrm{Pt}}_{20}{\mathrm{Co}}_{80}$ at 520 K was studied in details. Its structure was solved by quantitative analysis of the intensity distribution of crystal truncation rods. The outermost plane contains at least 80 at. % Pt, while the buried one is Co rich, with a concentration that is also close to 80%. No significant Co diffusion in the underlying substrate layers was observed. The stacking was found fcc in coherent continuation with the substrate. We believe that the growth mechanism observed here is responsible for clustering in Pt-rich films with perpendicular magnetic anisotropy.

The structure of a four monolayer deposit of Pd on Ni(110) has been determined by a combination of x-ray diffraction experiments and density-functional theory calculations. This Pd film presents, after annealing at 500 K, a (Nx2) reconstruction associated with a large enhancement of its catalytic activity. The N superstructure, along the dense [11;0] direction, comes from periodic edge dislocations initiated by a vacancy in the first Pd layer. In the perpendicular direction, the doubling of the period originates in a pairing-buckling displacement of the rows. This study evidences a new Pd atoms arrangement with quasi-four-fold hollow sites on the surface, which could play an important role in the exceptional catalytic activity.

Après une brève introduction portant sur l'étude de surfaces et d'interfaces par rayons X nous présentons dans cet article un équipement nouvellement constnllt à I'ESRF (European Synchrotron Radiation Facility) sur la ligne D32(CRG-IF IF i.e. Collaborating Reseach Group - Interface). Il s'agit d'un goniomètre équipé d'une chambre ultra-vide ainsi que d'un système de transfert et d'introduction rapide d'échantillon ; il permet de réaliser des études par diffraction de rayons X en incidence rasante de surfaces et d'interfaces in situ. Nous décrivons brièvement les appareils antérieurs en précisant quelles sont les améliorations apportées par notre équipement sans toutefois entrer dans des détails trop techniques. De premiers résultats sont aussi présentés.

The hexagonal structure of ferromagnetic Fe in ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Ru}}_{\mathit{y}}$ superlattices, x (y) being the number of Fe (Ru) atomic planes in one layer, is still controversial. From the x-ray-absorption near-edge-structure results of Beaudelet et al. it has been suggested that the Fe hexagonal stacking is not hcp but that the local environment of Fe atoms is a distorted bcc structure, despite the epitaxy on hcp ruthenium. This unusual Fe structure may allow an understanding of its ferromagnetic properties and thus is supported by recent theoretical investigations. To verify its existence, we have undertaken an x-ray-diffraction study performed in transmission mode for in-plane Bragg reflections and with grazing incidence angle for out-of-plane measurements. Unexpectedly, all our results convincingly support the regular hcp structure and seem to rule out the ``bcc-like'' model.

The hexagonal structure of ferromagnetic Fe in Fe/Ru superlattices is still controversial. The unexpected magnetic behaviour [1] as well as the X-ray absorption near-edge structure (XANES) results have led Beaudelet [2] to question the hcp structure for the Fe layers and to suggest another model of the Fe hexagonal stacking in which the local environment of Fe atoms looks like a distorted bcc one, despite the epitaxy on (0001) hcp Ru. To verify the existence of this unusual hexagonal stacking, we have undertaken an X-ray diffraction study performed with grazing incidence angle. Unexpectedly, our results support convincingly the regular hcp structure rather than the ‘bcc-like’ packing.

We report non linear transport properties below the metal-insulat transition temperature T° = 263 K in the halogened metal transition tetrachalcogenide (TaSe4)2I. These non linear properties are similar to those of NbSe3 and TaS3 and indicate that (TaSe4)2I is a new compound exhibiting charge density wave transport.

Recent works tend to prove that the time-dependent voltage detected in the non-linear state of a CDW conductor is generated by phase-slippage centers or vortices created at some interface where the CDW velocity undergoes a discontinuity ; this evidently is the case near the contacts but can also be achieved in some places in the bulk of the samples when inhomogeneous conditions are imposed. We report some results concerning the breaking of the CDW coherence 1) by electrode injection effects, 2) by thermal gradient.

We have studied the depinning conditions of a small part of a NbSe 3 sample with a length 1 (segment 1) when a current is flowing in a neighbouring segment of length L (segment L), the distance d between both segments being variable. We show that when the charge density wave is depinned in segment L, the threshold field in segment 1 is reduced. This reduction is maximum when segments L and 1 have a common contact (d = 0) and is measurable up to d around 60 μm. This non local effect is only present when segments L and 1 are polarized in this same direction. These results are explained with a simple phenomenological pinning model. We assume that a pinning force with a finite short range hinders the motion of phase dislocations under the electrodes. From the interplay between bulk pinning and dislocation mechanics we propose a general equation of motion of phase dislocations.

Electric field dependence of the d.c. electrical conductivity and of the fundamental frequency (i.e. the charge density wave velocity) for the transition metal tri-and tetrachalcogenides exhibiting CDW transport is compared to the tunneling model and to the classical collective pinning theory.

We show nonlocal effects in charge density wave depinning by the reduction of the threshold field of a short segment of a NbSe3 sample when the CDW slides in a neighbouring segment separated by a distance d. When d = 0 the CDW with the faster velocity pulls the slower one whatever the respective segments length which reveals the infinite coherence length of the moving wave.

X-ray anomalous diffraction in different geometries and X-ray resonant magnetic scattering have been used to investigate Fe x Mn 1− x /Ir(001) superlattices. The influence of the alloy stoichiometry on the structural and magnetic properties has been studied and compared with the bulk alloy properties. The structural results are compared with the magnetic results in order to understand the outstanding properties of these systems.