Ultrafast Structural Dynamics along the β − γ Phase Transition Path in MnAs

International audienceWe investigate the orthorhombic distortion and the structural dynamics of epitaxial MnAs layers on GaAs(001) using static and time-resolved x-ray diffraction. Laser-induced intensity oscillations of Bragg reflections allow us to identify the optical phonon associated with orthorhombic distortion and to follow its softening along the path towards an undistorted phase of hexagonal symmetry. The frequency of this mode falls in the THz range, in agreement with recent calculations. Incomplete softening suggests that the $\beta-\gamma$ transformation deviates from a purely second-order displacive transition

Local structure in a relaxor ferroelectric : BaTi(1-x)Zr(x)O3

Octobre 2007Relaxor ferroelectrics are characterized by a large, frequency-dependent peak of the dielectric permittivity as a function of temperature. This behaviour is generally attributed to the presence of nanometre-sized polar regions. One of the experimental challenges is the determination of the structural nature of such regions, which requires the use of local structural probes. This manuscript presents the study of the local structure in the perovskite-type relaxor ferroelectrics BaTi1-xZrxO3 (0.25 ≤ x ≤ 0.50), by means of X-ray absorption (EXAFS and XANES) and pair distribution function determination from neutron total scattering. We could determine the Ti4+ and Zr4+ cation displacements with respect to the centre of their oxygen cage. The main result is that the Ti4+ cations play a major role in the local polarisation in BaTi1-xZrxO3 relaxors. Moreover, it is shown that the deformation of the ZrO6 octahedra directly depends on the local distribution of the Ti and Zr atoms in the solid solution.Les ferroélectriques relaxeurs se caractérisent par un large pic de permittivité en fonction de la température, dépendant de la fréquence du champ de mesure. Ce comportement est généralement attribué à la présence de régions polaires de taille nanométrique. L'un des enjeux expérimentaux est la détermination de la nature structurale de ces régions, nécessitant entre autres l'utilisation de sondes de la structure locale. L'objet de ce travail est l'étude de la structure locale dans les pérovskites relaxeurs BaTi1-xZrxO3 (0.25 ≤ x ≤ 0.50), présentant une substitution isovalente Ti4+/Zr4+. Les techniques expérimentales utilisées sont l'absorption des rayons X (EXAFS et XANES) et la détermination de la fonction de distribution de paires par diffusion totale des neutrons. Les déplacements des cations Ti4+ et Zr4+ dans leur cage d'oxygènes ont pu être déterminés. Le principal résultat est que les cations Ti4+ jouent un rôle majeur dans la polarisation locale des relaxeurs BaTi1-xZrxO3. Par ailleurs, il est montré que la déformation des octaèdres ZrO6 dépend directement de la répartition locale des Ti et des Zr dans la solution solide

1983-1984 : des changements dans la nature des embauches

[fre] 1983-1984 : . des changements dans . la nature des embauches . Daniel Depardieu . et Marie-Claire Laulhé . Dénombrer les salariés et les chômeurs ne suffît pas pour décrire le marché du travail et comprendre son fonctionnement. La connaissance de la rotation de la main-d'œuvre dans les établissements et de ses causes serait très précieuse. Un pas est fait dans cette direction grâce à une exploitation approfondie, depuis 1983 et dans dix régions, des Déclarations de mouvements de main-d'œuvre des établissements de 50 salariés et plus. . Pour 1 000 travailleurs présents en début d'année, on compte en 1984, 194 entrées, dont 55 embauches fermes. En 1983, ce dernier taux était de 73, alors que la réduction du nombre des emplois était du même ordre qu'en 1984; mais l'écart est dû en grande partie aux contrats de solidarité. Les deux années, on a recensé moins de sorties que d'entrées liées à des contrats à durée déterminée : certains de ces contrats sont transformés en embauches fermes. [eng] 1983-1984 : changes in the nature of hiring . Daniel Depardieu and Marie-Claire Laulhé . Calculating the number of wage-earners and unemployed is not enough to describe the job market or to understand the way it functions. Knowledge of labor turnover in firms and its causes would be very precious. One step has been taken in this direction thanks to an in-depth examination of the Declarations of labor changes in firms of 50 wage-earners or more, since 1983 and in ten regions. . For 1 000 workers present at the beginning of the year, in 1984 194 entries into activity could be counted, of which 55 were permanent. In 1983, that rate was 73, while the reduction in the number of jobs was on the same order as in 1984; but the difference is due in great measure to early retirement. In both years, fewer departures than entries linked to contracts for a limited length of time were counted. Some of these contracts were transformed into permanent jobs. [spa] 1983-1984 : Cambios acaecidos en la naturaleza de las contratas . Daniel Depardieu / Marie-Claire Laulhé . Hacer el recuento de los asalariados y de los desocupados no basta para describir el mercado laboral y para entender su funcionamiento. El conocer la rotaciôn de la fuerza laboral en las empresas y sus causas serf a sumamente provechoso. Algo se adelantô en este sentido gracias a una explotaciôn profundizada, desde 1983 y en diez regiones, de las Declaraciones de movimientos de fuerza laboral en las empresas de 50 y mâs asalariados. Por 1 000 trabajadores présentes a primeras de anos, e recontaba en 1984, 194 entradas, de las cuales 55 contratas fijas. En 1983, esta ultima tasa era de 73, mientras que la aminoraciôn del numéro de empleos era parecida a la de 1984 : mas este descarte es imputable mayormente a los contratos de solidaridad. Durante ambos anos se recontô menos bajas que contratas vinculadas con contratos de deter- minada duradôn : algunos de dichos contratos remataron en contratas fijas.

Local structure in BaTi_1−<i>x</i>Zr_<i>x</i>O₃ relaxors from neutron pair distribution function analysis

International audienceThe pair distribution functions (PDF) of BaTi1−xZrxO3 (BTZ) relaxors (x=0.25,0.32,0.35), as well as those of the end members BaTiO3 and BaZrO3, were determined at 300 K from neutron powder scattering data. In the relaxors, the PDF provides direct evidence that the Ti and Zr atoms do not occupy the equivalent octahedral sites expected from the crystallographic cubic perovskite structure. It is shown that the TiO6 and ZrO6 octahedra in BTZ relaxors are instead similar to those observed in BaTiO3 and BaZrO3, respectively. In BaZrO3, the Zr atoms lie at the center of regular oxygen octahedra, forming nonpolar ZrO6 units. In the tetragonal ferroelectric phase of BaTiO3, the distribution of Ti-O distances within TiO6 octahedra is found compatible with a displacement of the Ti atoms in the [111]p direction of the pseudocubic perovskite cell. We conclude that the local polarization in BTZ relaxors is mainly due to the displacements of the Ti atoms and that moreover the Ti displacements are very similar in BTZ relaxors and in the classical ferroelectric BaTiO3

The TimIQ Synchronization for Sub-Picoseconds Delay Adjustment

International audienceSynchrotron facilities provides short, regular and high frequency flashes of light. These pulses are used by the scientific community for time resolved experiments. To improve the time resolution, demands for always shorter X-ray pulses are growing. To achieve this goal, Synchrotron SOLEIL and MAX IV laboratory have developed special operating modes such as low-alpha and femtoslicing, as well as a single pass linear accelerator. For the most demanding experiments, the synchronization between short light pulses and pump-probe devices requires sub-picoseconds delay adjustment. The TimIQ system has been developed for that purpose. It is a joint development between Synchrotron Soleil and MAX IV Laboratory. It is aimed to be used on three beamlines at Soleil and one at MAX IV. Based on IQ modulation technics, it allows shifting a radio frequency clock by steps of #100 fs. This paper is a description of this system and of its performances

Photoinduced charge density wave phase in 1T-TaS2: growth and coarsening mechanisms

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.ISSN:1631-0705ISSN:1878-153

Random local strain effects in the relaxor ferroelectric BaTi_1−<i>x</i>Zr_<i>x</i>O₃: experimental and theoretical investigation

We report an investigation of the local structure in homovalent-substituted BaTi1−x Zr x O3 relaxors by a combination of experimental and theoretical methods, namely neutron total scattering, X-ray absorption spectroscopy, and supercell ab-initio calculations. It is shown that unlike Zr atoms, Ti atoms are largely displaced in their octahedra, and are thus associated with strong local dipole moments. Besides, we give evidence that the difference in the size of Ti4+ and Zr4+ cations leads to a significant size mismatch of the Ti-O6 and Zr-O6 octahedra. When they link to form the perovskite structure of BaTi1−x Zr x O3, the O6 octahedra undergo slight distortions in order to accommodate their different sizes. It is shown that they are compressed in the direction of Zr neighbors, and expanded in the direction of Ti neighbors. The polar Ti displacements, which are sensitive to the octahedral distortions, then become constrained in their orientation according to the local Zr/Ti distribution. Such constraints impede a perfect alignment of all the Ti displacements as existing in the classic ferroelectric BaTiO3. Our results shed light on the structural mechanisms that lead to disordered Ti displacements in BaTi1−x Zr x O3 relaxors, and probably in other BaTiO3-based relaxors with homovalent substitution

Dynamics of Laser-Induced Magnetostructural Phase Transitions in MnAs/GaAs (001) Epitaxial Layers

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Temporal and spatial tracking of ultrafast light-induced strain and polarization modulation in a ferroelectric thin film

International audienceUltrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO 3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO 3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders