High-pressure study of X-ray diffuse scattering in ferroelectric perovskites

We present a high-pressure x-ray diffuse scattering study of the ABO$_3$ ferroelectric perovskites BaTiO_3 and KNbO_3. The well-known diffuse lines are observed in all the phases studied. In KNbO_3, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO_3 linearly decreases to zero at $\sim 11$ GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed.Comment: 4 pages, Accepted in PR

Imaging shape and strain in nanoscale engineered semiconductors for photonics by coherent x-ray diffraction

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

Charge order and suppression of superconductivity in HgBa2CuO4 at high pressures

New insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of the electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10GPa that gradually passes to an insulating state, typical of charge ordering, that totally suppresses superconductivity above 13GPa. 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 appearing at 10GPa of wavevector [0.25, 0, L]. As pressure induces a charge transfer of about 0.02 at 10GPa, our results are the first observation of charge order competing with superconductivity that develops in the over-doped region of the phase diagram of a cuprate.Comment: 9 pages, 3 figure

Erratum: A microscopic view on the Mott transition in chromium-doped V 2 O 3

Nature Communications 1, Article number: 105 (2010); published: 02 November 2010; updated: 17 January 2012. In Figure 2 of this Article, panel labels c and d were inadvertently switched. A typographical error was also introduced in the last sentence of the legend, which should have read 'The scale bar in panel c represents 10 μm'

Contribution à l'étude des systèmes de basse dimension par diffusion des rayons X

Jury : J. Klein Rapporteur P. Monceau Rapporteur Ch. Vettier Rapporteur S. Brazovskii J.-P. PougetThis thesis deals with x-ray diffraction methods applied to the study of structural instabilities, local ordering and phase treansitions in low-dimensionnal materials and molecular compounds.Ce mémoire d'habilitation traite de méthodes de diffraction des rayons X appliquées à l'étude des instabilités structurales, les ordres locaux et les transitions de phases structurales dans des matériaux de basse dimensionalité et les composés moléculaires

La diffraction cohérente des rayons X

Grâce à la brillance des sources synchrotron de troisième génération, on obtient des faisceaux de rayons X cohérents d’intensité supérieure à 109 photons par seconde. Ces faisceaux permettent des expériences de diffraction qui révèlent les franges d’interférence ou les tavelures, non visibles dans des expériences classiques. Cet article donne quelques exemples d’études en diffraction cohérente : reconstruction de déformations dans des nanocristaux, mise en évidence de défauts topologiques, et ptychographie. Des expériences de diffraction cohérente sont notamment réalisées sur la ligne CRISTAL à SOLEIL, ainsi qu’à l’ESRF

Etude des instabilites structurales dans les conducteurs organiques

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc