The isotropic Compton profile difference across the phase transition of VO2_2

We studied the isotropic Compton profile of the prototypical oxide VO$_2$ across the temperature induced electronic and structural phase transition at T$_C$ $\approx$ 340 K. We show that the phase transition leaves an observable signal, which facilitates Compton scattering studies of electronic structure and phase transitions in complex solids in powder form. We compare the experimental results with density functional theory calculations and find agreement in the shape of the difference profile, although the amplitude of the observed features is overestimated. The origin of the disagreement is discussed and we argue that it mainly originates mostly correlation effects beyond our current calculations and possibly to some extent, from thermal motion

Intramolecular structure and energetics in supercooled water down to 255 K

We studied the structure and energetics of supercooled water by means of X-ray Raman and Compton scattering. Under supercooled conditions down to 255 K, the oxygen K-edge measured by X-ray Raman scattering suggests an increase of tetrahedral order similar to the conventional temperature effect observed in non-supercooled water. Compton profile differences indicate contributions beyond the theoretically predicted temperature effect and provide a deeper insight into local structural changes. These contributions suggest a decrease of the electron mean kinetic energy by 3.3 +/- 0.7 kJ (mol K)(-1) that cannot be modeled within established water models. Our surprising results emphasize the need for water models that capture in detail the intramolecular structural changes and quantum effects to explain this complex liquid.Peer reviewe

Study of Mechanical Behaviour of Polycrystalline Materials at the Mesoscale Using High Energy X-Ray Diffraction

International audienceOwing to its selectivity, diffraction is a powerful tool for analysing the mechanical behaviour of polycrystalline materials at the mesoscale, i.e. phase and grain scale. In situ synchrotron diffraction (transmission mode) during tensile tests and modified self-consistent elastoplastic model were used to study elastic and plastic phenomena occurring in polycrystalline specimens during deformation. The evolution of stress for grains which belong to different phases of duplex stainless steel and pearlitic steel was analyzed

Elastoplastic deformation and damage process in duplex stainless steels studied using synchrotron and neutron diffractions in comparison with a self-consistent model

In situ time of flight neutron diffraction and X-ray synchrotron diffraction methods were applied to measure lattice strains in duplex steels during a tensile test. The experimental results were used to study slips on crystallographic planes and the mechanical effects of damage occurring during plastic deformation. For this purpose the prediction of an elastoplastic self-consistent model was compared with the experimental data. The used methodology allowed to determine the elastic limits and parameters describing work hardening in both phases of studied polycrystalline materials. In the second part of this work the developed elastoplastic model was applied to study damage occurring in the ferritic phase. The theoretical results showed a significant reduction of stresses localized in the damaged phase (ferrite) and confirmed the evolution of the lattice strains measured in the ferritic and austenitic phases

Hole depletion and localization due to disorder in insulating PrBa2Cu3O7-d: a Compton scattering study

The (mostly) insulating behaviour of PrBa2Cu3O7-d is still unexplained and even more interesting since the occasional appearance of superconductivity in this material. Since YBa2Cu3O7-d is nominally iso-structural and always superconducting, we have measured the electron momentum density in these materials. We find that they differ in a striking way, the wavefunction coherence length in PrBa2Cu3O7-d being strongly suppressed. We conclude that Pr on Ba-site substitution disorder is responsible for the metal-insulator transition. Preliminary efforts at growth with a method to prevent disorder yield 90K superconducting PrBa2Cu3O7-d crystallites.Comment: 4 pages, 3 figures, revised version submitted to PR

On the electronic structure and superconducting gap of Bi2Sr2CaCu2O8Bi_2Sr_2CaCu_2O_8

Photoemission spectra taken at room temperature with high energy and angle resolution on $Bi_2Sr_2CaCu_2O_8$ single crystals reveal a band near $E_F$ with weak dispersion along λX. It crosses $E_F$ at about 1/2 λX and the occupied part becomes visible up to the Brillouin zone boundary in angle-resolved inverse photoemission spectra. Starting for T>T$_c$ with a clear Fermi edge, for T$<$Tc spectral weight is shifted from EF to higher binding energy related to the opening of the superconducting gap of Δ=(30±4) meV. These findings strongly favour Fermi liquid based theories for high-Tc superconductivity in this material

Combined texture and microstructure analysis of deformed crystals by high-energy X-ray diffraction

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