Atomic surrounding of Co implanted in AlN at high energy

AlN bulk ceramic has been implanted with energetic Co ions. In order to accurately characterise the atomic surrounding of the implanted ions, X-ray absorption measurements were carried out at 80 K in the fluorescence mode at the Co K edge in the as-implanted and annealed states. Simulation of the EXAFS oscillations allowed us to identify a first stage where Co is inserted in the AlN matrix followed by a second stage where Co precipitates form.Fil: Traverse, Agnès. Lure, Universidad Paris-sud; FranciaFil: Delobbe, Anne. Lure, Universidad Paris-sud; FranciaFil: Zanghi, Didier. Lure, Universidad Paris-sud; FranciaFil: Rentería, Mario. Lure, Universidad Paris-sud; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Gailhanou, Marc. Lure, Universidad Paris-sud; Franci

Atomic surrounding of Co implanted in AlN at high energy

AlN bulk ceramic has been implanted with energetic Co ions. In order to accurately characterise the atomic surrounding of the implanted ions, X-ray absorption measurements were carried out at 80 K in the fluorescence mode at the Co K edge in the as-implanted and annealed states. Simulation of the EXAFS oscillations allowed us to identify a first stage where Co is inserted in the AlN matrix followed by a second stage where Co precipitates form.Facultad de Ciencias ExactasInstituto de Física La Plat

Structure of smectic defect cores: an X-ray study of 8CB liquid crystal ultra-thin films

We study the structure of very thin liquid crystal films frustrated by antagonistic anchorings in the smectic phase. In a cylindrical geometry, the structure is dominated by the defects for film thicknesses smaller than 150 nm and the detailed topology of the defects cores can be revealed by x-ray diffraction. They appear to be split in half tube-shaped Rotating Grain Boundaries (RGB). We determine the RGB spatial extension and evaluate its energy per unit line. Both are significantly larger than the ones usually proposed in the literatureComment: 4 page

Warping caused by surface elasticity in a nanowire under torsion

International audienceBy combining molecular statics simulations and continuum mechanics-based modeling, we show in this paper that the torsion of a 001 single-crystal copper nanowire with a circular cross section gives rise to a warp, i.e., to a displacement field along the wire axis that renders the cross section nonplanar. This behavior, which is in apparent contradiction with what is predicted by continuum mechanics for an isotropic cylinder, can be well explained if we take into account the elastic response of the wire lateral surface. The latter is characterized by the anisotropy of the surface elastic constants and, more specifically in the case of torsion, by the surface shear constant Cs44 whose strength as a function of the local orientation of the lateral surface is estimated independently from atomistic calculations on slabs presenting different vicinal surfaces. The solution of the torsion problem is then obtained by adopting a semi-inverse method in the framework of the finite strain theory in linear elasticity with Gurtin-Murdoch boundary conditions linking surface stress and bulk stress. It is shown that such an approach is well suited to explain quantitatively the warp obtained in our atomistic simulations and to prove the preponderant role played by the surface elastic constant Cs44

Comment on " Eshelby twist and correlation effects in diffraction from nanocrystals "

The aim of this comment is to show that anisotropic effects and image fields should not be omitted as they are in the publication of A. Leonardi, S. Ryu, N. M. Pugno, and P. Scardi (LRPS) [J. Appl. Phys. 117, 164304 (2015)] on Pd cylindrical nanowires containing an axial screw dislocation. Indeed, according to our previous study [Phys. Rev. B 88, 224101 (2013)], the axial displacement field along the nanowire exhibits both a radial and an azimuthal dependence with a twofold symmetry due the orientation. As a consequence, the deviatoric strain term used by LRPS is not suitable to analyze the anisotropic strain fields that should be observed in their atomistic simulations. In this comment, we first illustrate the importance of anisotropy in Pd nanowire by calculating the azimuthal dependence of the deviatoric strain term. Then the expression of the anisotropic elastic field is recalled in term of strain tensor components to show that image fields should be also considered. The other aspect of this comment concerns the supposedly loss of correlation along the nanorod caused by the twist. It is claimed for instance by LRPS that : "As an effect of the dislocation strain and twist, if the cylinder is long enough, upper/lower regions tend to lose correlation, as if the rod were made of different sub-domains.". This assertion appears to us misleading since for any twist the position of all the atoms in the nanorod is perfectly defined and therefore prevents any loss of correlation. To clarify this point, it should be specified that this apparent loss of correlation can not be ascribed to the twisted state of the nanowire but is rather due to a limitation of the X-ray powder diffraction. Considering for instance coherent X-ray diffraction, we show an example of high twist where the simulated diffractogram presents a clear signature of the perfect correlation

Understanding the warp in free 〈 111 〉 metal nanowires by modeling surface elasticity

International audienceIn this Letter, we combine atomistic simulations and a continuum model, based on the Gurtin-Murdoch theory of surface elasticity, to explain the warp previously observed in 111 gold nanowires. This phenomenon, which is characterized by the spontaneous formation of nonplanar cross sections, is primarily due to the crystallographic orientation of the surfaces that can form around a 111 wire. As a result, the surface stress tensor expressed in the wire cylindrical coordinates basis presents a nondiagonal shear component denoted S S Z () whose anisotropy (its variation with azimuth) induces bulk stress and warp in the wire. This result is demonstrated by computing S S Z as a function of and by integrating these atomistic inputs into the continuum model. By adopting this approach, the warp found in the atomistic simulations is well reproduced analytically for both Au and Cu nanowires having a circular cross section. This behavior is a fine example of the coupling between surface and bulk stress tensors involving nondiagonal components. An extension of this modeling to the case of nanowires with a hexagonal cross section is discussed