Effect of vibrations on the pre-edge features of x-ray absorption spectra

The influence of atomic vibrations on x-ray absorption near edge structure (XANES) is calculated by assuming that vibrational energies are small with respect to the instrumental resolution. The resulting expression shows that, at the K-edge, vibrations enable electric dipole transitions to 3s and 3d final states. The theory is applied to the K-edge of Al in Al2O3 corundum and of Ti in TiO2 rutile and compared with experiment. At the Al K-edge, sizeable transitions towards 3s final states are obtained, leading to a clear improvement of the agreement with experimental spectra. At the Ti K-edge, electric dipole transitions towards 3d final states explain the temperature dependence of the pre-edge features.Comment: 4 pages, 2 figure

Emergence of autopoietic vesicles able to grow, repair and reproduce in a minimalist particle system

This paper describes a 2D particle system in which autopoietic vesicles that are able to grow, repair, and reproduce can emerge from an initial random distribution of individual components. The reproduction process also exhibits inheritance of the vesicle content that could in principle change from one generation to the next and influence fitness, making those autopoietic vesicles eligible to be acted upon by natural selection. Video abstract: https://www.youtube.com/watch?v=JmCN_4jTwf

Photochemical synthesis of a “cage” compound in a microreactor: Rigorous comparison with a batch photoreactor

An intramolecular [2 + 2] photocycloaddition is performed in a microphotoreactor (0.81 mL) built by winding FEP tubing around a commercially available Pyrex immersion well in which a medium pressure mercury lamp is inserted. A rigorous comparison with a batch photoreactor (225 mL) is proposed by means of a simple model coupling the reaction kinetics with the mass, momentum and radiative transfer equations. This serves as a basis to explain why the chemical conversion and the irradiation time are respectively increased and reduced in the microphotoreactor relative to those in the batch photoreactor. Through this simple model reaction, some criteria for transposing photochemical synthesis from a batch photoreactor to a continuous microphotoreactor are defined

CONVERSION OF A LAMB PRODUCTION SYSTEM TO ORGANIC FARMING: HOW TO MANAGE, FOR WHICH RESULTS?

Two sheep flocks were managed organically (for two years from conversion onwards) under different lambing strategies (1 lambing/year vs. 3 lambings every two years). The second system was tested as there was a producer’s interest in high productivity that is a guarantee of good economic results in conventional. Reproduction, feeding, lamb production, carcass quality, health (particularly internal parasitism), economic return of the flock, grass production, and pasture biodiversity were evaluated. The lambs were bred under low therapeutic input. The economical advantage of increasing lambing frequency was not demonstrated, whereas this strategy complexified management and resulted in higher internal parasitic infection of the lambs, and finally showed a lower stability. There were difficulties in establishing a very high feed self-sufficiency in both systems, especially the more intensive system (4 points lower), due to harsh climatic conditions; following this five-year experiment, we are changing our strategy to provide nitrogen in the systems

Structural relaxation around substitutional Cr3+ in MgAl2O4

The structural environment of substitutional Cr3+ ion in MgAl2O4 spinel has been investigated by Cr K-edge Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopies. First-principles computations of the structural relaxation and of the XANES spectrum have been performed, with a good agreement to the experiment. The Cr-O distance is close to that in MgCr2O4, indicating a full relaxation of the first neighbors, and the second shell of Al atoms relaxes partially. These observations demonstrate that Vegard's law is not obeyed in the MgAl2O4-MgCr2O4 solid solution. Despite some angular site distortion, the local D3d symmetry of the B-site of the spinel structure is retained during the substitution of Cr for Al. Here, we show that the relaxation is accomodated by strain-induced bond buckling, with angular tilts of the Mg-centred tetrahedra around the Cr-centred octahedron. By contrast, there is no significant alteration of the angles between the edge-sharing octahedra, which build chains aligned along the three four-fold axes of the cubic structure.Comment: 7 pages, 4 figure

X-ray Linear Dichroism in cubic compounds: the case of Cr3+ in MgAl2O4

The angular dependence (x-ray linear dichroism) of the Cr K pre-edge in MgAl2O4:Cr3+ spinel is measured by means of x-ray absorption near edge structure spectroscopy (XANES) and compared to calculations based on density functional theory (DFT) and ligand field multiplet theory (LFM). We also present an efficient method, based on symmetry considerations, to compute the dichroism of the cubic crystal starting from the dichroism of a single substitutional site. DFT shows that the electric dipole transitions do not contribute to the features visible in the pre-edge and provides a clear vision of the assignment of the 1s-->3d transitions. However, DFT is unable to reproduce quantitatively the angular dependence of the pre-edge, which is, on the other side, well reproduced by LFM calculations. The most relevant factors determining the dichroism of Cr K pre-edge are identified as the site distortion and 3d-3d electronic repulsion. From this combined DFT, LFM approach is concluded that when the pre-edge features are more intense than 4 % of the edge jump, pure quadrupole transitions cannot explain alone the origin of the pre-edge. Finally, the shape of the dichroic signal is more sensitive than the isotropic spectrum to the trigonal distortion of the substitutional site. This suggests the possibility to obtain quantitative information on site distortion from the x-ray linear dichroism by performing angular dependent measurements on single crystals

CONVERSION OF A LAMB PRODUCTION SYSTEM TO ORGANIC FARMING: HOW TO MANAGE, FOR WHAT RESULTS?

Two sheep flocks were managed organically for two years from conversion under different lambing strategies (1 lambing/year vs. 3 lambings every two years). The second system was tested because pf a producer’s interest in high productivity, which is a guarantee of good economic results in conventional production.. Reproduction, feeding, lamb production, carcass quality, health (particularly internal parasitism), economic return of the flock, grass production, and pasture biodiversity were evaluated. The lambs were bred with low therapeutic inputs. No economic advantage of increasing lambing frequency was demonstrated, whereas this strategy complicated management and resulted in higher internal parasitic infection of the lambs, and finally showed lower stability. There were difficulties in establishing a very high feed self-sufficiency in both systems, especially the more intensive system (4 points lower), due to harsh climatic conditions. Following this five-year experiment, we are changing our strategy to provide nitrogen in the systems

Nonlinear Hysteretic Torsional Waves

We theoretically study and experimentally report the propagation of nonlinear hysteretic torsional pulses in a vertical granular chain made of cm-scale, self-hanged magnetic beads. As predicted by contact mechanics, the torsional coupling between two beads is found nonlinear hysteretic. This results in a nonlinear pulse distortion essentially different from the distortion predicted by classical nonlinearities, and in a complex dynamic response depending on the history of the wave particle angular velocity. Both are consistent with the predictions of purely hysteretic nonlinear elasticity and the Preisach-Mayergoyz hysteresis model, providing the opportunity to study the phenomenon of nonlinear dynamic hysteresis in the absence of other type of material nonlinearities. The proposed configuration reveals a plethora of interesting phenomena including giant amplitude-dependent attenuation, short term memory as well as dispersive properties. Thus, it could find interesting applications in nonlinear wave control devices such as strong amplitude-dependent filters

Reprise d’entreprise et compétences relationnelles du nouveau dirigeant (Entreprise revival and relational compentencies of the new manager)

A travers la reprise de PME le nouveau dirigeant va acquérir des biens matériels et physiques négociés et relativement quantifiés, toutefois en ce qui concerne le système relationnel interne et externe, il aura quasiment tout à découvrir et à redéfinir en termes de modes de fonctionnement. De plus le changement de dirigeant peut simultanément produire des niveaux multiples de déstabilisation qui auront tendance à se répercuter sur le fonctionnement et les résultats de l’entreprise. L’entrepreneur va être confronté à des mécanismes de résistance qu’il essaiera de gérer autant que possible et au mieux de les transformer en leviers d’actions La maîtrise de cette transition dépendra essentiellement de ses compétences relationnelles et influencera la réussite ou l’échec de la reprise. When taking over a SME, the new leader will get negotiated and relatively well quantified tangible and physical properties. However with regards to the internal and external relational systems, he will have almost everything to discover and redefine in terms of operating modes. Moreover the change of leader may simultaneously produce multiple levels of destabilization which will tend to modify the operation and the results of the company. The new leader will be facing « mechanisms of resistance » which he will try to manage as well as possible and possibly transform them into levers of actions. The control of this transition period will depend primarily on his relational abilities and will influence the success or failure of the taking over.enterprise revival, relational competence,manager, SME

Structural relaxations around Ti, Cr and Fe impurities in alpha-Al2O3 probed by x-ray absorption near edge structure combined with first-principles calculations

We determine the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3), by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations. The structural relaxations are found to be very local. We then show that XANES is sensitive to small variations in interatomic distances within the coordination shell of the absorbing atom. The experiments were carried out on single crystals of ruby and sapphires. Linear dichroic signals are essential to characterize the geometry of the impurity site. The calculations were performed within a self-consistent ``non muffin-tin'' framework, that uses pseudopotentials, plane-wave basis set, and the continued fraction for the absorption cross section