Taming the resistive switching in Fe/MgO/V/Fe magnetic tunnel junctions: An ab initio study

A possible mechanism for the resistive switching observed experimentally in Fe/MgO/V/Fe junctions is presented. Ab initio total energy calculations within the local density approximation and pseudopotential theory shows that by moving the oxygen ions across the MgO/V interface one obtains a metastable state. It is argued that this state can be reached by applying an electric field across the interface. In addition, the ground state and the metastable state show different electric conductances. The latter results are discussed in terms of the changes of the density of states at the Fermi level and the charge transfer at the interface due to the oxygen ion motion

Excitonic and Quasiparticle Life Time Effects on Silicon Electron Energy Loss Spectrum from First Principles

The quasiparticle decays due to electron-electron interaction in silicon are studied by means of first-principles all-electron GW approximation. The spectral function as well as the dominant relaxation mechanisms giving rise to the finite life time of quasiparticles are analyzed. It is then shown that these life times and quasiparticle energies can be used to compute the complex dielectric function including many-body effects without resorting to empirical broadening to mimic the decay of excited states. This method is applied for the computation of the electron energy loss spectrum of silicon. The location and line shape of the plasmon peak are discussed in detail.Comment: 4 pages, 3 figures, submitted to PR

Calculated iron L2,3L_{2,3} x-ray absorption and XMCD of spin-crossover Fe(phen)2_{2}(NCS)2_{2} molecule adsorbed on Cu(001) surface

The PAW method has been used to compute the iron L$_{2,3}$ edges of x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) of the spin-crossover Fe(phen)$_{2}$(NCS)$_{2}$ molecule when adsorbed on Cu(001) surface and in the gas phase, for both the high spin (HS) and low spin (LS) states. It is found that the calculated XAS and XMCD with the static core hole or the Slater transition state half hole are in less good agreement with experiment than those using the so called initial state. This disagreement is due to the reduction of the iron spin magnetic moment caused by the static screening of the core hole by the photo-electron. The L$_{2,3}$ XAS formula is found to be directly related to the unoccupied $3d$ density of states (DOS), and hence the symmetry broken $e_g$ and the $t_{2g}$ iron DOS are used to explain the XAS and XMCD results. It is demonstrated that the dependence of the HS XMCD on the direction of incident x-ray circularly polarized light with respect to the magnetization direction can be used to determine the iron octahedron deformation, while the XMCD for various magnetization directions is directly related to the anisotropy of the orbital magnetic moment and the magneto-crystalline energy. It is also shown that the magnetic dipole moment $T_z$ is very large due to the strong distortion of the iron octahedron and is necessary for an accurate determination of the sum rule computed spin magnetic moment.Comment: 39 pages, 5 figure

Full-Potential LMTO: Total Energy and Force Calculations

The essential features of a full potential electronic structure method using Linear Muffin-Tin Orbitals (LMTOs) are presented. The electron density and potential in the this method are represented with no inherent geometrical approximation. This method allows the calculation of total energies and forces with arbitrary accuracy while sacrificing much of the efficiency and physical content of approximate methods such as the LMTO-ASA method.Comment: 25 pages, 2 figures, Workshop on the TB-LMTO method, Monastery of Mont St. Odile, October 4-5, 199

Viabilité et vieillissement des semences d’arganier (Arganiaspinosa (L.) Skeels

La viabilité des semences est un facteur limitant de la production des plants d’arganier (Arganiaspinosa (L.) Skeels) en pépinière. Dans cette étude, on a comparé la viabilité des semences de trois âges (semences dedeux ans, d’un an et de l’année) conservées à la température ambiante. Des semences de six génotypes de pieds-mères différents ont été traitées au froid (4 °C) pendant un mois, scarifiées et traitées à l’acide  gibbérellique (GA3) pendant 24 heures avant leur mise en germination en présence de la lumière. La viabilité des semences a significativement diminué avec le vieillissement des semences. Ainsi, le pourcentage de germination est de 88,6 % pour les semences de l’année, de 74,7 % pour celles âgées d’un an et de 51,1 % pour celles âgées de deux ans. Le pourcentage de germination est corrélé hautement significativement avec le pourcentage de semences contaminées par les Fusarium (– 0,93) et par les Aspergillus (– 0,72). Les pourcentages de germination et de  contamination par les différents champignons sont très variables selon les génotypes des pieds-mères. Les semences de l’année conservées au froid dès leur récolte assurent une meilleure production de plants variant entre 75 % et 98,4 % selon les piedsmères.Mots-clés : âge des semences, Arganiaspinosa, Aspergillus,  contamination, Fusarium, génotypes des piedsmères, germination, Penicillium, viabilité