29 research outputs found
Charge redistribution at Pd surfaces: ab initio grounds for tight-binding interatomic potentials
A simplified tight-binding description of the electronic structure is often
necessary for complex studies of surfaces of transition metal compounds. This
requires a self-consistent parametrization of the charge redistribution, which
is not obvious for late transition series elements (such as Pd, Cu, Au), for
which not only d but also s-p electrons have to be taken into account. We show
here, with the help of an ab initio FP-LMTO approach, that for these elements
the electronic charge is unchanged from bulk to the surface, not only per site
but also per orbital. This implies different level shifts for each orbital in
order to achieve this orbital neutrality rule. Our results invalidate any
neutrality rule which would allow charge redistribution between orbitals to
ensure a common rigid shift for all of them. Moreover, in the case of Pd, the
power law which governs the variation of band energy with respect to
coordination number, is found to differ significantly from the usual
tight-binding square root.Comment: 6 pages, 2 figures, Latex; Phys.Rev. B 56 (1997
Analysis of Clean Transition Metal Surfaces by Core Level Spectroscopy
The shifts in the binding energy of core electrons detected by high resolution X-ray photoelectron spectroscopy are a very sensitive probe of the chemical bonding of the excited atom. Since the surface atoms have their geometrical environment perturbed, their core levels are shifted from their bulk positions. A very large number of experiments have been performed on the 4f core level positions of tantalum and tungsten for various orientations of the surface plane. Systematic trends have been put forward and explained by theoretical models. Furthermore, the analysis of the angular variation of the core level line intensities gives structural information when compared with theoretical calculations. In the case of W(100) a single scattering theory is sufficient to reproduce experimental data. Finally we show that, in some particular cases, the core level lineshapes may differ strongly from a Doniach Sunjic model. The temperature dependence of their widths due to core hole-phonon coupling can be reproduced within the independent boson theory
Le cas des marmites modelées e type M4.1. Nouvelles données archéologiques et archéometriques
Póster presentado en XI Congrès AIECM3 sur la céramique médiévale et moderne en Méditerranée, Antalya, TurquÃa, 19-23 octubre 2015Parmi les mobiliers présents en Méditerranée occidentale durant le haut Moyen Âge, la marmite modelée, à base plane et préhensions en languettes, correspondant au type M4.1 a été mise en évidence par S. Gutiérrez Lloret dans le sud-est de la péninsule Ibérique. Elle semble constituer un marqueur chronologique caractéristique des contextes archéologiques des VIIIe, IXe et Xe s. (Gutiérrez Lloret 1996, 2015). Ce type d’ustensile culinaire, assez générique, est présent sur les deux rives de la Méditerranée occidentale. En Espagne, ces marmites présentes dans de nombreux contextes précoces de la période islamique semble toutefois procéder de formes M.1 et M.2, issues de la tradition tardo antique et wisigothique. Dans le nord de l’Afrique, ce type s’inscrit vraisemblablement dans le prolongement des modèles de la Calcitic Ware de l’Antiquité tardive
Electron-correlation effects in appearance-potential spectra of Ni
Spin-resolved and temperature-dependent appearance-potential spectra of
ferromagnetic Nickel are measured and analyzed theoretically. The Lander
self-convolution model which relates the line shape to the unoccupied part of
the local density of states turns out to be insufficient. Electron correlations
and orbitally resolved transition-matrix elements are shown to be essential for
a quantitative agreement between experiment and theory.Comment: LaTeX, 6 pages, 2 eps figures included, Phys. Rev. B (in press
On the interpretation of spin-polarized electron energy loss spectra
We study the origin of the structure in the spin-polarized electron energy
loss spectroscopy (SPEELS) spectra of ferromagnetic crystals. Our study is
based on a 3d tight-binding Fe model, with constant onsite Coulomb repulsion U
between electrons of opposite spin. We find it is not the total density of
Stoner states as a function of energy loss which determines the response of the
system in the Stoner region, as usually thought, but the densities of Stoner
states for only a few interband transitions. Which transitions are important
depends ultimately on how strongly umklapp processes couple the corresponding
bands. This allows us to show, in particular, that the Stoner peak in SPEELS
spectra does not necessarily indicate the value of the exchange splitting
energy. Thus, the common assumption that this peak allows us to estimate the
magnetic moment through its correlation with exchange splitting should be
reconsidered, both in bulk and surface studies. Furthermore, we are able to
show that the above mechanism is one of the main causes for the typical
broadness of experimental spectra. Finally, our model predicts that optical
spin waves should be excited in SPEELS experiments.Comment: 11 pages, 7 eps figures, REVTeX fil
Theory of Spin-Resolved Auger-Electron Spectroscopy from Ferromagnetic 3d-Transition Metals
CVV Auger electron spectra are calculated for a multi-band Hubbard model
including correlations among the valence electrons as well as correlations
between core and valence electrons. The interest is focused on the
ferromagnetic 3d-transition metals. The Auger line shape is calculated from a
three-particle Green function. A realistic one-particle input is taken from
tight-binding band-structure calculations. Within a diagrammatic approach we
can distinguish between the \textit{direct} correlations among those electrons
participating in the Auger process and the \textit{indirect} correlations in
the rest system. The indirect correlations are treated within second-order
perturbation theory for the self-energy. The direct correlations are treated
using the valence-valence ladder approximation and the first-order perturbation
theory with respect to valence-valence and core-valence interactions. The
theory is evaluated numerically for ferromagnetic Ni. We discuss the
spin-resolved quasi-particle band structure and the Auger spectra and
investigate the influence of the core hole.Comment: LaTeX, 12 pages, 8 eps figures included, Phys. Rev. B (in press
Combining Solid State Physics Concepts and X-Ray Absorption Spectroscopy to Understand DeNOx Catalysis
Considering the NO adsorption process, starting from theoretical considerations, evaluation of the variation of the properties of metallic clusters versus their size and composition leads to a prediction of the NO adsorption mode on these nanoentities. Then, based on experimental results obtained mostly through X-ray absorption spectroscopy, we have connected the adsorption mode of the molecule to the behaviour of the metallic cluster following the adsorption process. A simple hypothesis leads us to discuss the catalytic activity of nanometer scale metallic clusters following NO adsorption