48 research outputs found
Excitations électroniques et magnétisme des matériaux : calcul ab initio de l'absorption X et du dichroïsme circulaire magnétique au seuil K
Experimental measurements of X-ray absorption (XAS) and magnetic circular dichroism(XMCD) are widely used to study electronic excitations and magnetism in materials.Theoretical interpretations of those measurements play a crucial role in the understandingof those studies. We have developped new methods to calculate X-ray absorptionand XMCD at the K edge based on the use of the density functionnal theory (DFT), ofpseudopotentials and of the continued fraction approach. Those methods allow a parameterfree treatment of the core hole.We applied those methods on a wide range of materials, from weak to strong electroniccorrelation, in order to see if a monoparticle approximation could describe XAS andXMCD spectra.We rst sudied the X-ray absorption of graphene, a weakly correlated material,in order to determine if the interlayer state responsible for the supraconductivity ofintercalated graphites could be seen in XAS, as suggested by recent experiments. We thencalculated the strongly correlated systems NiO, CoO, CuO, La2CuO4 and Ca2−xCuO2Cl2.We showed the presence of non-local excitations in the dipolar part of the absorption spectrain those systems, and proposed an independant method to measure the charge-transfergap based on the XAS of a hole-doped system. We showed the importance of many-bodyeects in the edge region and attributed them to multi-determinant eects with the helpof a concolution with photoemission formula. Last, we successfully calculated the XMCDspectra of iron and cobalt under pressure, and discussed of the relative importance ofdipole and quadrupole transitions, of anisotropy and of the accuracy of sum rules.Les mesures d'absorption X et de dichroïsme circulaire magnétique sont largementutilisées pour étudier les excitations électroniques et le magnétisme des matériaux. Cesétudes dépendent de manière cruciale de l'interprétation théorique des mesures expérimentales.Nous avons développé de nouvelles méthodes de calcul de l'absorption X (XAS)au seuil K et du dichroïsme magnétique circulaire (XMCD) au seuil K basées sur l'utilisationde la théorie de la fonctionnelle de densité (DFT), des pseudopotentiels et de lafraction continue. Ces méthodes permettent le traitement des eets du trou de coeur dansune approche sans paramètre libre.Nous avons appliqué ces méthodes à toute une gamme de matériaux, en allant de lafaible corrélation vers la forte corrélation électronique, an de voir si une approximationmonoparticule pouvait décrire les spectres XAS et XMCD. Nous avons d'abord étudiéles spectres XAS du graphène, matériau faiblement corrélé, an de déterminer si l'état interlayer , responsable de la supraconductivité dans les graphites intercalés, pouvaitêtre vu en XAS comme suggéré par des mesures récentes. Nous avons ensuite abordé lessystèmes à forte corrélation que sont NiO, CoO, CuO, La2CuO4 et Ca2−xCuO2Cl2. Nousavons montré la présence d'excitations non locales dans la partie dipolaire des spectresXAS dans ces systèmes, ce qui nous a permis de proposer une nouvelle méthode indépendantede mesure de la largeur de la bande interdite basée sur le XAS dans un système dopéavec des trous. Nous avons montré l'importance des effets à N corps dans la région duseuil de ces spectres et avons attribué cela à des effets à plusieurs déterminants de Slaterà l'aide d'une formule de convolution par la photoémission. Enfin, le XMCD du fer et ducobalt sous pression sont calculés avec succès, ce qui permet de discuter de l'importancerelative des transitions dipolaires et quadrupolaires, de l'anisotropie et de la pertinencede l'utilisation des règles de somme
Hole-depletion of ladders in SrCuO induced by correlation effects
The hole distribution in SrCuO is studied by low
temperature polarization dependent O K Near-Edge X-ray Absorption Fine
Structure measurements and state of the art electronic structure calculations
that include core-hole and correlation effects in a mean-field approach.
Contrary to all previous analysis, based on semi-empirical models, we show that
correlations and antiferromagnetic ordering favor the strong chain
hole-attraction. For the remaining small number of holes accommodated on
ladders, leg-sites are preferred to rung-sites. The small hole affinity of
rung-sites explains naturally the 1D - 2D cross-over in the phase diagram of
(La,Y,Sr,Ca)CuOComment: 6 pages, 8 figure
K-edge X-ray absorption spectra in transition metal oxides beyond the single particle approximation: shake-up many body effects
The near edge structure (XANES) in K-edge X-ray absorption spectroscopy (XAS)
is a widely used tool for studying electronic and local structure in materials.
The precise interpretation of these spectra with the help of calculations is
hence of prime importance, especially for the study of correlated materials
which have a complicated electronic structure per se. The single particle
approach, for example, has generally limited itself to the dominant dipolar
cross-section. It has long been known however that effects beyond this approach
should be taken into account, both due to the inadequacy of such calculations
when compared to experiment and the presence of shake-up many-body satellites
in core-level photoemission spectra of correlated materials. This effect should
manifest itself in XANES spectra and the question is firstly how to account for
it theoretically and secondly how to verify it experimentally. By using
state-of-the-art first principles electronic structure calculations and 1s
photoemission measurements we demonstrate that shake-up many-body effects are
present in K-edge XAS dipolar spectra of NiO, CoO and CuO at all energy scales.
We show that shake-up effects can be included in K-edge XAS spectra in a simple
way by convoluting the single-particle first-principles calculations including
core-hole effects with the 1s photoemission spectra. We thus describe all
features appearing in the XAS dipolar cross-section of NiO and CoO and obtain a
dramatic improvement with respect to the single-particle calculation in CuO.
These materials being prototype correlated magnetic oxides, our work points to
the presence of shake-up effects in K-edge XANES of most correlated transition
metal compounds and shows how to account for them, paving the way to a precise
understanding of their electronic structure.Comment: 6 pages, 4 picture
Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
Quantum ESPRESSO is an integrated suite of computer codes for
electronic-structure calculations and materials modeling, based on
density-functional theory, plane waves, and pseudopotentials (norm-conserving,
ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn
Source Package for Research in Electronic Structure, Simulation, and
Optimization". It is freely available to researchers around the world under the
terms of the GNU General Public License. Quantum ESPRESSO builds upon
newly-restructured electronic-structure codes that have been developed and
tested by some of the original authors of novel electronic-structure algorithms
and applied in the last twenty years by some of the leading materials modeling
groups worldwide. Innovation and efficiency are still its main focus, with
special attention paid to massively-parallel architectures, and a great effort
being devoted to user friendliness. Quantum ESPRESSO is evolving towards a
distribution of independent and inter-operable codes in the spirit of an
open-source project, where researchers active in the field of
electronic-structure calculations are encouraged to participate in the project
by contributing their own codes or by implementing their own ideas into
existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
Orientation and stability of a bi-functional aromatic organic molecular adsorbate on silicon
In this work we combine scanning tunneling microscopy, near-edge X-ray absorption fine structure spectroscopy, X-ray photoemission spectroscopy and density functional theory to resolve a long-standing confusion regarding the adsorption behaviour of benzonitrile on Si(001) at room temperature. We find that a trough-bridging structure is sufficient to explain adsorption at low coverages. At higher coverages when steric hindrance prevents the phenyl ring lying flat on the surface, the 2+2 cycloaddition structure dominates
The electronic structure of iridium oxide electrodes active in water splitting
Iridium oxide based electrodes are among the most promising candidates for electrocatalyzing the oxygen evolution reaction, making it imperative to understand their chemical/electronic structure. However, the complexity of iridium oxide's electronic structure makes it particularly difficult to experimentally determine the chemical state of the active surface species. To achieve an accurate understanding of the electronic structure of iridium oxide surfaces, we have combined synchrotron-based X-ray photoemission and absorption spectroscopies with ab initio calculations. Our investigation reveals a pre-edge feature in the O K-edge of highly catalytically active X-ray amorphous iridium oxides that we have identified as O 2p hole states forming in conjunction with IrIII. These electronic defects in the near-surface region of the anionic and cationic framework are likely critical for the enhanced activity of amorphous iridium oxides relative to their crystalline counterparts
Excitations électroniques et magnétisme des matériaux (clacul ab initio de l'absorption X et du dichroïsme circulaire magnétique au seuil K)
A l'aide de nouvelles méthodes de calcul de l'absorption X (XAS) et du dichroïsme circulaire magnétique (XMCD) au seuil K ne comportant aucun paramètre libre développées durant cette thèse, nous avons d'abord étudié les spectres XAS du graphène, afin de déterminer si l'état interlayer pouvait être vu en XAS comme suggéré par des mesures récentes. Nous avons ensuite abordé les systèmes à forte corrélation que sont NiO, CoO, CuO, La2CuO4 et Ca2-xCuO2Cl2. Nous avons montré la présence d'excitations non locales dans leurs spectres XAS, ce qui nous a permis de proposer une nouvelle méthode de mesure du gap. Nous avons montré l'importance des effets à plusieurs déterminants de Slater à l'aide d'une formule de convolution par la photoémission. Enfin, le XMCD du fer et du cobalt sous pression sont calculés avec succès, ce qui permet de discuter de l'importance relative des transition dipolaires et quadrupolaires, de l'anisotropie et de la pertinence de l'utilisation des règles de somme.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF
First-principles calculations of x-ray absorption in a scheme based on ultrasoft pseudopotentials: From !-quartz to high-Tc compounds
International audienceWe develop a first-principles scheme based on the continued-fraction approach and ultrasoft pseudopotentials to calculate K-edge x-ray absorption spectra in solids, allowing such calculations in transition-metal and rare-earth compounds with substantially reduced cutoffs with respect to the norm-conserving case. We validate the method by calculating Si and O K edges in !-quartz, Cu K edge in copper and in La2CuO4. For the case of Si and O edges in !-quartz and in copper, we obtain good agreement with experimental data. In the Cu K-edge spectra of La2CuO4, a material considered a real challenge for density-functional theory, we attribute all the near-edge and far-edge peaks to single-particle excitations