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IRAMIS

Théorie IRAMIS
Réunion thématique IRAMIS

Structure électronique et Modélisation atomistique

Jeudi 3 Avril CEA Saclay


Journée Théorie IRAMIS 2008 : programme


Jeudi 3 avril

08:30 Accueil des participants / Installation posters
08:50
Ouverture de la journée : Pascal Boulanger (IRAMIS/Dir)

Structure et dynamique
(Modérateur : Marie-Catherine Desjonquères)

  • 09:00-09:30 Deformation in Silica via Molecular Dynamics Simulations. Cindy Rountree (SPCSI).
  • 09:30-10:00 Physique des matériaux : des défauts ponctuels aux interfaces.. Marc Hayoun (LSI).
  • 10:00-10:30 Dynamique de molécules d'alcalins immergées dans des agrégats et matrice de gaz rare. Benoit Gervais (CIMAP).


  • 10:30-11:00 Pause café et session posters.

    Chimie quantique (Modérateur : ?)
  • 11:00-11:30 Chimie théorique de composés de lanthanides et actinides . Jean-Pierre Dognon (SCM).
  • 11:30-12:00 Couplage théorie-expérience en dynamique réactionnelle: une nécessité pour l'expérimentateur. Jean-Michel Mestdagh (SPAM).
  • 12:00-12:30 Réactivité chimique et spectroscopie vibrationnelle/électronique: de la molécule isolée à la matière condensée. Rodolphe Pollet (SPAM) (Intranet seul).
  • ----------------------------------------------------------
    12:30-14:30 Déjeuner + POSTERS
    ----------------------------------------------------------

    Spectroscopie (Modérateur : Sylvain Latil)

  • 14:30-15:00 RMN premiers principes: un nouvel outil pour sonder la structure des verres. Thibault Charpentier (SCM) (Intranet seul).
  • 15:00-15:45 Spectroscopie théorique: décrire et comprendre les excitations électroniques. Eleonora Luppi et Matteo Gatti (LSI-ETSF).
  • 15:45-16:30 Pause café et discussion posters

    Transport, magnétisme et corrélations (Modérateur : Lucia Reining)

  • 16:30-17:00 Existe-t-il des métaux à deux dimensions ? Geneviève Fleury (SPEC).
  • 17:00-17:30 Transport, structure et magnétisme électronique dans les nano-objets. Cyrille Barreteau (SPCSI)
  • 17:30 Clôture de la journée. Guillaume Petite (IRAMIS).

  • Abstracts :

    Spectroscopie théorique: décrire et comprendre les excitations électroniques.
    Eleonora Luppi et Matteo Gatti (LSI-ETSF).
    The ability to devise new functionalities in techologically relevant systems (e.g. photovoltaic solar cells, materials for optical data storage, biological markers, etc.) crucially depends on the microscopic understanding of the electronic excitations induced by external perturbations (e.g. light, electronic currents, etc.). Spectroscopy probes rightly the response of the materials to those perturbations. For this reason, reliable theoretical approaches are necessary to interpret experimental data and help to design new experiments. First-principles theories, based on either many-body perturbation theory or time-dependent density-functional theory, are a fundamental tool in the intepretations of electronic excitations. In this seminar we will review the recent developments that have been achieved in the theoretical spectroscopy group of LSI, ranging from photoemission to energy loss spectroscopy, from linear to nonlinear optics. We will discuss some paradigmatic applications and show the current projects aimed at pushing further the frontier of these methods.

    Posters (auteur, titre et abstract)

    Ekaterina ANTOSHCHENKOVA (LSI) : Simulation of epitaxial growth: the case of the magnesium oxide

    Gabriel AUTES (SPCSI) : Transport électronique dans les contacts atomiques magnétiques

    Cyrille BARRETEAU (SPCSI): Contribution de la polarisation orbitale au magnétisme du fer: du volume à l'agrégat
    Nous avons étudié la contribution du magnétisme orbitale aux propriétés magnétique du Fer pour des systèmes de diverse dimensionnalité: volume, surface, fil, agrégat. Nous utilisons un modèle basé sur l'approximation des liaisons-fortes dans une base incluant les orbitales s, p et d Notre modèle inclut le couplage spin-orbite et le magnétisme est pris en compte par un Hamiltonien d'interaction électron-électron de type Hartree-Fock incluant toutes les interactions de Coulomb intra-atomique et leur dépendance orbitale.

    Arjan BERGER (LSI) : Theoretical spectroscopy for finite systems
    This work deals with the calculation of linear-response properties of finite systems using time-dependent density-functional theory as well as many-body perturbation theory, namely GW and the Bethe-Salpeter equation. In particular we are interested in the performance of these methods when applied to large finite systems, e.g. biological systems. The existing theory we use for extended systems needs to be reformulated in order to make these calculations for large finite systems feasible. In this work we will discuss how we can obtain an efficient formulation for these systems.

    Silvana BOTTI (LSI), A. Castro, X. Andrade, A. Rubio and M. A. L. Marques :
    Ab initio calculation and modelling of van der Waals interactions between nanostructures and surfaces
    We present fully ab-initio calculations of van der Waals coefficients for two different situations: i)~the interaction between hydrogenated silicon clusters; and ii)~the interactions between these nanostructures and a non metallic surface (a silicon or a silicon carbide surface). The methods used are very efficient, and allow the calculation of systems containing hundreds of atoms. The results obtained are further analyzed and understood with the help of simple models. These models can be of interest for molecular dynamics simulations of silicon nanostructures on surfaces, where they can give a very fast yet sufficiently accurate determination of the van der Waals interaction.

    Silvana BOTTI (LSI), H-Ch. Weissker, M. A. L. Marques :
    Alloying effects in the optical properties of Si-Ge nanocrystals
    Time-dependent density functional theory (TDDFT) allows studying ab-initio the electronic excitations involved in spectroscopic experiments, possibly conserving a computational effort comparable to that of ground-state density functional theory (DFT). For this reason, TDDFT is particularly suitable to treat large scale nanostructures. Here I will present a study within TDDFT of the composition dependence of the optical properties of SixGe(1-x) nanocrystals. The excitation energies and the Stoke shifts have a distinct non-linear dependence on the composition. The theoretical results are compared with previous independent-particle DFT calculations and experimental luminescence data.

    Gaëlle BRUANT (LSI) : What is ETSF-I3?
    L'ETSF, "European Theoretical Spectroscopie Facility", est un grand instrument de théorie, à l’image des centres expérimentaux de rayonnement synchrotron partagés avec succès par les chercheurs de toute l’Europe. Notre expertise scientifique s’articule autour des propriétés électroniques des états excités de la matière. Grâce à la puissante combinaison de la théorie de la mécanique quantique et de la simulation par ordinateur, des groupes européens de théoriciens de la matière condensée, dont le LSI à Palaiseau, sont aujourd’hui en mesure de simuler, et ainsi de comprendre et de prédire des phénomènes en lien avec les excitations électroniques dans une large gamme de matériaux. L’ETSF permet d’analyser et d’expliquer les données expérimentales, et de favoriser l’invention dans de nombreux domaines fondamentaux et technologiques. L’objectif de l’ETSF est ainsi de parvenir à une large dissémination des techniques dites de la spectroscopie théorique à travers toute une gamme de services depuis la simple expertise jusqu’à la collaboration avec les utilisateurs, en passant par des formations spécialisées. Je propose dans ce poster de présenter les différents services de l’ETSF, financés dans le cadre de ETSF-I3, un projet d’e-infrastructure de la Communauté européenne (1er janvier 2008 – 31 décembre 2010).

    Matteo GATTI (1) (LSI): Understanding Correlations in Vanadium Dioxide from First Principles
    Vanadium dioxide is a prototype material for the discussion of correlation effects in solids. First-principles density-functional theory does not describe the metal-insulator transition, whereas strongly correlated models reproduce the main features. Here we present a parameter-free GW calculation of VO2 and show that the correlation effects in the band structure of both the metallic and the insulating phases are correctly reproduced, provided that quasiparticle energies and wave functions are calculated self-consistently. Our calculations explain the satellite in the photoemission spectrum of the metal as due to a plasmon resonance in the energy-loss function and show that this feature disappears in the insulator.

    Matteo GATTI (2) (LSI): Transforming Nonlocality into a Frequency Dependence: A Shortcut to Spectroscopy
    Measurable spectra are often derived from contractions of many-body Green's functions. One calculates hence more information than needed. Here we present and illustrate an in principle exact approach to construct effective potentials and kernels for the direct calculation of electronic spectra. In particular, a dynamical but local and real potential yields the spectral function needed to describe photoemission. We discuss for model solids the frequency dependence of this “photoemission potential” stemming from the non-locality of the corresponding self-energy.

    Christine GIORGETTI (LSI) : Modelling Carbon Nanotubes: Graphene and Graphite
    Using the ab initio code DP, we study Electron Energy Loss Spectroscopy in order to understand electronic properties on carbon nanotubes.
    1) We can do calculations directly on the tubes : but for the moment, we are limited to very small nanotubes (4 ang diameter), while experimental ones are closer to 20 ang diameter.
    2) We can model "big" carbon nanotubes with graphene sheets (nono or bilayers systems): EEL spectra in the graphene sheet should represent EEL spectra measures along the the tube axis.
    3) In order to study interactions between tubes and sheets, we try to model the multiwall nanotubes with different kind of stacking of graphene sheets.

    Ralf HAMBACH (LSI) : Discontinuity of the energy loss function at Bragg reflexes: graphite.
    As an example for layered materials, the loss function of graphite was studied for momentum transfers q beyond the first Brillouin zone. Surprisingly, near Bragg reflexes, the spectra are highly dependent on very small changes in q, which reminds the non-analyticity of the loss function in the optical limit q0. The effect is investigated by means of first principle calculations within the random phase approximation (RPA) and is confirmed by inelastic x-ray spectroscopy (IXS) measurements. We find crystal local field effects to be crucial and propose a simple 2x2 model dielectric function for explanation.

    Olivier HARDOUIN DUPARC (LSI) : Structures and defects of S=3n grain boundaries in copper
    Copper polycrystals, as most materials with a low stacking fault energy, contain a high proportion of S=3n grain boundaries (GBs). The S=3 {111} (coherent twins) may represent more than 50% of the polycrystal GB network and the percentage of the S=9 and S=27 GBs which, together with S=1, are necessarily attached to the S=3 GBs at triple junctions, may reach 15% [1]. Thus, these GBs must play an important role in the polycrystal properties. In the present work, we investigate the S=3n GB structures and defects at the atomic level in order to better understand the elemental deformation mechanisms occurring at these GBs. The GB structures are described using the structural unit model [2]. They are determined by coupling high-resolution transmission electron microscopy (HRTEM) observations and numerical simulations.
    The atomic structure of the S=3 {111} GB is very simple, formed of one type of structural unit named D; each D unit is obtained by a 70.53° around <110> rotation of two perfect crystal units. The interaction of a dissociated lattice dislocation with a S=3 {111} GB giving rise to extrinsic dislocations is not easy. HRTEM observations reveal two hardening mechanisms: first the formation of a stair-rod dislocation, then the formation of a Frank dislocation attached to the GB. The near S=9 {221} GBs display symmetrical and asymmetrical facets, generally {11,11,1} // {111} with small parts of incommensurate {110} // {111}. The asymmetrical facets may be described by using two elemental units: A (or A') of S=1 and E (or E') of two structures of the symmetrical S=9 GBs [3]. Variations of orientations coming from incorporation and accommodation of extrinsic dislocations seem to be easily achieved within these GBs. In some regions, a 3D intergranular phase appears between two asymmetrical facets. The defects at the limits between this phase and the two copper crystals are analysed. [4]. Finally, the structure of an asymmetrical S=27 {11, 11, 1} // {111} GB is also described in terms of A, E (E') units.
    Knowing that the interaction of a lattice dislocation with a GB strongly depends on the structural units at the impact point, we may expect different answers coming from the S=3n GBs. The behaviour of all the S=3 {111} GBs is likely comparable, depending on the dislocation characteristics, but in any case they are "hard" GBs. The behaviour of the S=9 and S=27 GBs may greatly change from place to place according to the structural unit on which the crystal dislocation impinges, but generally they are more penetrable than S=3 by crystal dislocations.

    Marc HAYOUN (LSI): Migration and correlation in highly defective systems: Fast-diffusion in Li2O
    The high-temperature superionic phase of lithium oxide is characterised by a high concentration of Frenkel defects and a diffusion mechanism involving several types of atomic jumps. We have calculated the tracer correlation factor and analysed the migration paths of the Li ions obtained by molecular dynamics (MD). A kinetic Monte Carlo code, simulating the lithium vacancy diffusion, has been developed and used to predict the correlation factor as a function of the atomic fraction of defects. There is a good agreement with the result directly obtained by MD. The analysis of the jump paths shows that the direct exchange between a vacancy and a migrating atom is the main part of the diffusion mechanism. The other atomic jumps, although complex, mostly imply vacancies. The Li+ fast-diffusion proceeds by a vacancy mechanism involving several jump types.

    Federico IORI (LSI), R. Magri, E. Degoli, I. Marri, G. Cantele, D. Ninno, Trani, O. Pulci, M. Palummo and S. Ossicini : Doping goes to Nano
    Silicon transistors are moving towards dimension of just a few nanometers, and there are now device designs that incorporate Si nanocrystals (Si-nc).Quantum confinement effect give rise to high photoluminescence (PL) quantum yeld of Si nanostructures, nanocrystals or nanowires, as compared with bulk crystalline Si. The main limitation is related to the radiationless Auger recombinations which can be circumvented by introducing an isoelectronic impurity or by simultaneous
    n- and p- type impurity doping which are expected to change significantly the electronic band structure of Si nanostructures. In this work, starting from hydrogenated Si-nc and nanowires simultaneous n-and p-type doping with Boron and Phosphorous impurities have been considered. The B-P codoping results easier than single doping and the two impurities tend to occupy nearest neighbours sites at the interfaces rather than other positions inside the nanocrystal itself. The codoped Si-nc present band edge states localized on the impurities that are responsible of the red shifted absorption threshold with respect to that of pure undoped nc in fair agreement with the experimental outcomes. The emission spectra show then a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron-hole pair. Absorption and emission spectra have been calculated for a small co-doped nanocrystal and nanowires through GW correction and the Bethe-Salpeter equation scheme.

    Sylvain LATIL (SPCSI), S. Roche and J.-C. Charlier : Transport study of carbon nanotube with random coverage of Π-conjugated molecules
    We present a mesoscopic study of electronic transport in carbon nanotubes with physisorption of Π-conjugated molecules. Our approach adresses large structures in order to treat realistic randomly located adsorption. The results indicate that the conduction regime (and its caracteristic lengths, e.g. elastic mean-free-path) are sensitive of the HOMO-LUMO gap of the conjugated molecule. Hence adsorption of benzene (gap=5eV) does not affect conduction properties of carbon nanotubes, whereas azulene (gap=2eV) is responsible for a finite electronic mean-free-path at Fermi level.

    Xóchitl LOPEZ-LOZANO (LSI) : Electronic and Catalytic properties of MoS2 Nanoplatelets: An ab initio Study
    Catalysts based on MoS2 are the most commonly used layered transition-metals-sulfides catalysts in petroleum refining. The study of the local active sites of these systems is of fundamental interest to understand and enhance their catalytic activity. MoS2 nanostructures consisting on single/double-layer-structure are under experimental study because of their potential applications as nanocatalysts. In this work we have performed ab initio density functional theory calculations using the ABINIT code to determine the structural, electronic and catalytically active sites of MoS2 nanoplatelets. The calculated total energy of the optimized atomic structures reveals that the double-sheet model is more stable than the single-sheet model. The electronic band structures show the existence of one-dimensional metallic states located at the nanoplatelet edges. Our results provide theoretical support to employ such MoS2 nanostructures as a novel nanocatalyst.

    Rodolphe POLLET & Dominik MARX (SPAM) : Application de la théorie de la fonctionnelle de la densité aux complexes de lanthanides en solution aqueuse
    La simulation ab initio de complexes de lanthanides, caractérisés par des électrons 4f très contractés autour du noyau, en solution aqueuse nécessite l'emploi de pseudopotentiels "ultradoux" de Vanderbilt permettant de réduire les temps de calcul. Leur construction ainsi que l'établissement de leur validité pour des complexes microsolvatés ainsi que pour un agent de contraste utilisé en imagerie médicale seront détaillés.

    Lucia REINING (LSI) : A quoi bon crayon et papier ?
    Ce poster résumera quelques axes de développement de théorie que nous poursuivons dans le groupe de spectroscopie théorique au LSI: effets excitoniques dans l'équation de Bethe-Salpeter, théorie de la fonctionnelle de la densité dépendante du temps, GW et au-delà, et la recherche d'approches alternatives pour décrire les corrélations.

    Cindy ROUNTREE (SPCSI) : Deformation in Silica via Molecular Dynamics Simulations
    Molecular Dynamics (MD) simulations probing the atomistic aspects of dynamic fracture in amorphous silica (a-SiO2) reveal nanometer scale cavities nucleating, augmenting, and coalescing with one another up to 20 nm ahead of the crack tip [1, 2, 3]. After which these cavities were seen to merge with the advancing crack to cause mechanical failure. This scenario was also observed experimentally during stress corrosion ultra-slow fracture of glass using Atomic Force Microscopy (AFM) [4, 5]. In order to characterize the irreversible changes in structure taking place within the process zone (i.e. the zone ahead of the crack tip where pores are opening), a variety of simulations have been carried out using 1) cyclic loading and unloading in hydrostatic pressure and 2) cyclic loading and unloading in shear. Structural changes revealed by these simulations have been analyzed in various ways (static structure factor, analysis of the ring structure, evolution of the fabric tensor…).
    [1] C.L. Rountree, et. al. Annual Review of Materials Research, 32 (2002) 377-400.
    [2] L.Van Brutzel, C.L. Rountree et. al., Mat. Res. Soc. Symp. Proc. 703 (2002) V3.9.1- V3.9.6.

    Jelena SJAKSTE (LSI) :Ab initio method for the electron-phonon scattering times in semiconductors.
    The interaction of excited electrons with phonons plays a central role in nanoelectronics. When electrons are promoted to the conduction band of a semiconductor by an interband absorption process, only a small portion of the excited electrons can return into the valence states, giving rise to emitted radiations. Rather, the finite lifetime of the excitation in due to non-radiative scattering mechanisms such as collisional processes with surfaces, impurities and phonons. In a relatively pure sample, at excitation energies lower than twice the band gap energy, electron-phonon interaction is the dominant process limiting the excitation lifetime.
    Despite its importance, a reliable approach within ab initio methods was still lacking for phonon interaction with carriers in the conduction band. Moreover, the coupling of excitons with phonons has so far only been calculated through a semi-empirical approach [1]. On the other hand, ultrafast optical spectroscopy now provides an efficient tool for detailed investigations of the microscopic scattering processes related to hot carriers [2], and in particular of their de-excitation through phonon scattering [3]. To bridge this gap we study in this work the electron-phonon scattering times for collisions with short-wavelength (intervalley) phonons in semiconductors. Our fully ab initio approach is based on Density Functional Perturbation Theory and on the direct integration of electronic scattering probabilities over all possible final states with no ad hoc assumptions. We apply it to the de-excitation of hot electrons in GaAs [4,5], and calculate the lifetime of the direct exciton in GaP [5], both in excellent agreement with experiments.
    [1] S. Zollner et al, Solid-State Electr. 32 (1989) 1585.
    [2] F. Rossi, T. Kuhn, Rev. Mod. Phys. 74 (2002) 895.
    [3] J. Shah et al, Phys. Rev. Lett, 59 (1987) 2222.
    [4] J. Sjakste, V.~Tyuterev, N.~Vast, Appl. Phys. A 86 (2007) 301.
    [5] J. Sjakste, N. Vast, V. Tyuterev, Phys. Rev. Lett. 99 (2007) 236405.

    Francesco SOTTILE (LSI) : New frontiers for ab initio calculations: biomolecules
    Absorption or emission of light from biomolecules are crucial processes to understand the machinery of life. Photosynthesis, vision, bioluminescence or DNA damage are paradigmatic examples. A sound theoretical understanding of the photo-chemistry of biological molecules is not only needed to describe the mechanisms of Biology, but also because some of the key molecules can be employed for technological purposes at the nanoscale. However, despite the tremendous effort focused on this field, the first-principles theoretical description of the interaction of these molecules with time-dependent electromagnetic fields is still a challenging problem , lacking a definitive, systematic, methodology, allowing to bridge the different spatial and time scales that are relevant for the description of light-induced biological processes with predictive power. Density Functional Theory and Many-body Perturbation Theory has repeatedly shown in the last decade their usefulness when attempting this challenge, and we show here some examples.

    Lionel TRUFLANDIER (SCM) : Investigation of 3d Transition Metal NMR Shielding Tensors Using the GIPAW Method
    We present DFT based method for calculating NMR shielding tensors for 3d transition metal nuclei using periodic boundary conditions. Calculations employ the gauge-including projector augmented-wave pseudopotential method.[1] Effects induced by the use of ultrasoft pseudopotentials on the second-order magnetic response, as well as frozen core approximation, projector expansion or indirect relativistic effects, are presented [3]. The reliability and the strength of the approach for 49Ti, 51V and 55Mn nuclei are demonstrated by comparing to traditional quantum chemical methods, using benchmarks of finite organometallic systems. Application to infinite systems is validated for the 51V nucleus in various vanadium oxide based compounds. The successful agreement between experimental and theoretical isotropic chemical shifts [2] contrasts with the poor accuracy obtained for the whole shielding tensor eigenvalues. This reveals the limitation of pure exchange-correlation functionals compared to their exact-exchange corrected analogues [3].
    [1] C..J.Pickard and F.Mauri,Phys.Rev.B 63 ,245101 (2001).
    [2] L.Truflandier,M.Paris,C.Payen,and F.Boucher,J.Phys.Chem. B 110 ,21403 (2006)
    [3] L.Truflandier,M.Paris and F.Boucher,Phys.Rev.B 76 ,35102 (2007)

    Nathalie VAST (LSI): Supraconductivité conventionnelle par le dopage des icosaèdres de bore.
    Les éléments de faible numéro atomique ont été intensivement étudiés pour trouver un couplage électron-phonon avec une forte température critique. Parmi eux, le bore est un exemple caractéristique. Les phonons de fréquence élevée des couches de bore métalliques dans la structure du diborure de bore MgB2 sont principalement responsables de la température critique de supraconductivité de 39K. D'autre part, les géométries de type "boule" sont les constituants des fullerènes C60, dont l'intercalation avec des atomes alcalins produit la supraconductivité : c'est alors la grande courbure de la molécule dans cette géométrie qui accroît l'importance du couplage électron-phonon.
    Dans le cadre théorique de la DFT, le carbure de bore B13C2 est métallique: il combine alors les 2 propriétés - large couplage électron-phonon par la géométrie icosaédrique, et grandes fréquences de vibration par les atomes qui le composent - qui en font un candidat à la supraconductivité. Nous avons estimé la température de supraconductivité dans ce matériau, et discutons quel type de dopage est nécessaire pour rendre supraconducteurs les échantillons de B13C2 qui sont naturellement semiconducteurs.

    Valerie VENIARD (LSI) : Optique non-linéaire dans les solides : génération de seconde harmonique
    La connaissance des propriétés optiques des solides est fondamentale pour l'amélioration des matériaux et des dispositifs non-linéaires. De plus, elle offre la possibilité de chercher de nouveaux matériaux ayant des propriétés bien spécifiques. Un des processus particulièrement important est la génération d'harmoniques d'ordre deux, au cours duquel deux photons sont absorbés par le matériau et un photon d'énergie double est émis. Ce processus, grâce à sa grande sensitivité aux symétries du système, est souvent utilisé comme sonde pour l'étude des surfaces et des interfaces,. Les premiers calculs de génération d'harmoniques ont été faits avec des théories à électrons indépendants dans un champ moyen, mais l'accord obtenu avec les résultats expérimentaux pour des semi-conducteurs n'est pas satisfaisant. La prise en compte des effets à l'échelle microscopique est nécessaire pour améliorer la détermination théorique des susceptibilités d'ordre deux. En principe, la réponse optique d'un matériau est fortement modifiée par les effets de champs locaux et les corrélations électroniques. Des progrès importants ont été réalisés en optique linéaire, liés à la Théorie de la Fonctionnelle de la Densité Dépendante du Temps (TDDFT). Mais au delà de la réponse linéaire, l'impact des champs locaux et des excitons sur la réponse non linéaire reste encore très mal connu, faute de formalisme adapté. Nous présenterons le formalisme que nous avons développé pour tenir compte à la fois des effets macroscopiques et microscopiques dans la susceptibilité d'ordre deux. Ce formalisme est en cours d'implémentation et nous présenterons aussi les premiers résultats que nous avons obtenus pour le carbure de silicium.

    Julien VIDAL (LSI) : Ab initio modeling of optoelectronic properties of chalcopyrites for photovoltaic conversion
    Ab initio calculations have been instrumental in the understanding of important structural and chemical properties of chalcopyrites, such as non stoichiometry, self compensation and stability. The benefit of such methods has been specially important as chalcopyrites are a very complex class of semiconductors, in which the interpretation of experiments is far from straightforward. The objective of the present work is to obtain missing values on fundamental opto- electronic properties of chalcopyrite alloys and of their main defects to feed device level modeling. The calculations are based on, and go beyond, density functional theory within the two frameworks of all electron and pseudopotentials-planewaves calculations. We compare standard DTF results to hybrid approaches where a fixed amount of exact exchange has been introduced, to LDA+U and to SC-GW calculations within different approximations. The differences between these approaches and their reliabilities will be discussed. Only GW using self-consistency was able to give band gaps within 10% error. We calculate effective masses and show the contribution of the GW corrections to the band dispersion, as well as their importance for the different types of electronic states that are present in these complex compounds. Finally, we discuss the influence of many-body corrections on the defect levels. In this work we will also present results on how the different computing schemes affect intrinsic and extrinsic point defect formation energy.

    Hans-Christian WEISSKER (LSI) : Dielectric and Loss Function for Finite Momentum Transfer~-- Answers and Open Questions
    Both the EEL spectrum and the dynamic structure factor as measured in inelastic x-ray scattering (IXS) are given by the imaginary part of the inverse dielectric function. Our combined experimental and theoretical work [1] of IXS and ab initio calculations carried out on silicon at different levels of approximation shows that time-dependent density-functional theory in adiabatic local-density approximation describes both the loss function and the dielectric function for non-zero momentum transfer very well for valence excitations in semi-conductors. The remaining differences are shown to be mainly lifetime related. We have also demonstrated the importance of crystal local-field effects and of the coupling between resonant and anti-resonant contributions to the imaginary part of the dielectric function. For sodium, on the other hand, the TDLDA fares much less well. In our present poster we discuss several open questions which have to be solved in order to obtain a coherent and precise description of the response functions. In particular, we will focus on the absorption edges which represent the contributions of the (semi-) core states. The influence of the core polarization on the spectra will be addressed as well. Moreover, we show how the problem of interpolation between spectra calculated for different momentum transfers can be solved. This enables the description of experiments with a finite resolution in this parameter.
    [1] H.-Ch. Weissker et al., Phys Rev Lett 97 (2006) 237602.

    Wojciech WELNIC (LSI), Simo Huotari, Giulio Monaco and Lucia Reining : Electronic excitations at finite momentum transfer and short-range order changes in covalent materials - a joint theoretical and experimental approach
    Time-dependent-density-functional theory (TDDFT) within the adiabatic local density approximation (ALDA) is successfully employed to calculate the dynamical structure factor S(q,ω) nowadays. Good agreement with experimental data obtained from inelastic x-ray scattering has been reported for different materials such as silicon, aluminum or transition metal oxides. In this work we explore the potential of TDLDA calculations of S(q,ω) to detect changes in the local geometry of a material. For this purpose we performed TDLDA calculations for amorphous and crystalline GeTe. This covalently bonded material has been reported to exhibit a significant change in the local geometry upon amorphisation. For q=0 it has been shown that the change in local geometry results in a decrease of the matrix elements of the optical transitions upon amorphisation. However, our calculations show that for finite momentum transfer q the differences in the spectra of ε2(q, ω) as well as of S(q, ω) vanish. Inelastic x-ray scattering experiments confirm our results. Here we present an explanation for these unexpected results.

    Alberto ZOBELLI (LSI): Shaping nanotubes with electron nano-beam. Theoretical and experimental aspects
    La nature et le rôle de défauts est de première importance pour la compréhension des propriétés physiques des nanotubes monoparoi (SWNT) de carbone et nitrure de bore. La microscopie électronique en transmission (TEM) est un outil très puissant pour l'étude des défauts dans les matériaux, mais dans le cas de SWNT les atomes peuvent être aussi être éjectés par l’irradiation électronique. Cet effet peut changer la structure initiale du tube mais peut être également vu comme un outil potentiel pour “usiner” des structures nanométriques.
    Nous avons développé un outil théorique pour la description du mécanisme d'irradiation. Dans un premier temps, nous avons dérivé, par des calculs basés sur la théorie de la fonctionnelle de densité, la carte des seuils d'énergie d'émission. Successivement, nous avons dérivé numériquement la section efficace total de Mott pour différents sites d'émission dans des nanotubes de carbone et nitrure de bore. Utilisant un microscope STEM, nous avons été capables de contrôler la génération de défauts dans des systèmes nanotubulaires avec des conditions expérimentales optimisées sur la base de nos calculs de section efficace. Défauts ponctuels ou ligne de dislocation peuvent ainsi être obtenu avec une résolution spatiale de quelques nanomètres. La structure, l'énergie et les propriétés électroniques des défauts ponctuels et des lignes de défauts ont été étudiées dans les systèmes de BN. L'énergie d'activation et les chemins réactionnels pour la diffusion de mono et de bi-lacunes dans du BN hexagonal ont été dérivés en utilisant le “nudged elastic band method” combiné avec les techniques basées sur la fonctionnelle de la densité. Nous avons aussi démontré que l'apparition de défauts étendus est plus favorable qu’une distribution aléatoire de défauts ponctuels et que cela est dû à l'existence de sites préférentiels pour l'émission d'atomes en présence de défauts préexistants plutôt qu’à des phénomènes de migration et nucléation thermique des lacunes.


     

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