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 q-»0.
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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