Role of Dimensionality and Size in Controlloing the Drag Seebeck Coefficient of Doped Silicon Nanostructures: A Fundamental Understanding

In this theoretical study, we examine the influence of dimensionality, size reduction, and heattransport direction on the phonon-drag contribution to the Seebeck coefficient of silicon nanostructures. Phonon-drag contribution arises from the momentum transfer between out-of-equilibrium phonon populations and charge carriers, and significantly enhances the thermoelectric coefficient. Our implementation of the phonon drag term accounts for the anisotropy of nanostructures such as thin films and nanowires through the boundary- and momentum-resolved phonon lifetime. Our approach also takes into acconout the spin-orbit coupling, which turns out to be crucial for hole transport. We reliably quantify the phonon drag contribution at various doping levels, temperatures, and nanostructure geometries for both electrons and holes in silicon nanstructures. Our results support the recent experimental findings, showing that a part of phonon drag contribution survives in 100 nm silicon nanostructures

Simulation of electron energy loss spectra with the turboEELS and thermo-pw codes

For some materials like noble metals, electron energy loss spectra have a complex structure that makes them difficult to analyze without the help of ab initio calculations. Various theoretical approaches can be used for this purpose, among which the time-dependent density functional perturbation theory (TDDFPT) which has been widely used to study plasmons in a number of bulk and surface systems. In the present paper we present a comparison of the results and performance of two different numerical implementations of TDDFPT: the Sternheimer and Liouville-Lanczos methods. The former approach is implemented in the thermo-pw module and the latter one in the turboEELS code of the QUANTUM ESPRESSO package for electronic structure calculations. In the present paper a comparison is made for bulk bismuth, a semimetal, taking into account spin-orbit coupling, as well as for bulk gold, a noble metal. We show that for these two examples, both codes gives identical results and the turboEELS code has a better performance than the thermo-pw code, and point out in which cases the usage of thermo-pw alone or of both codes can be advantageous

Ab initio study of electron energy loss spectra of bulk bismuth up to 100 eV

The dynamical charge-density response of bulk bismuth has been studied within time-dependent density functional perturbation theory, explicitly accounting for spin-orbit coupling. The use of the Liouville-Lanczos approach allows us to calculate electron energy loss spectra for excitation energies as large as 100 eV. Effects of 5d semicore electronic states, spin-orbit coupling, exchange and correlation, local fields, and anisotropy are thoroughly investigated. The account of the 5d states in the calculation turns out to be crucial to correctly describe the loss spectra above 10 eV and, in particular, the position and shape of the bulk-plasmon peak at 14.0 eV at vanishing transferred momentum. Our calculations reveal the presence of interband transitions at 16.3 eV, which had never been discussed before. The origin of the peak at 5.8 eV is revisited as due to mixed interband and collective excitations. Finally, our study supplements the lack of experiments at finite transferred momenta

Stand growth and structure of mixed-species and monospecific stands of Scots pine (Pinus sylvestris L.) and oak (Q. robur L., Quercus petraea (Matt.) Liebl.) analysed along a productivity gradient through Europe

19 Pág.Past failures of monocultures, caused by wind-throw or insect damages, and ongoing climate change currently strongly stimulate research into mixed-species stands. So far, the focus has mainly been on combinations of species with obvious complementary functional traits. However, for any generalization, a broad overview of the mixing reactions of functionally different tree species in different mixing proportions, patterns and under different site conditions is needed, including assemblages of species with rather similar demands on resources such as light. Here, we studied the growth of Scots pine and oak in mixed versus monospecific stands on 36 triplets located along a productivity gradient across Europe, reaching from Sweden to Spain and from France to Georgia. The set-up represents a wide variation in precipitation (456–1250 mm year−1), mean annual temperature (6.7–11.5 °C) and drought index by de Martonne (21–63 mm °C−1). Stand inventories and increment cores of trees stemming from 40- to 132-year-old, fully stocked stands on 0.04–0.94-ha-sized plots provided insight into how species mixing modifies stand growth and structure compared with neighbouring monospecific stands. On average, the standing stem volume was 436 and 360 m3 ha−1 in the monocultures of Scots pine and oak, respectively, and 418 m3 ha−1 in the mixed stands. The corresponding periodical annual volume increment amounted to 10.5 and 9.1 m3 ha−1 year−1 in the monocultures and 10.5 m3 ha−1 year−1 in the mixed stands. Scots pine showed a 10% larger quadratic mean diameter (p < 0.05), a 7% larger dominant diameter (p < 0.01) and a 9% higher growth of basal area and volume in mixed stands compared with neighbouring monocultures. For Scots pine, the productivity advantages of growing in mixture increased with site index (p < 0.01) and water supply (p < 0.01), while for oak they decreased with site index (p < 0.01). In total, the superior productivity of mixed stands compared to monocultures increased with water supply (p < 0.10). Based on 7843 measured crowns, we found that in mixture both species, but especially oak, had significantly wider crowns (p < 0.001) than in monocultures. On average, we found relatively small effects of species mixing on stand growth and structure. Scots pine benefiting on rich, and oak on poor sites, allows for a mixture that is productive and most likely climate resistant all along a wide ecological gradient. We discuss the potential of this mixture in view of climate change.The authors wish to thank the European Union for funding the project ?Mixed species forest management. Lowering risk, increasing resilience (REFORM)? (#2816ERA02S, PCIN2017-026) under the framework of Sumforest ERA-NET. All contributors thank their national funding institutions to establish, measure and analyse data from the triplets. The first author also thanks the Bayerische Staatsforsten (BaySF) for supporting the establishment of the plots and the Bavarian State Ministry for Nutrition, Agriculture, and Forestry for permanent support of the project W 07 ?Long-term experimental plots for forest growth and yield research? (#7831-22209-2013). The French site (FR-1) belongs to the OPTMix experimental site (https://optmix.irstea.fr), which is supported annually by Ecofor, Allenvi, and the French national research infrastructure ANAEE-F. Research on the Lithuanian triplets was made possible by the national funding institution Research Council of Lithuania (LMTLT), agreement No. S-SUMFOREST-17-1. Thanks are also due to Ulrich Kern for the graphical artwork. Finally, we thank two anonymous reviewers for their constructive criticism.Peer reviewe

Propriétés vibrationelles du bore alpha et du carbure de bore

Atomic structural properties and lattice dynamics of alpha boron and of B4C boron carbide have been studied by Density Functional Theory (D.F.T.) and Density Functional Perturbation Theory (D.F.P.T.).The bulk modulus of the unit-cell and of the icosahedron have been investigated, and the equation of state at zero temperature has been determined. In alpha boron, Raman diffusion and infrared absorption have been studied under pressure, and the theoretical and experimental Grüneisen coefficients have been compared. In boron carbide, inspection of the theoretical and experimental vibrational spectra has led to the determination of the atomic structure of B4C. Finally, the effects of isotopic disorder have been modelized by an exact method beyond the mean-field approximation, and the effects onto the Raman lines has been investigated. The method has been applied to isotopic alloys of diamond and germanium.Les propriétés statiques et vibrationnelles du bore alpha et du carbure de bore B4C ont été étudiées par Théorie de la Fonctionnelles de la Densité (D.F.T.) et Théorie de Perturbation de la Fonctionnelle de la Densité (D.F.P.T.).Les propriétés de compressibilité de la structure icosaédrique sont étudiées en détail, et l'équation d'état à température nulle théorique est déterminée, pour la maille et pour l'icosaèdre.Dans le bore alpha, les propriétés de diffusion Raman et d'absorption infrarouge expérimentales et théoriques sont étudiées en fonction de la pression, et les coefficients de Grüneisen en centre de zone de Brillouin sont comparés. Dans le carbure de bore, l'examen des spectres vibrationnelles et théoriques conduit à la détermination de la structure atomique de B4C.Enfin, les effets du désordre isotopique ont été modélisés de façon ab initio, et leur effet sur les raies de diffusion Raman examinés. La méthode a été appliquée aux alliages binaires de diamant et de germanium

Etude ab initio des propriétés physiques des matériaux

My activities are in fundamental research in the group of theory of the Laboratoire des Solides Irradiés. It includes the study of the physical properties of some of those materials which are of interest to the CEA, to nuclear research and to nanoelectronics. The objective is to achieve a parameter-free description of processes which control electronic excitations or electronic relaxations. This covers : - The study of the ground state of materials; - The investigation of the properties of the excited state, from the viewpoint of the spectroscopy of valence electrons; - The lattice dynamics, its coupling with phonons, and the effect of electron-phonon coupling on electronic transport and electronic relaxation. These studies involves high performance computing and require computer time from big facilities like the GENCI in France. In the manuscript, I recall first how the inverse dielectric function is calculated within time dependent, density functional theory, and the link with the measurement of the electronic loss function. Theoretical results are presented for titania TiO$_2$ and zirconia ZrO$_2$ which are uncorrelated oxides. Then main theoretical results on optical absorption of cuprous oxide Cu$_2$O and on zirconia ZrO$_2$ are presented. A new interpretation on the kernel which allows to model excitonic effects in time dependent density functional theory is presented. Finally, recent calculations on boron carbides, and in particular B$_4$C are reviewed.Mon activité de recherche fondamentale dans le groupe de théorie du Laboratoire des Solides Irradiés concerne l'étude des propriétés des matériaux d'intérêt pour le CEA, dans les domaines du nucléaire ou de la nanoélectronique. Elle a pour objectif d'atteindre une description théorique -sans paramètre ajustable- des processus contrôlant l'excitation électronique, ainsi que la relaxation -ou désexcitation- électronique, et couvre: - Les propriétés de la matière hors excitation - l'état fondamental; - Les propriétés de l'état excité, abordées sous l'angle de la spectroscopie pour les électrons de valence; - Les vibrations collectives des atomes, leur couplage avec les électrons, et leurs effets sur le transport électronique ou la relaxation électronique. Ces études requièrent un environnement de calcul intensif et l'accès aux ordinateurs du Grand Equipement National de Calcul Intensif GENCI. Dans ce manuscrit, est d'abord rappelé comment calculer la fonction diélectrique inverse en théorie de la fonctionnelle de la densité dépendante du temps, et quel est le lien avec la fonction de perte électronique observée. Des résultats théoriques sur la fonction diélectrique inverse dans des oxydes non corrélés représentés par le dioxyde de titane TiO$_2$ et la zircone ZrO$_2$ sont décrits. Ensuite sont donnés les principaux résultats théoriques pour les calculs de spectres d'absorption optique pour l'oxyde de cuivre Cu$_2$O et la zircone ZrO$_2$. J'y présente une nouvelle interprétation de travail sur le noyau permettant de modéliser les effets excitoniques en théorie de la fonctionnelle de la densité dépendante du temps. Enfin, les derniers calculs menés sur les carbures de bore sont rappelés

Étude ab initio des plasmons et du couplage électron-phonon dans le bismuth: de la modélisation de l'absorption des porteurs libres à une nouvelle méthode pour le calcul de spectre de perte d'énergie électronique

Ce travail a été consacré à l'étude théorique du bismuth semi-métallique à l'aide de méthodes basées sur la théorie de la fonctionnelle de la densité (DFT). Les effets de couplage spin-orbite et d'échange et de corrélation dans l'approximation de densité locale (LDA) et de gradient généralisé (GGA) ont été approfondis de façon systématique. J'ai trouvé que les poches d'électrons et de trous au niveau de Fermi sont correctement décrites, ce qui m'a permis d'interpréter avec succés les expériences pompe-sonde dans le bismuth photoexcité menées au laboratoire des Solides Irradiés. Le calcul du couplage électron-phonon a montré la forte dépendance, par rapport au vecteur d'onde électronique, du couplage de la bande de valence la plus haute avec le phonon A1g LO de centre de zone, ce qui explique l'observation de la forte dépendance en k de l'amplitude d'oscillation de l'énergie de liaison de cette même bande en photoémission résolue en temps. J'ai aussi montré que la présence d'extréma dans les bandes de valence et de conduction, où la masse des porteurs peut atteindre 18 m0, favorise une accumulation des porteurs et conduit à une augmentation de leur fréquence plasma au cours du temps aprés photoexcitation, un effet qui n'a pas (encore) été observé dans d'autres matériaux. Enfin, j'ai développé une nouvelle méthode en théorie de perturbation de la fonctionnelle de la densité dépendante du temps (TDDFPT), qui permet de calculer la réponse électronique du matériau pour n'importe quelle valeur du moment transféré. Cette approche basée sur la méthode de récursion de Lanczos m'a permis de calculer les spectres de perte d'énergie électronique de Bi dans la gamme d'énergie 0-100 eV et de combler l'intervalle d'énergie entre les pertes des électrons de valence et celles des électrons de cœur. Cette méthode ouvre des perspectives considérables, comme le calcul des plasmons de surface.This work has been devoted to the theoretical study of bulk semimetallic bismuth with methods based on the density functional theory (DFT). Effects of spin-orbit coupling and of the exchange-and-correlation functionals in the local density (LDA) and generalized gradient approximation (GGA) have been systematically investigated. I have found that electron and hole pockets at the Fermi level are accurately reproduced, which has enabled me to successfully interpret the pump-probe experiments in the photoexcited bismuth performed in the Laboratoire des Solides Irradiés. The strong dependence on the electronic wave vector, of the calculated electronic coupling of the upper valence band with the zone-center A1g LO phonon, explains the observation of a strongly k-dependent oscillation amplitude of the upper valence band in time-resolved photoemission experiments upon activation of the coherent A1g phonon under photoexcitation. I have also shown that the presence of local extrema in the conduction and valence bands structure, where the carrier mass can be as large as 18 m0, favours an accumulation of photoexcited carriers in these extrema and contributes to the augmentation of the plasma frequency as a function of time after the photoexcitation, an effect which has no analogy in other materials (as yet). Finally, I have developed a new ab initio approach in the time-dependent density functional perturbation theory (TDDFPT), which allows us to calculate the electronic response of materials for any momentum transfer. This approach based on the Lanczos recursion method has enabled me to calculate for the first time the electron energy-loss spectrum of Bi in the 0-100 eV energy range, bridging the gap between valence and core losses. This method opens the way to the routine calculation of surface plasmons.PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Optical and loss spectra of carbon nanotubes: Depolarization effects and intertube interactions

We performed ab initio calculations of the anisotropic dielectric response of small-diameter single-walled carbon nanotubes in the framework of time-dependent density-functional theory. The calculated optical spectra are in very good agreement with experiment, both concerning absolute peak positions and anisotropy effects. The latter can only be described correctly when crystal local-field effects (“depolarization” effects) are fully taken into account. Moreover, interactions between the tubes can strongly modify their absorption and electron energy-loss spectra.This work was supported by the EC-RTN program NANOPHASE (Contract No. HPRN-CT-2000-00167). A. R. acknowledges support from the Ecole Polytechnique during a sabbatical leave and MCyT (MAT2001-0946). Computer time was granted by IDRIS (Project No. 544).Peer reviewe

Etude théorique des phases du titane

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Hot electron relaxation and energy loss rate in silicon: Temperature dependence and main scattering channels

International audienceIn this work, we revisit the density functional theory (DFT)-based results for electron–phonon scattering in highly excited silicon. Using the state-of-the-art ab initio methods, we examine the main scattering channels, which contribute to the total electron–phonon scattering rate and the energy loss rate of photoexcited electrons in silicon as well as their temperature dependence. Both temperature dependence and the main scattering channels are shown to strongly differ for the total electron–phonon scattering rate and the energy loss rate of photoexcited electrons. While the total electron–phonon scattering rate increases strongly with temperature, the temperature dependence of the energy loss rate is negligible. Also, while acoustic phonons dominate the total electron–phonon scattering rate at 300 K, the main contribution to the energy loss rate comes from optical modes. In this respect, DFT-based results are found to disagree with conclusions of Fischetti et al. [Appl. Phys. Lett. 114, 222104 (2019)]. We explain the origin of this discrepancy, which is mainly due to differences in the description of the electron–phonon scattering channels associated with transverse phonons