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Even faster integer multiplication
No full textWe give a new proof of Fürer's bound for the cost of multiplying n-bit integers in the bit complexity model. Unlike Fürer, our method does not require constructing special coefficient rings with ''fast'' roots of unity. Moreover, we establish the improved bound O(n log n K^(log^∗ n)) with K=8. We show that an optimised variant of Fürer's algorithm achieves only K=16, suggesting that the new algorithm is faster than Fürer's by a factor of 2^(log^∗ n). Assuming standard conjectures about the distribution of Mersenne primes, we give yet another algorithm that achieves K=4
Interplay between structure and electronic properties of layered transition-metal dichalcogenides: Comparing the loss function of 1T and 2H polymorphs
No full textInternational audienceTransition-metal dichalcogenides (TMD) share the same global layered structure, but distinct polymorphs are characterized by different local coordinations of the transition-metal atoms. Here we compared the 1T and 2H families of metallic TMD, both in the bulk and in the two-dimensional forms. By means of first-principles time-dependent density functional calculations of the loss function, we established the direct connection between the low-energy plasmon properties and the crystal-structure symmetry. The different atomic environments affect the d − d electron-hole excitations, which are prominent at low energies, resulting in distinct in-plane plasmon dispersions in the two families. Conversely, the different periodicity of the plasmon reappearance along the c axis perpendicular to the layers can be used to distinguish the various crystal structures of TMD
Croissance épitaxiale de Si(Ge) sur Si et GaAs à basse température par PECVD, pour cellules solaires tandem
No full textThis thesis focuses on epitaxial growth of Si and SiGe at low temperature (200°C) by Plasma Enhanced Chemical Vapor Deposition (PECVD), and its application in thin film crystalline solar cells. Our goal is to gain insight into this unusual growth process, as well as to investigate the potential of such low temperaturedeposited material for single and multi-junction solar cells.First, we have proposed a one pump-down plasma process to clean out-of-the-box c-Si wafer surface and grow epitaxial layers of up to 8µm thick, without ultra-high vacuum, in a standard RF-PECVD reactor. By exploring the experimental parameters space, the link between layer quality and important physical variables, such as silane dilution, ion energy, or deposition pressure, has been confirmed. Both material and electrical properties were analyzed, and we found that epitaxial quality improves with film thickness. Furthermore, we could bring evidence of SiGe and Ge epitaxial growth under similar conditions. Then, with the whole process steps <200°C, we have achieved PIN heterojunction solar cells on highly doped substrates with 1-4µm epitaxial absorber, reaching 8.8% efficiency (without light trapping) and 80.5% FF. Replacing Si absorber by epitaxial Si0:73Ge0:27 resulted in 11% boost in Jsc. The use of an engineered wafer/epitaxial layer interface and stress enables easy lift-off: e.g. we successfully bonded 1.5µm thick 10cm^2 epi-Si to glass. Additionally, we have considered the impact of photonic nanostructures on device properties. Together, the control of growth, transfer and advanced light trapping are paving the way toward highly efficient, ultrathin (<10µm) and low cost c-Si cells. Finally, in contrast with general trend of growing III-V semiconductors on Si, we have studied the hetero-epitaxial growth of Si on III-V. Good crystal quality was achieved by direct Si deposition on GaAs, thanks to reduced thermal load and suppressed polarity issues in this approach. Using MOCVD, we could build GaAs cells with 20% efficiency and III-V tunnel junctions reaching 55A/cm^2. Tunneling improvement upon H-plasma exposure was shown. Those results, combined with III-V layer lift-off, validate milestones toward high efficiency tandem AlGaAs(MOVD)/SiGe(PECVD) metamorphic solar cells.Cette thèse s'intéresse à la croissance épitaxiale de Si et SiGe à basse température (200°C) par dépôt chimique en phase vapeur assistée par plasma (PECVD), et à l'utilisation de ces matériaux cristallins dans les cellules solaires en couches minces. L'objectif était de mieux comprendre cette croissance inattendue et d'étudier le potentiel de ces matériaux pour les cellules simples et multijonctions. Nous avons d'abord démontré qu'il est possible d'effectuer, avec un réacteur PECVD standard, un nettoyage efficace de la surface du c-Si et de poursuivre par une croissance épitaxiale de couches de Si jusqu'à 8µm d'épaisseur. L'impact des paramètres du procédé tels que la dilution du SiH4 dans l'H2, l'énergie des ions ou encore la pression totale, sur la qualité des couches a été mis en évidence. Les propriétés électriques et structurelles des couches ont été analysées, et nous avons démontré une amélioration de la qualité cristalline avec l'épaisseur de la couche. La croissance épitaxiale de Ge et SiGe sur c-Si dans des conditions similaires a également été établie. Ensuite, par une séquence d'étapes à moins de 200°C, des hétérojonctions PIN sur substrats très dopés, avec une couche absorbante épitaxiée de 1-4µm ont été réalisées, atteignant 8.8% de rendement (sans piégeage optique) et 80% de FF. Le remplacement du Si par du Si0:73Ge0:27 a permis un gain de 11% sur le Jsc. Le contrôle de l'interface wafer/épi et des contraintes permet de favoriser le décollement : des couches epi-Si de 1.5µm/10cm^2 ont été reportées sur verre avec succés. Nous avons également analysé l'influence de nanostructures photoniques sur les propriétés des dispositifs. L'étude conjointe de la croissance, du transfert et du piégeage optique ouvre la voie aux cellules c-Si ultra-minces (<10µm) bas côut. Enffin, contrairement au scénario classique de dépôt des matériaux III-V sur Si, nous avons étudié l'hétéroépitaxie de Si sur III-V. Avec cette approche, une bonne qualité cristalline de Si déposé directement sur GaAs est obtenue grâce aux faibles contraintes thermiques et à l'absence de problèmes de polarité à l'interface. Nous avons fabriqué des cellules GaAs avec 20% d'efficacité et des jonctions tunnel atteignant 55A/cm^2 par dépôt MOVPE. Une augmentation du courant tunnel par exposition au plasma d'hydrogène a aussi été démontrée. Ces résultats de croissance, cellule et jonction tunnel, couplés aux techniques de report, valident les briques élémentaires pour atteindre une cellule tandem AlGaAs(MOVPE)/SiGe(PECVD) à haut rendement
Solar irradiances measured using SPN1 radiometers: uncertainties and clues for development
No full textInternational audienceThe fast development of solar radiation and energy applications, such as photovoltaic and solar thermodynamic systems, has increased the need for solar radiation measure-ment and monitoring, for not only the global but also the diffuse and direct components. End users look for the best compromise between getting close to state-of-the-art mea-surements and keeping low capital, maintenance and operat-ing costs. Among the existing commercial options, SPN1 is a relatively low cost solar radiometer that estimates global and diffuse solar irradiances from seven thermopile sensors under a shading mask and without moving parts. This work presents a comprehensive study of SPN1 accu-racy and sources of uncertainty, drawing on laboratory ex-periments, numerical modelling and comparison studies be-tween measurements from this sensor and state-of-the art in-struments for six diverse sites. Several clues are provided for improving the SPN1 accuracy and agreement with state-of-the art measurements
Points fixes d’opérateur de Shapley sans paiement et propriétés structurelles des jeux à paiement moyen
No full textNational audienc
Le radar BASTA, une alternative aux radars pulsés à 95GHz avec de multiples applications
No full textInternational audienceUn radar pour étudier les nuages et le brouillar
Adaptation in a stochastic multi-resources chemostat model
No full textInternational audienceWe are interested in modeling the Darwinian evolution resulting from the interplay of phenotypic variation and natural selection through ecological interactions, in the specific scales of the biological framework of adaptive dynamics. Adaptive dynamics so far has been put on a rigorous footing only for direct competition models (Lotka-Volterra models) involving a competition kernel which describes the competition pressure from one individual to another one. We extend this to a multi-resources chemostat model, where the competition between individuals results from the sharing of several resources which have their own dynamics. Starting from a stochastic birth and death process model, we prove that, when advantageous mutations are rare, the population behaves on the mutational time scale as a jump process moving between equilibrium states (the polymorphic evolution sequence of the adaptive dynamics literature). An essential technical ingredient is the study of the long time behavior of a chemostat multi-resources dynamical system. In the small mutational steps limit this process in turn gives rise to a differential equation in phenotype space called canonical equation of adaptive dynamics. From this canonical equation and still assuming small mutation steps, we prove a rigorous characterization of the evolutionary branching points
On the origin of falling-tone chorus elements in Earth's inner magnetosphere
No full textInternational audienceGeneration of extremely/very low frequency (ELF/VLF) chorus waves in Earth's inner magnetosphere has received increased attention recently because of their significance for radiation belt dynamics. Though past theoretical and numerical models have demonstrated how rising-tone chorus elements are produced, falling-tone chorus element generation has yet to be explained. Our new model proposes that weak-amplitude falling-tone chorus elements can be generated by magnetospheric reflection of rising-tone elements. Using ray tracing in a realistic plasma model of the inner magnetosphere, we demonstrate that rising-tone elements originating at the magnetic equator propagate to higher latitudes. Upon reflection there, they propagate to lower L-shells and turn into oblique falling tones of reduced power, frequency, and bandwidth relative to their progenitor rising tones. Our results are in good agreement with comprehensive statistical studies of such waves, notably using magnetic field measurements from THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft. Thus, we conclude that the proposed mechanism can be responsible for the generation of weak-amplitude falling-tone chorus emissions
Améliorer les prévisions de la dispersion d'un panache volcanique de dioxyde de soufre par assimilation progressive d'observations satellitaires.
No full textInternational audienceForecasting the dispersal of volcanic clouds during an eruption is of primary importance, especially for ensuring aviation safety. As volcanic emissions are characterized by rapid variations of emission rate and height, the (generally) high level of uncertainty in the emission parameters represents a critical issue that limits the robustness of volcanic cloud dispersal forecasts. An inverse modeling scheme, combining satellite observations of the volcanic cloud with a regional chemistry-transport model, allows reconstructing this source term at high temporal resolution. We demonstrate here how a progressive assimilation of freshly acquired satellite observations, via such an inverse modeling procedure, allows for delivering robust sulfur dioxide (SO2) cloud dispersal forecasts during the eruption. This approach provides a computationally cheap estimate of the expected location and mass loading of volcanic clouds, including the identification of SO2-rich parts
The place of agricultural sciences in the literature on ecosystem services
No full textCABI:20153168804International audienceWe performed a quantitative and qualitative analysis of the scientific literature on ecosystem services in order to help tracing a research agenda for agricultural sciences. The ecosystem services concept now lies at the heart of current developments to address global environmental change. Do agricultural sciences generate knowledge that covers this emerging theme? An analysis of scientific production allowed us to return to the ecological origins of this concept and see how little it has been appropriated by agricultural sciences until now, despite major focus on the issue of agro-ecosystems in the literature. Agricultural sciences tend to be more active in the field of environmental services, defined as services rendered by humans to ecosystems. The main studied services are those which have already been clearly identified and which act in synergy. Less attention is paid to the antagonisms between different services. These findings call for the implementation of agricultural research programmes that will consider the socio-agro-ecosystem as a whole and broaden the traditional issues addressed by agricultural sciences. We insist on three main management and operational issues that needs to be overcome if this is to be done: working at the landscape scale, increasing inter-disciplinary collaborations and take uncertainties into account