243 research outputs found
Histoire et humanités numériques : nouveaux terrains de dialogue entre les archives et la recherche
Comme dans toute discipline au contact de nouvelles technologies, on assiste au sein des sciences historiques, et plus largement au sein des sciences humaines et sociales, Ă un questionnement sur lâadoption de nouvelles pratiques de recherche. Alors que les outils numĂ©riques offrent des perspectives dâanalyse renouvelĂ©es â des mĂ©thodes quantitatives complexes aux bases de donnĂ©es multimĂ©dia en passant par la visualisation â ceux-ci modifient Ă©galement les canaux de diffusion et de popularisation du patrimoine, quand ils ne deviennent pas eux-mĂȘmes de nouveaux objets dâĂ©tude, Ă lâimage du Web ou des mĂ©dias sociaux. Sous le titre dâ« humanitĂ©s numĂ©riques » se rassemble depuis quelques annĂ©es une communautĂ© dâindividus et dâinstitutions qui se prĂ©occupent de ces transformations et sâapproprient petit Ă petit un outillage numĂ©rique transdisciplinaire. Sur le terrain, loin dâune rĂ©volution annoncĂ©e avec grandiloquence, câest par lâexpĂ©rimentation et la collaboration, pas Ă pas, que ces nouvelles modalitĂ©s se mettent en place
SAMCO: Adaptation de la société aux risques en montagne dans un contexte de changement global
National audienceLe projet SAMCO (Society Adaptation for coping with Mountain risks in a global change COntext, 2013-2016) a pour but de développer un ensemble d'outils méthodologiques permettant d'améliorer la résilience des sociétés soumises aux impacts des risques en milieux montagneux. Ces outils visent à mesu-rer la capacité d'adaptation des écosystÚmes et des sociétés à partir de l'analyse de cas réels provenant de 3 sites pilotes représentatifs
Bell inequalities for three systems and arbitrarily many measurement outcomes
We present a family of Bell inequalities for three parties and arbitrarily
many outcomes, which can be seen as a natural generalization of the Mermin Bell
inequality. For a small number of outcomes, we verify that our inequalities
define facets of the polytope of local correlations. We investigate the quantum
violations of these inequalities, in particular with respect to the Hilbert
space dimension. We provide strong evidence that the maximal quantum violation
can only be reached using systems with local Hilbert space dimension exceeding
the number of measurement outcomes. This suggests that our inequalities can be
used as multipartite dimension witnesses.Comment: v1 6 pages, 4 tables; v2 Published version with minor typos correcte
SAMCO: Society adaptation for coping with mountain risks in a global change context
International audienceThe SAMCO project is intended to contribute to the development of a proactive resilience framework enhancing the overall resilience of societies on the impacts of mountain risks
Unifying the Framework of Multi-Layer Network and Visual Analytics
International audienceThe notion of multi-layer networks introduces a general framework and common vocabulary for existing ideas in complex network theory. In doing so, it is possible to understand and compare these dierent ideas in a new and more fruitful manner. However, to make this operationalizable to the visualization and visual analytics community, we need more clarity. For example: What is a layer? What are the semantics of interlayer edges, and specifically, identity links between layers? Can dierent multilayered networks be expressed or implemented in the same way? And vice versa, can one multilayered network be expressed or implemented in dierent ways
Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence.
International audienceWe present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs
Thermal carrier emission and nonradiative recombinations in nonpolar(Al,Ga)N/GaN quantum wells grown on bulk GaN.
International audienceWe investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320K range. We first show that the effective lifetime of QW excitons tau increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing tau depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs
Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities
We report on the direct observation of biexcitons in a III nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a
more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.This work was supported by the NCCR Quantum Photonics, research instrument of the Swiss National Science Foundation, through Grant No. 129715 and Grant No. 200020-113542, and by the EU-project Clermont4 (Grant No. FP7-235114)
Joined optical and thermal characterization of a III-nitride semiconductor membrane by micro-photoluminescence spectroscopy and Raman thermometry
We present the simultaneous optical and thermal analysis of a freestanding
photonic semiconductor membrane made from wurtzite III-nitride material. By
linking micro-photoluminescence (PL) spectroscopy with Raman thermometry,
we demonstrate how a robust value for the thermal conductivity can be
obtained using only optical, non-invasive means. For this, we consider the
balance of different contributions to thermal transport given by, e.g.,
excitons, charge carriers, and heat carrying phonons. Further complication is
given by the fact that this membrane is made from direct bandgap
semiconductors, designed to emit light based on an InGaN
() quantum well embedded in GaN. To meet these challenges, we designed
a novel experimental setup that enables the necessary optical and thermal
characterizations in parallel. We perform micro-Raman thermometry, either based
on a heating laser that acts as a probe laser (1-laser Raman thermometry), or
based on two lasers, providing the heating and the temperature probe separately
(2-laser Raman thermometry). For the latter technique, we obtain temperature
maps over tens of micrometers with a spatial resolution less than
, yielding
for the
-plane of our -thick membrane at around
room temperature, which compares well to our calculations
applied to a simplified structure. Based on these calculations, we explain the
particular relevance of the temperature probe volume, as quasi-ballistic
transport of heat-carrying phonons occurs on length scales beyond the
penetration depths of the heating laser and even its focus spot radius. The
present work represents a significant step towards non-invasive, highly
spatially resolved, and still quantitative thermometry performed on a photonic
membrane.Comment: 28 pages, 14 figures, and Supplemental Materia
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