Ref HAL: hal-01308220_v1
Ref Arxiv: 1510.06716
DOI: 10.1103/PhysRevB.92.235308
WoS: 000367064700005
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7 Citations
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The formation and propagation of a polariton condensate under tightly focused excitation is investigated in a ZnO microcavity both experimentally and theoretically. 2D near-field and far-field images of the condensate are measured under quasi-continuous non-resonant excitation. The corresponding spatial profiles are compared to a model based on the Gross-Pitaevskii equation under cylindrical geometry. This work allows to connect the experiments performed with a small excitation laser spot and the previous kinetic models of condensation in a 2D infinite microcavity, and to determine the relevant parameters of both the interaction and the relaxation between the reservoir and the condensate. Two main parameters are identified: the exciton-photon detuning through the polariton effective mass and the temperature, which determines the efficiency of the relaxation from the reservoir to the condensate.

Commentaires: 13 pages, 3 tables and 9 figures. Réf Journal: Phys. Rev. B 92, 235308 (2015)

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Theory of crystallization and structure of small virus capsids and ribonucleoprotein nanoparticles

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An indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character. Current research on transport properties of IXs is opening a pathway to the development of novel optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1-3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly promising because of much larger exciton binding energies and natural dipoles induced by strong built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (µ-PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long distances from the excitation spot (30 < r < 100µm) turned out to be a secondary PL, excited by the light emitted at the central spot, guided along the plane, due to the refractive index contrast between the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance reached by the measurable PL is limited by recombination of excitons at non-radiative defects. Using GaN substrates instead of sapphire should both suppress the secondary emission and the nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures, except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed observe the mere propagation of excitons up to 35 µm away from the excitation spot and up to 250 K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole-dipole repulsion in high-density regions and for disorder along the sample plane.[1]Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A. C. Gossard. Optics Letters 35 (10), 1587 (2010).[2]A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard. Science 321, 229 (2008).[3]A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard. Optics Letters 32, 2466 (2007).[4]F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T. Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre. Phys. Rev. B 91, 205424 (2015).

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This lecture will present the specific features of GaN- and ZnO-based quantum wells that make them interesting candidates for the creation and manipulation of dipolar, indirect excitons. Conceived as a comparison with what is currently obtained in biased GaAs-based structures, the presentation will emphasize the consequences of a number of properties such as: (i) much larger exciton binding, (ii) much larger Mott densities, (iii) built-in electric fields larger by one order of magnitude, (iv) the subsequent radiative lifetimes that can easily reach several tens of µs… Actual material properties that can be considered detrimental, at present, will also be discussed, like the current crystalline quality of the materials, inducing high densities of nonradiative defects.Theoretical and experimental results will be commented, in order to illustrate the current understanding of the physics of the dipolar excitons that appear naturally in these systems, with no need of an externally applied electric field. In particular, the change of exciton energies under high-density conditions will be discussed. The complex recombination dynamics, measured by time-resolved spectroscopy, will be explained. Recent results on the in-plane propagation of those dipolar excitons will be shown.

Ref HAL: hal-01241397_v1
DOI: 10.1515/corrrev-2015-0040
WoS: 000364578400019
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Fracture propagation involves the coupling of many length scales ranging from the sample loading geometry to the molecular level. In brittle materials, the length scales of the damage process zone are reduced to a submicrometric scale and the coupling with the macroscopic scale is expected to be the domain of linear elastic fracture mechanics (LEFM). However, although 2D elastic analyses are generally adequate to describe the sample deformation at macroscopic scales, local investigations of failure mechanisms at the sample free surface require the use of 3D mechanical tools due to the crack front local curvature and to the corner point singularities at the intersection between the crack front and the external surfaces of the sample. We present here a thorough multiscale investigation of-2-the slow crack growth of a sharp crack in oxide glasses in the stress-corrosion regime, combining experimental and numerical analyses of the displacement fields from the millimeter scale to the nanoscale range. The principal aim of the study is identifying the length and time scales of the mechanisms of damage and interaction between water and glass, which have been the subject of an extensive debate in the last decades.

Ref HAL: hal-01300109_v1
PMID 25241396
DOI: 10.1016/j.cortex.2014.08.010
WoS: 000349573300005
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44 Citations
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Right brain damage often provokes deficits of visuospatial attention. Although the spatial attention networks have been widely investigated in stroke patients as well as in the healthy brain, little is known about the impact of slow growing lesions in the right hemisphere. We here present a longitudinal study of 20 patients who have been undergoing awake brain surgery with per-operative line bisection testing. Our aim was to investigate the impact of tumour presence and of tumour resection on the functional (re)organization of the attention networks. We assessed patients' performance on lateralized target detection, visual exploration and line bisection before surgery, and in the acute and post-acute operative phases after surgery. Clear evidence for transient neglect signs was observed in the acute post-operative phase, although full recovery had invariably occurred in all patients. The resection of the right angular gyrus was associated with transient neglect-like symptoms in all tasks, whereas resection of more anterior regions correlated with transient deficits only in visual exploration or detection (but not in line bisection). The attentional networks showed substantial functional recovery. This impressive pattern of recovery is discussed in terms of involvement of the contralateral left hemisphere and of preservation of long-range white matter pathways within the right hemisphere.

Ref HAL: hal-01297480_v1
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Les nanocomposites de polymère sont des matériaux dont la matrice polymère est renforcée par des nanoparticules (NPs), comme par exemple des NPs de silice. Pouvoir contrôler la structuration de ces dernières au sein de la matrice est un vieux rêve, parce que l’on espère pouvoir ajuster les propriétés macroscopiques via la microstructure. Dans un premier temps, nous montrerons quelques exemples d’analyse structurale de nanocomposites par diffusion de rayonnement aux petits angles, simulation numérique, et microscopie électronique [1]. Ces études mettent en évidence le caractère en partie hors équilibre de ces structures. Ces dernières années, un paramètre de contrôle des structuration des NPs à l’équilibre est apparu [2-4] : le greffage de chaînes de polymère à la surface des NPs. Il permet de contrôler l’agrégation des NPs, et d’obtenir des structures anisotropes essentiellement visualisées par TEM. Même si cela n’a pas encore trouvé d’écho dans la littérature de mode, la coiffure semble donc être liée au rangement, au moins pour les toutes petites billes. Pour finir, nous discuterons d’un modèle structural développé pour des systèmes plus proches des applications industrielles, et nettement moins enclins à une structuration ordonnée, malgré les efforts de greffage [5]. Références: 1 A. Banc, A.-C. Genix, M. Chirat, C. Dupas, S. Caillol, M. Sztucki, J. Oberdisse, Macromolecules 2014, 47 (9), 3219–32302 A. El Harrak, G. Carrot, J. Oberdisse, Ch. Eychenne-Baron, F. Boué, Macromolecules 2004, 37, 6376-63843 P. Akcora et al, Nature Mat 2009, 8, 354-3594 S. Kumar, N. Jouault, B. Benicewicz, T. Neely Macromolecules 2013, 46, 3199-32145 G. P. Baeza, A.-C. Genix, C. Degrandcourt, L. Petitjean, J. Gummel, M. Couty, J. Oberdisse, Macromolecules 2013, 46, 317−329