Exciton dynamics in WSe2 bilayers

We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with $\tau_{D}$ ~ 3 ps and the indirect optical transition with $\tau_{i}$ ~ 25 ps. For temperatures between 4 to 150 K $\tau_{i}$ remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignment during the short emission time $\tau_{D}$. The evolution of the direct exciton polarization and intensity as a function of excitation laser energy is monitored in PL excitation (PLE) experiments.Comment: 4 pages, 3 figure

Walking into the sunset : how criminal achievement shapes the desistance process

According to the criminal career paradigm, the link between past and future criminal activities is important and the desistance process may vary for individuals whose criminal trajectories were punctuated by failure compared to those who achieved a certain level of success. This study, based on the life narratives of 27 individuals who maintained a state of non-offending for more than a year, examines how criminal achievement modulates the desistance process. The aim is to understand whether criminal achievement acts as a barrier or a facilitator in the desistance process. A short questionnaire based on the life-history calendar method was used to classify individuals according to the parameters of their criminal careers. Narrative life stories were then used to look at the obstacles and frustrations encountered during desistance. The results show the relation between criminal achievement and desistance is complex: success in criminal activities is not always hindering desistance

Exocomets in the circumstellar gas disk of HD 172555

The source HD172555 is a young A7V star surrounded by a debris disk with a gaseous component. Here, we present the detection of variable absorption features detected simultaneously in the Ca II K and H doublet lines (at 3,933 and 3,968 Angstrom). We identified the presence of these absorption signatures at four different epochs in the 129 HARPS high-resolution spectra gathered between 2004 and 2011. These transient absorption features are most likely due to Falling Evaporating Bodies (FEBs, or exocomets) that produce absorbing gas observed transiting in front of the central star. We also detect a stable Ca II absorption component at the star's radial velocity. With no corresponding detection in the Na I line, the resulting very low upper limit for the NaI/CaII ratio suggests that this absorption is due to circumstellar gas.Comment: Accepted for publication in Astronomy&Astrophysics Letter

Physically Based Rigid Registration of 3-D Free-Form Objects : application to Medical Imaging

The registration of 3-D objects is an important problem in computer vision and especially in medical imaging. It arises when data acquired by different sensors and/or at different times have to be fused. Under the basic assumption that the objects to be registered are rigid, the problem is to recover the six parameters of a rigid transformation. If landmarks or common characteristics % between both objects to register are not available, the problem has to be solved by an iterative method. However such methods are inevitably attracted to local minima. This paper presents a novel iterative method designed for the rigid registration of 3-D objects. Its originality lies in its physical basis: instead of minimizing an energy function with respect to the parameters of the rigid transformation (the classical approach) the minimization is achieved by studying the motion of a rigid object in a potential field. In particular, we consider the kinetic energy of the solid during the registration process, which allows it to «jump over» some local maxima of the potential energy and so avoid some local minima of that energy. We present extensive experimental results on real 3-D medical images. In this particular application, we perform the matching process with the whole segmented volumes

Threshold selection, mitosis and dual mutation in cooperative co-evolution: application to medical 3d tomography

International audienceWe present and analyse the behaviour of specialised operators designed for cooperative coevolution strategy in the framework of 3D tomographic PET reconstruction. The basis is a simple cooperative co-evolution scheme (the "fly algorithm"), which embeds the searched solution in the whole population, letting each individual be only a part of the solution. An individual, or fly, is a 3D point that emits positrons. Using a cooperative co-evolution scheme to optimize the position of positrons, the population of flies evolves so that the data estimated from flies matches measured data. The final population approximates the radioactivity concentration. In this paper, three operators are proposed, threshold selection, mitosis and dual mutation, and their impact on the algorithm efficiency is experimentally analysed on a controlled test-case. Their extension to other cooperative co-evolution schemes is discussed

Detection of seismic site effects: Development of experimental methods and application to the city of Nice

Les effets de site représentent un enjeu important pour la prévention parasismique, puisqu’ils peuvent aggraver considérablement les dommages lors d’un séisme. La connaissance de la réponse des sols aux séismes permet d’adapter la réglementation parasismique à ces contraintes lors de l’élaboration de microzonages et de Plan de Prévention des risques. Les méthodes expérimentales de détermination des effets de site visent à obtenir les paramètres de l’amplification sismique par la mesure de séismes ou de bruit de fond sismique. Cet article présente les principaux résultats obtenus jusqu’en 2005 par l’ERA « Risque sismique » dans la mise au point de ces techniques et leur application comparée sur la ville de Nice. Cette ville a effectivement constitué le principal chantier d’application grâce à de multiples campagnes d’enregistrements sismiques. De nombreux séismes ont d’abord été analysés par la méthode des fonctions de transfert. La méthode « H/V bruit de fond » a été appliquée sur plus de 600 points et les résultats interpolés spatialement. Un modèle géotechnique du sous-sol a ensuite permis de comparer les résultats obtenus avec la géologie et des simulations numériques de la propagation des ondes. La complémentarité des méthodes et leurs atouts respectifs sont mis en valeur sur ce territoire à fort enjeu.Site effects represent a critical challenge in the field of earthquake prevention since these effects are capable of seriously exacerbating damage whenever an earthquake strikes. Knowledge of the soil response to seismic activity allows adapting earthquake protection regulations to better incorporate these constraints during the process of defining microzones and producing the Risk Prevention Plan. The experimental methods employed to determine site effects seek to obtain seismic amplification parameters by means of measuring earthquakes or seismic background noise. This article presents the main set of results derived until 2005 by the “Seismic risk” ERA research team during development of these techniques as well as their comparative application to Nice, a city that has constituted the main field application site thanks to its hosting of multiple seismic recording campaigns. For starters, many earthquakes have been analyzed according to the transfer function method. The “Horizontal/Vertical background noise” method was in particular applied to over 600 points, with results being spatially interpolated. A geotechnical model of the subsoil then served to compare the results output with both the geology and numerical simulations of wave propagation. The complementarity of methods and their respective advantages are highlighted in this seismically-active setting

A Palaeozoic open shelf benthic assemblage in a protected marine environment

International audienceA Late Ordovician benthic assemblage containing trilobites (Colpocoryphe, Onnia) and diploporid echinoderms (Aristocystites) taxa that are usually typical of open shelf environments is preserved in protected marine settings in the lower part of the Kermeur Formation (Armorican Massif, NW France). A facies analysis carried out on the Veryac'h section (Crozon Peninsula) allows identification of four sedimentary facies (SF), ranging from the bay/lagoon domain (SF1 and SF2) to the back-barrier (SF3) and barrier (SF4) domains. A benthic fauna comprising of trilobites, brachiopods and diploporids is found in the SF1 and SF2. In SF1, fossils are randomly scattered in sediments (taphofacies 1, TF1) showing two different preservation stages: TF1a with complete articulated skeletons and TF1b with disarticulated shells. These two preservation stages can be related either to the different modes of life of the taxa (i.e., recumbent, burrowing or nektobenthic) or to a composite assemblage with both autochthonous and allochthonous taxa. In this benthic assemblage, some trilobites and echinoderms taxa are mostly preserved either as complete exoskeletons and exuviae (Colpocoryphe grandis) or complete thecae (Aristocystites sp.), indicating an autochthonous origin. These taxa are usually found in low-energy open shelf settings. Their occurrence in a protected environment results from the combination of both allocyclic factors (i.e., change in sea level) and local conditions (i.e., shelf morphology, environmental conditions). Indeed, the lower part of the Kermeur Formation was deposited during a transgressive phase following a significant sea-level fall (Sandbian-Katian boundary). During the transgressive phase, barrier-lagoon systems were formed on the flat surface inherited from the sandbian shelf. Under non-restricted ecological conditions (i.e., normal oxygenation and salinity) in the protected settings, the new ecological niches were suitable for benthic organisms accustomed to open shelf environments. The settlement of these open shelf benthic species in coastal and protected environments may represent an offshore-onshore expansion, contrary to that described in the onshore-offshore diversification model of Phanerozoic shelf communities (Jablonski et al., 1983; Sepkoski, 1991)

Exciton states in monolayer MoSe2: impact on interband transitions

We combine linear and non-linear optical spectroscopy at 4K with ab initio calculations to study the electronic bandstructure of MoSe2 monolayers. In 1-photon photoluminescence excitation (PLE) and reflectivity we measure a separation between the A- and B-exciton emission of 220 meV. In 2-photon PLE we detect for the A- and B-exciton the 2p state 180meV above the respective 1s state. In second harmonic generation (SHG) spectroscopy we record an enhancement by more than 2 orders of magnitude of the SHG signal at resonances of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our post-Density Functional Theory calculations show in the conduction band along the $K-\Gamma$ direction a local minimum that is energetically and in k-space close to the global minimum at the K-point. This has a potentially strong impact on the polarization and energy of the excitonic states that govern the interband transitions and marks an important difference to MoS2 and WSe2 monolayers.Comment: 8 pages, 3 figure

The Impact of Ly49-NK Cell-Dependent Recognition of MCMV Infection on Innate and Adaptive Immune Responses

Clinical and experimental data indicate that a subset of innate lymphocytes, natural killer (NK) cells, plays a crucial role in the response against herpesviruses, especially cytomegaloviruses (CMV). Indeed, in mice, NK cells, due to the expression of germline encoded Ly49 receptors, possess multiple mechanisms to recognize CMV infection. Classically, this results in NK cell activation and the destruction of the infected cells. More recently, however, this unique host-pathogen interaction has permitted the discovery of novel aspects of NK cell biology, implicating them in the regulation of adaptive immune responses as well as in the development of immunological memory. Here, we will concisely review the newly acquired evidence pertaining to NK cell Ly49-dependent recognition of MCMV-infected cell and the ensuing NK cell regulatory responses

Comparison of the internalization efficiency of LDL and transferrin receptors on L2C guinea pig lymphocytes

AbstractWe demonstrate that L2C lymphocytes have about 10-times more receptors for transferrin (TO than healthy lymphocytes, as has been shown in the case of LDL receptors. The dissociation constant is the same in the two cell types (about 4 × 10−7 M). In contrast to LDL, Tf enters L2C lymphocytes with very rapid kinetics. It is shown by cross-reaction that each receptor is internalized independently of the other