New data on Angustiphyllum Altevogt 1965, an automobile free-living rugose coral from the Eifelian of the Western Armorican Massif (France)

accessible à l'adresse suivante : http://popups.ulg.ac.be/Geol/document.php?id=3761International audienceThe genus Angustiphyllum is recorded for the first time in the Armorican Massif (Rade de Brest) and two new species are erected: A. stylophorum and A. vidalae. The well preserved material shows the indisputable presence of costae on the exterior side of the corallum, which, besides the wedge-like morphology of the corallum, constitutes the main diagnostic feature of the genus. Angustiphyllum seems to be a good index for the Upper Eifelian of the Ibero-Armorican Domain

Hybrid chiral domain walls and skyrmions in magnetic multilayers

Noncollinear spin textures in ferromagnetic ultrathin films are currently the subject of renewed interest since the discovery of the interfacial Dzyaloshinskii-Moriya interaction (DMI). This antisymmetric exchange interaction selects a given chirality for the spin textures and allows stabilising configurations with nontrivial topology. Moreover, it has many crucial consequences on the dynamical properties of these topological structures, including chiral domain walls (DWs) and magnetic skyrmions. In the recent years the study of noncollinear spin textures has been extended from single ultrathin layers to magnetic multilayers with broken inversion symmetry. This extension of the structures in the vertical dimension allows very efficient current-induced motion and room-temperature stability for both N\'eel DWs and skyrmions. Here we show how in such multilayered systems the interlayer interactions can actually lead to more complex, hybrid chiral magnetisation arrangements. The described thickness-dependent reorientation of DWs is experimentally confirmed by studying demagnetised multilayers through circular dichroism in x-ray resonant magnetic scattering. We also demonstrate a simple yet reliable method for determining the magnitude of the DMI from static domains measurements even in the presence of these hybrid chiral structures, by taking into account the actual profile of the DWs. The advent of these novel hybrid chiral textures has far-reaching implications on how to stabilise and manipulate DWs as well as skymionic structures in magnetic multilayers.Comment: 22 pages, 5 figure

Combinaison cohérente de lasers à fibre (étude en régime impulsionnel et mise en phase d'un grand nombre d'émetteurs)

Les lasers à fibre suscitent un grand intérêt dans tous les domaines, pour des applications allant du médical aux télécommunications, car ils permettent d'obtenir une bonne cohérence spatiale et temporelle et sont compacts et robustes. La limite se situe dans le domaine des lasers à fortes puissances. La combinaison de plusieurs sources lasers est alors une solution prometteuse pour surpasser les limites individuelles de ces sources, en particulier les techniques de combinaisons cohérentes de faisceaux. Dans cette famille de combinaison, la technique de contrôle actif de la phase par marquage en fréquence est particulièrement intéressante, mais deux limitations majeures ont été identifiées : le cas du régime impulsionnel ainsi que le nombre limité de voies pouvant être mise en phase. Dans cette thèse des solutions sont proposées pour lever ces limitations. Dans un premier temps, nous avons réalisé la première démonstration expérimentale d'une combinaison cohérente impulsionnelle utilisant cette technique. Ensuite, en adaptant des techniques de codage orthogonal issues du domaine des télécommunications, nous avons augmenté le nombre de voies pouvant être combinées. Un outil de simulation du système a été développé pour étudier l'utilisation des techniques de codage et pour étudier la propagation des faisceaux à travers la turbulence atmosphérique. Pour évaluer les performances des systèmes à grand nombre d'émetteur, nous avons mis en place une méthodologie, basée sur l'utilisation de plans d'expériences et de métamodèles, permettant d'identifier les fibres ou les interactions entre fibres les plus influentes, en fonction des paramètres initiaux de la combinaison.A great interest exists for fibre lasers in all domains, from medical to telecommunication applications, as they enable to achieve a good spatial and spectral coherence and they are compact and robust. The limit comes from high-power applications. The combination of several laser sources is therefore a solution to overcome the limitations of individual emitter, especially coherent combining techniques. In this combination form, the active phase control by frequency tagging technique looks the most promising one, but two main limitations were identified : the pulse regime and the limited number of channels, which can be phase-locked. In this thesis, solutions are proposed to overcome these limitations. At first, we realised the first experimental demonstration of a pulse coherent combination using this technique. Then by adapting orthogonal coding tehniques from the telecommunication domain, we increased the number of channels which can be combined. A simulation tool was developped to study the coding techniques as well as the propagation of the beams through a turbulent atmosphere. To evaluate the performances of systems with a large number of emitters, we developped a methodology based on the use of numerical designs of experiments and metaodels, enabling us to identify the most influent fibres or interactions between fibres, as a function of the initial parameters of the combination.PARIS-Télécom ParisTech (751132302) / SudocSudocFranceF

Quantization of Neural Network Equalizers in Optical Fiber Transmission Experiments

The quantization of neural networks for the mitigation of the nonlinear and components' distortions in dual-polarization optical fiber transmission is studied. Two low-complexity neural network equalizers are applied in three 16-QAM 34.4 GBaud transmission experiments with different representative fibers. A number of post-training quantization and quantization-aware training algorithms are compared for casting the weights and activations of the neural network in few bits, combined with the uniform, additive power-of-two, and companding quantization. For quantization in the large bit-width regime of $\geq 5$ bits, the quantization-aware training with the straight-through estimation incurs a Q-factor penalty of less than 0.5 dB compared to the unquantized neural network. For quantization in the low bit-width regime, an algorithm dubbed companding successive alpha-blending quantization is suggested. This method compensates for the quantization error aggressively by successive grouping and retraining of the parameters, as well as an incremental transition from the floating-point representations to the quantized values within each group. The activations can be quantized at 8 bits and the weights on average at 1.75 bits, with a penalty of $\leq 0.5$~dB. If the activations are quantized at 6 bits, the weights can be quantized at 3.75 bits with minimal penalty. The computational complexity and required storage of the neural networks are drastically reduced, typically by over 90\%. The results indicate that low-complexity neural networks can mitigate nonlinearities in optical fiber transmission.Comment: 15 pages, 9 figures, 5 table

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four of today's most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts (Fe–N–Cs). Our analysis reaches far beyond previous such attempts in linking kinetic performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activity with previously elusive kinetic metrics such as the active metal site density (SD) and the catalytic turnover frequency (TOF). Kinetic ORR activities, SD and TOF values were evaluated using in situ electrochemical NO2− reduction as well as an ex situ gaseous CO cryo chemisorption. Experimental ex situ and in situ Fe surface site densities displayed remarkable quantitative congruence. Plots of SD versus TOF (“reactivity maps”) are utilized as new analytical tools to deconvolute ORR reactivities and thus enabling rational catalyst developments. A microporous catalyst showed large SD values paired with low TOF, while mesoporous catalysts displayed the opposite. Trends in Fe surface site density were linked to molecular nitrogen and Fe moieties (D1 and D2 from 57Fe Mössbauer spectroscopy), from which pore locations of catalytically active D1 and D2 sites were established. This cross-laboratory analysis, its employed experimental practices and analytical methodologies are expected to serve as a widely accepted reference for future, knowledge-based research into improved PGM-free fuel cell cathode catalysts.EC/H2020/779366/EU/Critical Raw material ElectrocatalystS replacement ENabling Designed pOst-2020 PEMFC/CRESCENDOTU Berlin, Open-Access-Mittel - 202

Skyrmions in Magnetic Multilayers: Chirality, Electrical Detection and Current-induced Motion

Sub-100-nm skyrmions are stabilized in magnetic metallic multilayers and observed using transmission electron microscopy, magnetic force microscopy, scanning transmission X-ray microscopy and X-ray resonant magnetic scattering. All these advanced imaging techniques demonstrate the presence of 'pure' Neel skyrmion textures with a determined chirality. Combining these observations with electrical measurements allows us to demonstrate reproducible skyrmion nucleation using current pulses, and measure their contribution to the transverse resistivity to detect them electrically. Once nucleated, skyrmions can be moved using charge currents. We find predominantly a creep-like regime, characterized by disordered skyrmion motion, as observed by atomic force microscopy and scanning transmission X-ray microscopy. These observations are explained qualitatively and to some extent quantitatively by the presence of crystalline grains of about 20nm lateral size with a distribution of magnetic properties

Sécurité de l’information en PME : les déterminants des actions de protection et de soutien des dirigeants

This research addresses the determinants of CEOs’ actions regarding the information security (ISS) of small and medium enterprises (SMEs). This article aims to (a) identify factors influencing CEOs’ ISS actions, (b) examine the relevance of protection motivation theory (PMT) in explaining top management support (TMS, i.e., supportive actions), and (c) find potential differentiated effects on protective vs. supportive actions.The results of a questionnaire-based survey (N=200) show that the PMT and social influence constructs, while explaining a significant amount of variance, exert differentiated effects: in contrast with protective actions, which are influenced mainly by self-efficacy, SME CEOs’ supportive actions are strongly affected by the social influence of peers (partners and competitors) and customers.At a theoretical level, this research validates the relevance of the PMT framework for the study of TMS determinants in the context of ISS. This study is also the first to distinguish between these two types of actions and offers new insights on CEOs’ ISS-related behavior literature. For practitioners, the results imply that even when CEOs do not exert protective actions, it is important to build on their professional relations to trigger and enhance their supportive actions.Cette recherche porte sur les déterminants des comportements en sécurité de l’information (SSI) des dirigeants de PME, en distinguant les actions de protection des actions de soutien. Cet article vise à (a) identifier certains des facteurs qui influencent leurs comportements en SSI, (b) examiner la pertinence de la Protection Motivation Theory (PMT) pour expliquer le Top Management Support (TMS, c.-à-d. le soutien du dirigeant) et (c) mettre en évidence d’éventuels effets différenciés selon les actions de protection et de soutien.Les résultats, provenant d’une étude quantitative (N=200), montrent que les construits de la PMT et l’influence sociale expliquent une part significative de la variance, mais exercent également des effets différenciés : alors que les actions de protection sont prioritairement influencées par l’auto-efficacité, les actions de soutien des dirigeants sont essentiellement affectées par l’influence sociale des pairs (partenaires et concurrents) et des clients.Au niveau théorique, cette recherche établit la pertinence de la PMT pour étudier les déterminants du TMS dans le contexte de la SSI. Cette étude est également la première qui distingue ces deux types d’actions et offre ainsi de nouveaux éléments de compréhension des comportements des dirigeants de PME en SSI. Pour les praticiens, nos résultats mettent en évidence que même si les dirigeants n’agissent pas directement, il est important de prendre en considération l’entourage professionnel du dirigeant de PME de manière à développer ses actions de soutien

Optimisation des architectures de réseau optique multiplexé en longueur d'onde à base d'amplificateur optique semi-conducteur

PARIS-Télécom ParisTech (751132302) / SudocSudocFranceF