Magnetic excitations in a new anisotropic Kagom\'{e} antiferromagnet

The Nd-langasite compound contains planes of magnetic Nd3+ ions on a lattice topologically equivalent to a kagom\'{e} net. The magnetic susceptibility does not reveal any signature of long-range ordering down to 2 K but rather a correlated paramagnetism with significant antiferromagnetic interactions between the Nd and a single-ion anisotropy due to crystal field effect. Inelastic neutron scattering on Nd-langasite powder and single-crystal allowed to probe its very peculiar low temperature dynamical magnetic correlations. They present unusual dispersive features and are broadly localized in wave-vector Q revealing a structure factor associated to characteristics short range-correlations between the magnetic atoms. From comparison with theoretical calculations, these results are interpreted as a possible experimental observation of a spin liquid state in an anisotropic kagom\'{e} antiferromagnet.Comment: to appear in Physica

Comportement non linéaire de l'aimantation de FeCl2, FeBr2 et FeI2 en champ perpendiculaire a l'axe d'anisotropie

We present, for ferrous halogenides, FeCl2, FeBr2, FeI2, the magnetization isotherms obtained, at low temperature, in a perpendicular magnetic field (static and pulsed fields). These experimental results show a divergence from the linear magnetization law. We give a natural interpretation of our results by using a rigorous treatment of a theoretical model in which we consider an assembly of triplet ions with a single ion uniaxial anisotropy, the magnitude of which is of the same order as the intralayer ferromagnetic exchange interaction (J1) and the intralayer antiferromagnetic one (J2) (molecular field-approximation). This study allows us to calculate the divergence from the linear law for all values of D, J1 and J2 and shows the fundamental role played by the anisotropy. The theoretical model gives a more satisfactory explanation of the experimental results obtained for FeCl 2, FeBr2 and FeI2 than given by previous models.Nous présentons, pour les halogénures ferreux FeCl2, FeBr 2, FeI2, les isothermes d'aimantation obtenues, à basse température, en champ magnétique perpendiculaire (champ statique et champ pulsé). Ces résultats expérimentaux mettent en évidence un écart à la loi linéaire d'aimantation. Nous donnons une interprétation naturelle de nos résultats par le traitement rigoureux d'un modèle théorique où l'on considère une assemblée d'ions triplets caractérisés par une anisotropie uniaxiale dont l'amplitude D est du même ordre de grandeur que les interactions d'échange, ferromagnétiques dans le plan (J 1) et antiferromagnétiques entre plans (J2) (approximation du champ moléculaire). Cette étude théorique permet d'évaluer l'écart à la loi linéaire pour toute valeur de D, J1 et J 2 et montre le rôle fondamental joué par l'anisotropie. Ce modèle théorique donne une explication plus satisfaisante des résultats obtenus pour FeCl2, FeBr2 et FeI2 que ceux donnés par les modèles précédents

Improving QoS in Satellite Networks using Cross-Layer Mechanisms

International audienceQuality of service (QoS) support can be enforced across several protocol layers by enhancing specific mechanisms such as resource management, power control, scheduling, queuing models and transport protocols. However, even if designed independently, one single layer adaptation impacts other layers, for example, coding rate adaptation at the physical layer impacts the available rate for transport protocols and can induce buffer overflow and congestion. We argue throughout the present study that collaborative mechanisms are efficient and allow an adaptation to network changes at all layers more than single layer standard mechanisms. The main idea is to use available information from layers that are not adjacent in order to optimize the overall performance of a system. The network is seen as a collaborative layer system, where decisions are taken to increase the QoS offered to end-users. We search to bring to light specific layer mechanisms and illustrate how QoS can be improved on the forward and return link in bent-pipe geostationary satellite systems

On the Virtualization and Dynamic Orchestration of Satellite Communication Services

Key features of satellite communications such as wide-scale coverage, broadcast/multicast support and high availability, together with significant amounts of new satellite capacity coming online, anticipate new opportunities for satellite communications services as an integral part within upcoming 5G systems. To materialize these opportunities, satellite communications services have to be provisioned and operated in a more flexible, agile and cost-effective manner than done today. In this context, this paper describes a solution for the virtualization and dynamic orchestration of satellite communication services that builds on the introduction of Software Defined Networking (SDN) and Network Function Virtualization (NFV) technologies within the satellite ground segment systems. Along with the description of the main system architecture traits, the flowchart of a general procedure for the dynamic instantiation of virtualized satellite networks on top of a SDN/NFV-enabled satellite ground segment system is provided. The paper also presents experimental results for the dynamic customization of satellite network services through the implementation of a set of virtualized satellite network functions that can be orchestrated over general purpose open virtual platforms

Magnetic Frustration on a Kagomé Lattice in R3_{3}Ga5_{5}SiO14_{14} Langasites with R = Nd, Pr

9 pages. 1 table, 5 figures.International audienceIn the R$_{3}$Ga$_{5}$SiO$_{14}$ compounds, the network R of rare earth cations form well separated planes of corner sharing triangles topologically equivalent to a kagomé lattice. Powder samples and single crystals with R = Nd and Pr were prepared and magnetostatic measurements were performed under magnetic field up to 10 T in the temperature range from 1.6 K to 400 K. Analysis of the magnetic susceptibility at the high temperatures where only the quadrupolar term of the crystal electric field prevails, suggests that the Nd and Pr magnetic moments can be modeled as coplanar elliptic rotators perpendicular to the three fold axis of the crystal structure that interact antiferromagnetically. Nonetheless, a disordered phase that can be ascribed to geometric frustration persists down to the lowest temperature which is about 25 times smaller than the energy scale for the exchange interactions

On the Virtualization and Dynamic Orchestration of Satellite Communication Services

Key features of satellite communications such as wide-scale coverage, broadcast/multicast support and high availability, together with significant amounts of new satellite capacity coming online, anticipate new opportunities for satellite communications services as an integral part within upcoming 5G systems. To materialize these opportunities, satellite communications services have to be provisioned and operated in a more flexible, agile and cost-effective manner than done today. In this context, this paper describes a solution for the virtualization and dynamic orchestration of satellite communication services that builds on the introduction of Software Defined Networking (SDN) and Network Function Virtualization (NFV) technologies within the satellite ground segment systems. Along with the description of the main system architecture traits, the flowchart of a general procedure for the dynamic instantiation of virtualized satellite networks on top of a SDN/NFV-enabled satellite ground segment system is provided. The paper also presents experimental results for the dynamic customization of satellite network services through the implementation of a set of virtualized satellite network functions that can be orchestrated over general purpose open virtual platforms

PIETE : étude de la partition entre évaporation du sol et transpiration par utilisation de la géochimie isotopique. Mesures et modélisation

Actual evapotranspiration (ETR) is one of the major component of the hydrological cycle, but also one of the most difficult to measure. Its quantification requires the knowledge of its components: evaporation from soils, transpiration from plants and evaporation of intercepted water. The study focuses on the partition between evaporation and transpiration and is based on the use of stable water isotopes (oxygen 18 and deuterium). Indeed, the isotopic signatures of water vapour produced by evaporation and transpiration are in general different. In combination with the sampling of the isotopic signature of liquid water in the soil and in the plant, its measurement allows the quantification of the importance of each reservoir. The associated processes are studied within the PIETE (Isotopic Partition Evaporation Transpiration Evapotranspiration) using both modelling and laboratory and field experiments. The methodology used is described in this paper and some preliminary results are presented.L`évapotranspiration réelle (ETR) constitue l`un des termes essentiels du cycle hydrologique et l'un des plus difficiles à mesurer. Sa quantification requiert la connaissance de ses composantes: évaporation du sol, évaporation directe de la pluie ou de l'irrigation interceptée par le couvert végétal, transpiration des couverts végétaux. L'étude se concentre sur la séparation entre évaporation directe du sol et transpiration des couverts végétaux et s'appuie sur les isotopes stables de l'eau (oxygène 18 et deutérium). En effet, les signatures isotopiques de la vapeur issue de l'évaporation du sol nu et de la transpiration sont en général distinctes, ce qui permet de quantifier l'importance de chaque réservoir, en combinant les mesures dans la vapeur atmosphérique, dans l'eau du sol et des végétaux. C'est ce que se propose d'étudier le projet PIETE (Partition Isotopique Evaporation Transpiration Evapotranspiration) en combinant modélisation mécaniste et expérimentations de laboratoire et de terrain. La méthodologie utilisée est présentée dans cet article ainsi que quelques résultats préliminaires