Réalisation d'écrans magnétiques supraconducteurs

Low fields and low frequency shielding properties of YBCO magnetic shields are measured at 77 K. They compare favourably with shielding properties of mumetal shields. Therefore high-$T_{\rm c}$ superconducting magnetic shields can already be used to shield small volumes. The case of magnetic shields for large volumes is also discussed.Nous mesurons à 77 K les caractéristiques d'écrantage basse fréquence et bas champ d'écrans supraconducteurs en YBaCuO. Celles-ci se comparent favorablement à celles d'écrans en mumétal. La réalisation pratique d'écrans supraconducteurs est dès lors possible pour l'écrantage de petits volumes. Les géométries de réalisation d'écrans pour les grands volumes sont également discutées

Stretched exponential relaxation in systems with random free energies

A spin glass phase is characterized by a large number of quasi degenerate states (or valley bottoms). Their free energies Fa = F0 + fa, where fa is a non extensive fluctuation, have recently been shown to be random independent variables, as in Derrida's model. Relaxation to equilibrium of such systems is considered and a simple approximation to the transition probability (in the master equation) is considered that leads to a (random) relaxation time τ ∼ exp β ΔF with — β(F0 + ΔF) = In Σ exp( — β Fa). The ensuing relaxation to equilibrium is shown to be a stretched exponential a behaviour common to a wide class of materials. The dependence of the result on the particular choice of the transition probability is touched upon.Un verre de spin est caractérisé par un grand nombre d'états (ou fond de vallées) quasidégénérés. Il a été montré récemment que leurs énergies libres Fa = F0 + fa, où fa est une fluctuation non extensive, se comportent en variables aléatoires indépendantes, comme dans le modèle de Derrida. Nous considérons la relaxation vers l'équilibre de tels systèmes. En particulier nous considérons une simple approximation pour la probabilité de transition (dans l'équation maîtresse) qui conduit à un temps de relaxation aléatoire de la forme τ ∼ exp β Δ F où - β(F0 + AF) = ln Σ exp(— β Fa). La relaxation vers l'équilibre qui en découle, a la forme d'une exponentielle étirée, un comportement commun à une large classe de matériaux. La façon dont un tel résultat dépend du choix de la probabilité de transition, est abordée

Natural Killer (NK) Cells in Antibacterial Innate Immunity: Angels or Devils?

Natural killer (NK) cells were first described as immune leukocytes that could kill tumor cells and soon after were reported to kill virus-infected cells. In the mid-1980s, 10 years after their discovery, NK cells were also demonstrated to contribute to the fight against bacterial infection, particularly because of crosstalk with other leukocytes. A wide variety of immune cells are now recognized to interact with NK cells through the production of cytokines such as interleukin (IL)-2, IL-12, IL-15 and IL-18, which boost NK cell activities. The recent demonstration that NK cells express pattern recognition receptors, namely Toll-like and nucleotide oligomerization domain (NOD)-like receptors, led to the understanding that these cells are not only under the control of accessory cells, but can be directly involved in the antibacterial response thanks to their capacity to recognize pathogen-associated molecular patterns. Interferon (IFN)-γ is the predominant cytokine produced by activated NK cells. IFN-γ is a key contributor to antibacterial immune defense. However, in synergy with other inflammatory cytokines, IFN-γ can also lead to deleterious effects similar to those observed during sepsis. Accordingly, as the main source of IFN-γ in the early phase of infection, NK cells display both beneficial and deleterious effects, depending on the circumstances