381 research outputs found
Orientability of Fredholm families and topological degree
We construct a degree theory for oriented Fredholm mappings of index zero between open subsets of Banach spaces and between Banach manifolds. Our approach is based on the orientation of Fredholm mappings: it does not use Fredholm structures on the domain and target spaces. We provide a computable formula for the change in degree through an admissible homotopy that is necessary for applications to global bifurcation. The notion of orientation enables us to establish rather precise relationships between our degree and many other degree theories for particular classes of Fredholm maps, including the Elworthy-Tromba degree, which have appeared in the literature in a seemingly unrelated manner
First principles determination of the Peierls stress of the shuffle screw dislocation in silicon
The Peierls stress of the a/2 screw dislocation belonging to the shuffle
set is calculated for silicon using density functional theory. We have checked
the effect of boundary conditions by using two models, the supercell method
where one considers a periodic array of dislocations, and the cluster method
where a single dislocation is embedded in a small cluster. The Peierls stress
is underestimated with the supercell and overestimated with the cluster. These
contributions have been calculated and the Peierls stress is determined in the
range between 2.4 x 10-2 and 2.8 x 10-2 eV {\AA}-3. When moving, the
dislocation follows the {111} plane going through a low energy metastable
configuration and never follows the 100 plane, which includes a higher energy
metastable core configuration
Stability of undissociated screw dislocations in zinc-blende covalent materials from first principle simulations
The properties of perfect screw dislocations have been investigated for
several zinc-blende materials such as diamond, Si, -SiC, Ge and GaAs, by
performing first principles calculations. For almost all elements, a core
configuration belonging to shuffle set planes is favored, in agreement with low
temperature experiments. Only for diamond, a glide configuration has the lowest
defect energy, thanks to an sp hybridization in the core
Theoretical study of dislocation nucleation from simple surface defects in semiconductors
Large-scale atomistic calculations, using empirical potentials for modeling
semiconductors, have been performed on a stressed system with linear surface
defects like steps. Although the elastic limits of systems with surface defects
remain close to the theoretical strength, the results show that these defects
weaken the atomic structure, initializing plastic deformations, in particular
dislocations. The character of the dislocation nucleated can be predicted
considering both the resolved shear stress related to the applied stress
orientation and the Peierls stress. At low temperature, only glide events in
the shuffle set planes are observed. Then they progressively disappear and are
replaced by amorphization/melting zones at a temperature higher than 900 K
Undissociated screw dislocations in silicon: calculations of core structure and energy
The stability of the perfect screw dislocation in silicon has been
investigated using both classical potentials and first-principles calculations.
Although a recent study by Koizumi et al . stated that the stable screw
dislocation was located in both the 'shuffle' and the 'glide' sets of {111}
planes, it is shown that this result depends on the classical potential used,
and that the most stable configuration belongs to the 'shuffle' set only, in
the centre of one hexagon. We also investigated the stability of an sp 2
hybridization in the core of the dislocation, obtained for one metastable
configuration in the 'glide' set. The core structures are characterized in
several ways, with a description of the three-dimensional structure,
differential displacement maps and derivatives of the disregistry
Theoretical study of kinks on screw dislocation in silicon
Theoretical calculations of the structure, formation and migration of kinks
on a non-dissociated screw dislocation in silicon have been carried out using
density functional theory calculations as well as calculations based on
interatomic potential functions. The results show that the structure of a
single kink is characterized by a narrow core and highly stretched bonds
between some of the atoms. The formation energy of a single kink ranges from
0.9 to 1.36 eV, and is of the same order as that for kinks on partial
dislocations. However, the kinks migrate almost freely along the line of an
undissociated dislocation unlike what is found for partial dislocations. The
effect of stress has also been investigated in order to compare with previous
silicon deformation experiments which have been carried out at low temperature
and high stress. The energy barrier associated with the formation of a stable
kink pair becomes as low as 0.65 eV for an applied stress on the order of 1
GPa, indicating that displacements of screw dislocations likely occur via
thermally activated formation of kink pairs at room temperature
Dislocation core field. I. Modeling in anisotropic linear elasticity theory
Aside from the Volterra field, dislocations create a core field, which can be
modeled in linear anisotropic elasticity theory with force and dislocation
dipoles. We derive an expression of the elastic energy of a dislocation taking
full account of its core field and show that no cross term exists between the
Volterra and the core fields. We also obtain the contribution of the core field
to the dislocation interaction energy with an external stress, thus showing
that dislocation can interact with a pressure. The additional force that
derives from this core field contribution is proportional to the gradient of
the applied stress. Such a supplementary force on dislocations may be important
in high stress gradient regions, such as close to a crack tip or in a
dislocation pile-up
F-02: Antibiothérapies anti-SARM dans le traitement probabiliste des bactériémies à Staphylococcus aureus au CHU d'Angers
Introduction – objectifs
La résistance à la méticilline du Staphylococcus aureus (SA) ne cesse de diminuer en France. Nous avons évalué la prescription des antibiothérapies probabilistes dans les bactériémies à SA et leur impact clinique.
Matériels et méthodes
Il s’agit d’une étude rétrospective incluant les bactériémies à Staphylococcus aureus sur le CHU d’Angers entre novembre 2012 et septembre 2013 avec recueil de l’antibiothérapie prescrite au rendu « cocci gram (CG)+ », à J5, hospitalisation depuis plus de 5 jours ou dans les six derniers mois, signes de gravité clinique, mortalité à J5.
RĂ©sultats
Nous avons inclus 146 épisodes de bactériémies. A l’annonce de CG+, 10 patients étaient décédés, 38,2 % ont reçu un anti-SARM (n = 52), 54,4 % un anti-SASM (n = 74), 7,4 % aucun antibiotique (n = 10). Parmi les bactériémies à SARM (n = 23), 60,9 % (n = 14) ont reçu une antibiothérapie initiale à visée SARM contre 36,3 % (n = 41) pour les bactériémies à SASM (n = 113) (OR = 2,71, p = 0,03). Cette adaptation semble influencée par une hospitalisation récente (OR = 1,82 ; p = 0,33) et la présence de signes de gravité (OR = 1,87, p = 0,23). Dans 21,4 % (n = 6) des bactériémies à SASM, l’antibiothérapie anti-SARM n’a pas été désecaladée. La mortalité à J5 est 13 %. L’absence d’antibiothérapie dans les 24 premières heures (OR = 2,97 ; p = 0,046) et une prescription de C3G (OR = 5,2, p = 0,05) en monothérapie augmentent le risque de décès à J5.
Conclusion
L’antibiothérapie initale semble adaptée à l’histoire et la présentation des patients. L’initiation sans délai, la visée staphylococcique et la désescalade sont les éléments importants du traitement
An update on THORPEX-related research in data assimilation and observing strategies
International audienceThe international programme "THORPEX: a World Weather Research Programme" provides a framework in which to tackle the challenge of improving the forecast skill of high-impact weather through international collaboration between academic institutions, operational forecast centres, and users of forecast products. The objectives of the THORPEX Data Assimilation and Observation Strategy Working Group (DAOS-WG) are two-fold. The primary goal is to assess the impact of observations and various targeting methods to provide guidance for observation campaigns and for the configuration of the Global Observing System. The secondary goal is to setup an optimal framework for data assimilation, including aspects such as targeted observations, satellite data, background error covariances and quality control. The Atlantic THORPEX Regional campaign, ATReC, in 2003, has been very successful technically and has provided valuable datasets to test targeting issues. Various data impact experiments have been performed, showing a small but very slightly positive impact of targeted observations. Projects of the DAOS-WG include working on the AMMA field experiment, in the context of IPY and to prepare the future THORPEX-PARC field campaign in the Pacific by comparing sensitivity of the forecasts to observations between several groups
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