Recherches sur les écosystemes des réserves biologiques de la Forêt de Fontainebleau. VII - Structure et fonctions des peuplements de houx (Ilex aquifolium L.)

Holly bushes form an important structural unit in some natural reserves of the Fontainebleau forest (60 km S.E. of Paris). Due to the high stem density of the holly, its great basal area and the longevity of its leaves (about two years), the biomass and the global productivity of the mature beech-oak forest are appreciably increased. This additional production brings about a large immobilisation and important turn-over of nutrients. Holly plays an important role as potassium accumulator and magnesium cycle accelerator. The leaf litter, falling alternately from holly and trees, is appreciably increased in volume and its mineral and nitrogen contents are enriched. During the first months following leaf drop, decomposition of the holly litter is very rapid. The actual evolution of holly populations tend to be extended by the peripheral formation of suckers and by seedlings. Under the bushes, deep shade and thick litter accumulation inhibit any regeneration. The progressive substitution of old oaks by beeches is unfavorable to holly. The presence of holly stands could represent a preterminal stage of this forest successio

Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering

Diffuse X-ray data for mixed stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure

Neutron Laue and X-ray diffraction study of a new crystallographic superspace phase in n-nonadecane-urea

International audienceAperiodic composite crystals present long-range order without translational symmetry. These materials may be described as the intersection in three dimensions of a crystal which is periodic in a higher-dimensional space. In such materials, symmetry breaking must be described as structural changes within these crystallographic superspaces. The increase in the number of superspace groups with the increase in the dimension of the superspace allows many more structural solutions. This is illustrated in n-nonadecane-urea, revealing a fifth higher-dimensional phase at low temperature

fbl-typing of Staphylococcus lugdunensis: a frontline tool for epidemiological studies

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Apport de la MLVA dans l'épidémiologie moléculaire des infections à Staphylococcus lugdunensis

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Neutral-ionic phase transition : a thorough ab-initio study of TTF-CA

The prototype compound for the neutral-ionic phase transition, namely TTF-CA, is theoretically investigated by first-principles density functional theory calculations. The study is based on three neutron diffraction structures collected at 40, 90 and 300 K (Le Cointe et al., Phys. Rev. B 51, 3374 (1995)). By means of a topological analysis of the total charge densities, we provide a very precise picture of intra and inter-chain interactions. Moreover, our calculations reveal that the thermal lattice contraction reduces the indirect band gap of this organic semi-conductor in the neutral phase, and nearly closes it in the vicinity of the transition temperature. A possible mechanism of the neutral-ionic phase transition is discussed. The charge transfer from TTF to CA is also derived by using three different technics.Comment: 11 pages, 9 figures, 7 table

Epidémie d'hépatite A aiguë parmi des hommes ayant des rapports sexuels avec des hommes (HSH) Caractéristiques cliniques et épidémiologiques des cas diagnostiqués au CHU de Rouen

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