Low density instability in a nuclear Fermi liquid drop

The instability of a Fermi-liquid drop with respect to bulk density distortions is considered. It is shown that the presence of the surface strongly reduces the growth rate of the bulk instability of the finite Fermi-liquid drop because of the anomalous dispersion term in the dispersion relation. The instability growth rate is reduced due to the Fermi surface distortions and the relaxation processes. The dependence of the bulk instability on the multipolarity of the particle density fluctuations is demonstrated for two nuclei $^{40}Ca$ and $^{208}Pb$.Comment: 12 pages, latex, 3 ps-figures, submitted to Phys. Rev.

Functionnal biodiversity in mango orchards on Reunion. Ecosystemic and landscape effects on epigeous predatory arthropods.

The knowledge of factors implicated in agroecosystem diversity is necessary to better evaluate their role. The study aims to link within-field plant diversity, cultural practices, and landscape context with terrestrial predatory arthropods in mango orchards on Reunion. Twenty-four plots distributed within the mango production area have been studied. Arthropods were sampled with pitfall traps. The land plots were grouped into three groups of agricultural practice intensity. The landscape was mapped within a circle of 400 meters around the orchards, which permitted to distinguish three different landscape contexts. Based on 83181 arthropods collected, 65124 were determined as predators, their communities were mainly composed of ants (Hymenoptera: Formicinae) and spiders (Araneae). The species richness and their equitability were influenced by factors at three different scales: (1) the within-field plant diversity improved evenness of predatory arthropods communities; (2) the moderate farming practices, as organic farming practices, had higher species richness than those obtained with “conventionnal” practices; (3) a landscape context with relevant heterogeneity and fragmentation increased the species richness of epigeal predatory arthropods in mango orchards

Mechanical and chemical spinodal instabilities in finite quantum systems

Self consistent quantum approaches are used to study the instabilities of finite nuclear systems. The frequencies of multipole density fluctuations are determined as a function of dilution and temperature, for several isotopes. The spinodal region of the phase diagrams is determined and it appears that instabilities are reduced by finite size effects. The role of surface and volume instabilities is discussed. It is indicated that the important chemical effects associated with mechanical disruption may lead to isospin fractionation.Comment: 4 pages, 4 figure

Improvement in the reconstrution method for VAMOS Spectrometer

International audienceThe VAMOS spectrometer operational at GANIL is a large acceptance variable mode spectrometer designed for nuclear reaction studies using radioactive and stable ion beams. The spectrometer coupled with ancillary detectors like EXOGAM has been successfully used in recent experiments on (in)elastic, few nucleon transfer reactions in inverse kinematics and search for nuclei far from stability using deep inelastic transfer reactions In large acceptance spectrometers, the experimental resolution is worsened by aberrations of third and higher orders. Hardware corrections are limited and cannot completely correct the non-linear effects. Thus software reconstruction of trajectories (ray tracing) is essential to obtain the required resolution and identification of the products. A numerical method has been developed for reconstruction of ion trajectories and correction of aberrations in VAMOS. We have devised a procedure to select an optimum subset of closest trajectories for each focal plane event (x, θ, y, ϕ) from the database (generated by an ion-optics calculation). A polynomial fit to the momentum vector of the reaction product in terms of (x, θ, y, ϕ) is made only to this subset. Such an approach is found to give improved resolutions compared to fitting a single polynomial over the entire phase space. Extraction of charge state and angular distributions are rather difficult due to the variation of acceptance over the large phase space. Mass identification of the reaction products and characteristics of the spectrometer acceptance with its variation for different rigidities have been obtained. Applications to 238U+48Ca and 238U+58Ni systems at 5.5 MeV/u will be presented

Sending femtosecond pulses in circles: highly non-paraxial accelerating beams

We use caustic beam shaping on 100 fs pulses to experimentally generate non-paraxial accelerating beams along a 60 degree circular arc, moving laterally by 14 \mum over a 28 \mum propagation length. This is the highest degree of transverse acceleration reported to our knowledge. Using diffraction integral theory and numerical beam propagation simulations, we show that circular acceleration trajectories represent a unique class of non-paraxial diffraction-free beam profile which also preserves the femtosecond temporal structure in the vicinity of the caustic

Structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin film

The structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin films are investigated for potential application in microthermoelectric devices. Nanocrystalline Si0.8Ge0.2 thin films are grown by low-pressure chemical vapor deposition on a sandwich of Si3N4/SiO2/Si3N4 films deposited on a Si (100) substrate. The Si0.8Ge0.2 film is doped with boron by ion implantation. The structure of the thin film is studied by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the film has column-shaped crystal grains ~100 nm in diameter oriented along the thickness of the film. The electrical conductivity and Seebeck coefficient are measured in the temperature range between 80–300 and 130–300 K, respectively. The thermal conductivity is measured at room temperature by a 3 method. As compared with bulk silicon-germanium and microcrystalline film alloys of nearly the same Si/Ge ratio and doping concentrations, the Si0.8Ge0.2 nanocrystalline film exhibits a twofold reduction in the thermal conductivitity, an enhancement in the Seebeck coefficient, and a reduction in the electrical conductivity. Enhanced heat carrier scattering due to the nanocrystalline structure of the films and a combined effect of boron segregation and carrier trapping at grain boundaries are believed to be responsible for the measured reductions in the thermal and electrical conductivities, respectively