Crystallization of Ge2Sb2Te5 nanometric phase change material clusters made by gas-phase condensation

International audienceThe crystallization behavior of Ge2Sb2Te5 nanometric clusters was studied using X-ray diffraction with in situannealing. Clusters were made using a sputtering gas-phase condensation source, which allowed for the growth of well-defined, contaminant-free, and isolated clusters. The average size for the clusters is 5.7 ± 1 nm. As-deposited amorphous clusters crystallize in the fcc cubic phase at 180 °C, while for thin films, the phase change temperature is 155 °C. This observation illustrates the scalability of the Ge2Sb2Te5phase change from the amorphous to the cubic state in three-dimensionally confined systems in this size range

Asymmetric response of forest and grassy biomes to climate variability across the African Humid Period : influenced by anthropogenic disturbance?

A comprehensive understanding of the relationship between land cover, climate change and disturbance dynamics is needed to inform scenarios of vegetation change on the African continent. Although significant advances have been made, large uncertainties exist in projections of future biodiversity and ecosystem change for the world's largest tropical landmass. To better illustrate the effects of climate–disturbance–ecosystem interactions on continental‐scale vegetation change, we apply a novel statistical multivariate envelope approach to subfossil pollen data and climate model outputs (TraCE‐21ka). We target paleoenvironmental records across continental Africa, from the African Humid Period (AHP: ca 14 700–5500 yr BP) – an interval of spatially and temporally variable hydroclimatic conditions – until recent times, to improve our understanding of overarching vegetation trends and to compare changes between forest and grassy biomes (savanna and grassland). Our results suggest that although climate variability was the dominant driver of change, forest and grassy biomes responded asymmetrically: 1) the climatic envelope of grassy biomes expanded, or persisted in increasingly diverse climatic conditions, during the second half of the AHP whilst that of forest did not; 2) forest retreat occurred much more slowly during the mid to late Holocene compared to the early AHP forest expansion; and 3) as forest and grassy biomes diverged during the second half of the AHP, their ecological relationship (envelope overlap) fundamentally changed. Based on these asymmetries and associated changes in human land use, we propose and discuss three hypotheses about the influence of anthropogenic disturbance on continental‐scale vegetation change

Status report of the accelerator for multiply charged ions in Grenoble

International audienceThe status report of the Multiply Charged Ion Accelerator of the CEA-Grenoble is presented. This facility is devoted to the production of multiply charged ions (up to Xe31+) in the low energy domain (1–20 q keV). A new 14.5 GHz high magnetic field electron cyclotron resonance ion source of the Caprice type has been installed, the transmission of the beam line increased, resulting in a large enhancement of the available beam intensity. Ion selection and deceleration are simplified. Emittances as low as before are achieved. Performances for various beams are presented, together with updated beam emittance measurements. The intensity enhancement is especially large for highly charged ion beams like Ar17+ (100 times more intensity), and enables routine delivery to experiments of more than 10 μA of Xe25+ at 20 q keV in 20 π mm mrad. The range of available charge states was extended to Xe31+ (with a reasonable intensity of 180 nA). Future prospects include the extension of the energy range down to a few q eV

Growth and properties of cobalt clusters made by sputtering gas-aggregation

A study of the different operation modes of a sputtering gas-aggregation source is presented. The size distributions of small cobalt clusters shed some light on the first steps of the growth process. Large clusters of 2 to 6 nm diameter with narrow distributions are obtained. Their icosahedral structure is identified by HRTEM observations

Stopping power and nano-particles: Collisions of ions in low charge states with metallic clusters

In an experimental study, the multi-ionisation of metallic clusters (Nan) has been analysed in collisions with light ions in low charge states (H+, He+, He2+, O3+) at collision velocities below 1 a.u. Cluster ions are produced in charge states up to 5+. The average charge of the nano-particles is found to increase linearly with the variation of projectile velocity and the square of the effective projectile charge, well in agreement with the electronic stopping power of the bulk material. A fraction of 50% to 30% of the total projectile energy loss (decreasing with velocity) is transferred into vibrational modes in good agreement with recent theoretical predictions