Early stages of the HFCVD process on multi-vicinal silicon surfaces studied by electron microscopy probes (SEM, TEM)

In this paper, we show that silicon dimples are suitable samples to study diamond nucleation on a controlled distribution of defects by SEM FEG and HRTEM observations. Indeed, multi-vicinal surfaces generated by a UHV thermal treatment have been characterised by STM experiments. On these terraces, we observed a strong increase of the nucleation density higher than two orders of magnitude compared to pristine silicon samples. Moreover, a preferential location of diamond nuclei along the steps is reported. This result is explained by the large surface diffusion length of carbon species compared to the terrace's width. Indeed, during the early stages of growth, oriented silicon carbide nano-crystals are observed with the relationship SiC(220)//Si(220)

In situ study of the initial stages of diamond deposition on 3C-SiC (100) surfaces: Towards the mechanisms of diamond nucleation

The mechanisms involved in the diamond nucleation on 3C-SiC surfaces have been investigated using a sequential in situ approach using electron spectroscopies (XPS, XAES and ELS). Moreover, diamond crystals have been studied by HRSEM. The in situ nucleation treatment allows a high diamond nucleation density close to 4 x 10(10) cm(-2). During the in situ enhanced nucleation treatment under Plasma, a negative bias was applied to the sample. The formation of an amorphous carbon phase and the roughening of the 3C-SiC surface have been observed. The part of these competing mechanisms in diamond nucleation is discussed

The Low CO Content of the Extremely Metal Poor Galaxy I Zw 18

We present sensitive molecular line observations of the metal-poor blue compact dwarf I Zw 18 obtained with the IRAM Plateau de Bure interferometer. These data constrain the CO J=1-0 luminosity within our 300 pc (FWHM) beam to be L_CO < 1 \times 10^5 K km s^-1 pc^2 (I_CO < 1 K km s^-1), an order of magnitude lower than previous limits. Although I Zw 18 is starbursting, it has a CO luminosity similar to or less than nearby low-mass irregulars (e.g. NGC 1569, the SMC, and NGC 6822). There is less CO in I Zw 18 relative to its B-band luminosity, HI mass, or star formation rate than in spiral or dwarf starburst galaxies (including the nearby dwarf starburst IC 10). Comparing the star formation rate to our CO upper limit reveals that unless molecular gas forms stars much more efficiently in I Zw 18 than in our own galaxy, it must have a very low CO-to-H_2 ratio, \sim 10^-2 times the Galactic value. We detect 3mm continuum emission, presumably due to thermal dust and free-free emission, towards the radio peak.Comment: 5 pages in emulateapj style, accepted by the Astrophysical Journa

Convection-permitting fully coupled WRF-Hydro ensemble simulations in high mountain environment: impact of boundary layer- and lateral flow parameterizations on land–atmosphere interactions

Numerical climate models have been upgraded by the improved description of terrestrial hydrological processes across different scales. The goal of this study is to explore the role of terrestrial hydrological processes on land–atmosphere interactions within the context of modeling uncertainties related to model physics parameterization. The models applied are the Weather Research and Forecasting (WRF) model and its coupled hydrological modeling system WRF-Hydro, which depicts the lateral terrestrial hydrological processes and further allows their feedback to the atmosphere. We conducted convection-permitting simulations (3 km) over the Heihe River Basin in Northwest China for the period 2008–2010, and particularly focused on its upper reach area of complex high mountains. In order to account for the modeling uncertainties associated with model physics parameterization, an ensemble of simulations is generated by varying the planetary boundary layer (PBL) schemes. We embedded the fully three-dimensional atmospheric water tagging method in both WRF and WRF-Hydro for quantifying the strength of land–atmosphere interactions. The impact of PBL parameterization on land–atmosphere interactions is evaluated through its direct effect on vertical mixing. Results suggest that enabled lateral terrestrial flow in WRF-Hydro distinctly increases soil moisture and evapotranspiration near the surface in the high mountains, thereby modifies the atmospheric condition regardless of the applied PBL scheme. The local precipitation recycling ratio in the study area increases from 1.52 to 1.9% due to the description of lateral terrestrial flow, and such positive feedback processes are irrespective of the modeling variability caused by PBL parameterizations. This study highlights the non-negligible contribution of lateral terrestrial flow to local precipitation recycling, indicating the potential of the fully coupled modeling in land–atmosphere interactions research

Potential of the Coupled WRF/WRF-Hydro Modeling System for Flood Forecasting in the Ouémé River (West Africa)

Since the beginning of the 2000s, most of the West-African countries, particularly Benin, have experienced an increased frequency of extreme flood events. In this study, we focus on the case of the Ouémé river basin in Benin. To investigate flood events in this basin for early warning, the coupled atmosphere–hydrology model system WRF-Hydro is used, and analyzed for the period 2008–2010. Such a coupled model allows exploration of the contribution of atmospheric components into the flood event, and its ability to simulate and predict accurate streamflow. The potential of WRF-Hydro to correctly simulate streamflow in the Ouémé river basin is assessed by forcing the model with operational analysis datasets from the European Centre for Medium-Range Weather Forecasts (ECMWF). Atmospheric and land surface processes are resolved at a spatial resolution of 5 km. The additional surface and subsurface water flow routing are computed at a resolution of 500 m. Key parameters of the hydrological module of WRF-Hydro are calibrated offline and tested online with the coupled WRF-Hydro. The uncertainty of atmospheric modeling on coupled results is assessed with the stochastic kinetic energy backscatter scheme (SKEBS). WRF-Hydro is able to simulate the discharge in the Ouémé river in offline and fully coupled modes with a Kling–Gupta efficiency (KGE) around 0.70 and 0.76, respectively. In the fully coupled mode, the model captures the flood event that occurred in 2010. A stochastic perturbation ensemble of ten members for three rain seasons shows that the coupled model performance in terms of KGE ranges from 0.14 to 0.79. Additionally, an assessment of the soil moisture has been developed. This ability to realistically reproduce observed discharge in the Ouémé river basin demonstrates the potential of the coupled WRF-Hydro modeling system for future flood forecasting applications

Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters

As a first step for studying star formation in the extreme outer Galaxy (EOG), we obtained deep near-infrared images of two embedded clusters at the northern and southern CO peaks of Cloud 2, which is one of the most distant star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the IRCS imager at the Subaru telescope, we detected cluster members with a mass detection limit of < 0.1 M_{sun}, which is well into the substellar regime. These high quality data enables a comparison of EOG to those in the solar neighborhood on the same basis for the first time. Before interpreting the photometric result, we have first constructed the NIR color-color diagram (dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the Mauna Kea Observatory filter system and also for the low metallicity environment since the metallicity in EOG is much lower than those in the solar neighborhood. The estimated stellar density suggests that an ``isolated type'' star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation is ongoing in Cloud 2-S. Despite the difference of the star formation mode, other characteristics of the two clusters are found to be almost identical: (1) K-band luminosity function (KLF) of the two clusters are quite similar, as is the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the estimated star formation efficiencies (SFEs) for both clusters are typical compared to those of embedded clusters in the solar neighborhood (~ 10 %). The similarity of two independent clusters with a large separation (~ 25 pc) strongly suggest that their star formation activities were triggered by the same mechanism, probably the supernova remnant (GSH 138-01-94).Comment: 14pages, 11 figures; Accepted for publication in Ap

Electronic and physico-chemical properties of nanmetric boron delta-doped diamond structures

Heavily boron doped diamond epilayers with thicknesses ranging from 40 to less than 2 nm and buried between nominally undoped thicker layers have been grown in two different reactors. Two types of [100]-oriented single crystal diamond substrates were used after being characterized by X-ray white beam topography. The chemical composition and thickness of these so-called deltadoped structures have been studied by secondary ion mass spectrometry, transmission electron microscopy, and spectroscopic ellipsometry. Temperature-dependent Hall effect and four probe resistivity measurements have been performed on mesa-patterned Hall bars. The temperature dependence of the hole sheet carrier density and mobility has been investigated over a broad temperature range (6K<T<450 K). Depending on the sample, metallic or non-metallic behavior was observed. A hopping conduction mechanism with an anomalous hopping exponent was detected in the non-metallic samples. All metallic delta-doped layers exhibited the same mobility value, around 3.660.8 cm2/Vs, independently of the layer thickness and the substrate type. Comparison with previously published data and theoretical calculations showed that scattering by ionized impurities explained only partially this low common value. None of the delta-layers showed any sign of confinement-induced mobility enhancement, even for thicknesses lower than 2 nm.14 page

PHIL photoinjector test line

LAL is now equiped with its own platform for photoinjectors tests and Research and Developement, named PHIL (PHotoInjectors at LAL). This facility has two main purposes: push the limits of the photoinjectors performances working on both the design and the associated technology and provide a low energy (MeV) short pulses (ps) electron beam for the interested users. Another very important goal of this machine will be to provide an opportunity to form accelerator physics students, working in a high technology environment. To achieve this goal a test line was realised equipped with an RF source, magnets and beam diagnostics. In this article we will desrcibe the PHIL beamline and its characteristics together with the description of the first two photoinjector realised in LAL and tested: the ALPHAX and the PHIN RF Guns

Coastal Observations of Weather Features in Senegal during the AMMA SOP-3 Period

During 15 August through 30 September 2006, ground and aircraft measurements were obtained from a multi-national group of students and scientists in Senegal. Key measurements were aimed at investigating and understanding precipitation processes, thermodynamic and dynamic environmental conditions, cloud, aerosol and microphysical processes and spaceborne sensors (TRMM, CloudSat/Calipso) validation. Ground and aircraft instruments include: ground based polarimetric radar, disdrometer measurements, a course and a high-density rain gauge network, surface chemical measurements, a 10 m flux tower, broadband IR, solar and microwave measurements, rawinsonde and radiosonde measurements, FA-20 dropsonde, in situ microphysics and cloud radar measurements. Highlights during SOP3 include ground and aircraft measurements of squall lines, African Easterly Waves (AEWs), Saharan Air Layer advances into Senegal, and aircraft measurements of AEWs -- including the perturbation that became Hurricane Isaac

Measurement of XUV-absorption spectra of ZnS radiatively heated foils

Time-resolved absorption of zinc sulfide (ZnS) and aluminum in the XUV-range has been measured. Thin foils in conditions close to local thermodynamic equilibrium were heated by radiation from laser-irradiated gold spherical cavities. Analysis of the aluminum foil radiative hydrodynamic expansion, based on the detailed atomic calculations of its absorption spectra, showed that the cavity emitted flux that heated the absorption foils corresponds to a radiation temperature in the range 55 60 eV. Comparison of the ZnS absorption spectra with calculations based on a superconfiguration approach identified the presence of species Zn6+ - Zn8+ and S5+ - S6+. Based on the validation of the radiative source simulations, experimental spectra were then compared to calculations performed by post-processing the radiative hydrodynamic simulations of ZnS. Satisfying agreement is found when temperature gradients are accounted for