2,409 research outputs found
Understanding the nucleation mechanisms of Carbon Nanotubes in catalytic Chemical Vapor Deposition
The nucleation of carbon caps on small nickel clusters is studied using a
tight binding model coupled to grand canonical Monte Carlo simulations. It
takes place in a well defined carbon chemical potential range, when a critical
concentration of surface carbon atoms is reached. The solubility of carbon in
the outermost Ni layers, that depends on the initial, crystalline or
disordered, state of the catalyst and on the thermodynamic conditions, is
therefore a key quantity to control the nucleation
Importance of carbon solubility and wetting properties of nickel nanoparticles for single wall nanotube growth
Optimized growth of Single Wall Carbon Nanotubes requires a full knowledge of
the actual state of the catalyst nanoparticle and its interface with the tube.
Using Tight Binding based atomistic computer simulations, we calculate carbon
adsorption isotherms on nanoparticles of nickel, a typical catalyst, and show
that carbon solubility increases for smaller nanoparticles that are either
molten or surface molten under experimental conditions. Increasing carbon
content favors the dewetting of Ni nanoparticles with respect to sp2 carbon
walls, a necessary property to limit catalyst encapsulation and deactivation.
Grand Canonical Monte Carlo simulations of the growth of tube embryos show that
wetting properties of the nanoparticles, controlled by carbon solubility, are
of fundamental importance to enable the growth, shedding a new light on the
growth mechanisms
Polarization analysis of K-edge resonant x-ray scattering of germanium
The polarization of K-edge resonant scattering at the space group
``forbidden'' (0 0 6) reflection of Ge was measured as function of the
azimuthal angle, psi. The experimental results are compared to model
calculations based on symmetry analysis of the resonant scattering tensors.Comment: 4 pages, 3 figures. Submitted to Phys. Rev. B Rapid Communications
V2: Updated after referee report
High-contrast Imaging with Spitzer: Deep Observations of Vega, Fomalhaut, and epsilon Eridani
Stars with debris disks are intriguing targets for direct imaging exoplanet
searches, both due to previous detections of wide planets in debris disk
systems, as well as commonly existing morphological features in the disks
themselves that may be indicative of a planetary influence. Here we present
observations of three of the most nearby young stars, that are also known to
host massive debris disks: Vega, Fomalhaut, and eps Eri. The Spitzer Space
Telescope is used at a range of orientation angles for each star, in order to
supply a deep contrast through angular differential imaging combined with
high-contrast algorithms. The observations provide the opportunity to probe
substantially colder bound planets (120--330 K) than is possible with any other
technique or instrument. For Vega, some apparently very red candidate point
sources detected in the 4.5 micron image remain to be tested for common proper
motion. The images are sensitive to ~2 Mjup companions at 150 AU in this
system. The observations presented here represent the first search for planets
around Vega using Spitzer. The upper 4.5 micron flux limit on Fomalhaut b could
be further constrained relative to previous data. In the case of eps Eri,
planets below both the effective temperature and the mass of Jupiter could be
probed from 80 AU and outwards, although no such planets were found. The data
sensitively probe the regions around the edges of the debris rings in the
systems where planets can be expected to reside. These observations validate
previous results showing that more than an order of magnitude improvement in
performance in the contrast-limited regime can be acquired with respect to
conventional methods by applying sophisticated high-contrast techniques to
space-based telescopes, thanks to the high degree of PSF stability provided in
this environment.Comment: 11 pages, 12 figures, accepted for publication in A&
EVALUATION OF GASTROPROTECTIVE POTENTIAL OF CAJANUS CAJAN SEEDS EXTRACT ON ETHANOL-INDUCED GASTRIC ULCER IN ALBINO RATS
Objective: The present study was undertaken to evaluate the gastroprotective potential of Cajanus cajan seed aqueous extract (CCSE) on experimentally induced gastric ulcer in albino rats.
Methods: Preliminary acute toxicity study was conducted to determine the oral median lethal dose (LD50). Twenty (20) male rats divided into 5 groups (A-E) of 4 animals each were used. Group A (normal control) and B (ulcer model) received 10ml/kg distilled water. Groups C, D and E received Cimetidine (100mg/kg b.w intraperitoneally), 250mg/kg and 500mg/kg b.w of CCSE orally respectively, once daily for seven (7) days. On Day 8, gastric ulcers were induced in groups B – E using 80% ethanol (1ml/rat) prior to sacrifice. Ulcer indices were determined on excised stomach tissues prior to further processing for histological examination.
Results: The LD50 of CCSE is greater than 5g/kg b.w. Cimetidine and CCSE treatments showed statistically significant decrease (p<0.05) in ulcer indices compared to group B (ulcer model). Histological features revealed noticeable preservation of the gastric mucosa in consistence with the macroscopical findings.
Conclusion: The present study revealed that C. cajan seed extract possesses gastroprotective properties against ethanol-induced gastric ulcers
Interaction of carbon clusters with Ni(100) : Application to the nucleation of carbon nanotubes
In order to understand the first stages of the nucleation of carbon nanotubes
in catalytic processes, we present a tight-binding Monte Carlo study of the
stability and cohesive mechanisms of different carbon structures deposited on
nickel (100) surfaces. Depending on the geometry, we obtain contrasted results.
On the one hand, the analysis of the local energy distributions of flat carbon
sheets, demonstrate that dangling bonds remain unsaturated in spite of the
presence of the metallic catalyst. Their adhesion results from the energy gain
of the surface Ni atoms located below the carbon nanostructure. On the other
hand, carbon caps are stabilized by the presence of carbon atoms occupying the
hollow sites of the fcc nickel structure suggesting the saturation of the
dangling bonds
A simple thermodynamical witness showing universality of macroscopic entanglement
We show that if the ground state entanglement exceeds the total entropy of a
given system, then this system is in an entangled state. This is a universal
entanglement witness that applies to any physical system and yields a
temperature below which we are certain to find some entanglement. Our witness
is then applied to generic bosonic and fermionic many body systems to derive
the corresponding "critical" temperatures that have a very broad validity.Comment: 3 pages, Torun conference, June 25-28, 200
Genetic variation of durum wheat landraces using morphological and protein markers
Genetic variations of cultivars are very interesting in reducing genetic vulnerability and lead to stable control of production. The aim of this research was to study genetic diversity among six durum wheat cultivars. For the first assay we evaluated seven morphological traits which are: spikelet per spike, spike length, spike width, beard length, plant height, width of truncation and barb length. The tested genotypes were classified in three groups according to the linkage distance analysis. The genetic variability was also evaluated for seed storage-proteins by sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE). Electrophoregram allowed the estimation of the durum wheat genetic similarity (GS). This GS analysis based on Unweighted Pair Group Method with Arithmetic averages (UPGMA), permits to obtain the same genotypic clustering. No significant correlation was observed among the two methods tested. It is concluded that seed storage protein profiles could be useful markers in the studies of genetic diversity and genotypes classification, which can be used to improve the efficiency of wheat breeding programs.Key words: Wheat genotypes, SDS-PAGE, genetic diversity, cluster analysis
Magnetic structure of antiferromagnetic NdRhIn5
The magnetic structure of antiferromagnetic NdRhIn5 has been determined using
neutron diffraction. It has a commensurate antiferromagnetic structure with a
magnetic wave vector (1/2,0,1/2) below T_N = 11K. The staggered Nd moment at
1.6K is 2.6mu_B aligned along the c-axis. We find the magnetic structure to be
closely related to that of its cubic parent compound NdIn3 below 4.6K. The
enhanced T_N and the absence of additional transitions below T_N for NdRhIn5
are interpreted in terms of an improved matching of the
crystalline-electric-field (CEF), magnetocrystalline, and exchange interaction
anisotropies. In comparison, the role of these competing anisotropies on the
magnetic properties of the structurally related compound CeRhIn5 is discussed.Comment: 4 pages, 4 figure
Imaging the symmetry breaking of molecular orbitals in carbon nanotubes
Carbon nanotubes have attracted considerable interest for their unique
electronic properties. They are fascinating candidates for fundamental studies
of one dimensional materials as well as for future molecular electronics
applications. The molecular orbitals of nanotubes are of particular importance
as they govern the transport properties and the chemical reactivity of the
system. Here we show for the first time a complete experimental investigation
of molecular orbitals of single wall carbon nanotubes using atomically resolved
scanning tunneling spectroscopy. Local conductance measurements show
spectacular carbon-carbon bond asymmetry at the Van Hove singularities for both
semiconducting and metallic tubes, demonstrating the symmetry breaking of
molecular orbitals in nanotubes. Whatever the tube, only two types of
complementary orbitals are alternatively observed. An analytical tight-binding
model describing the interference patterns of ? orbitals confirmed by ab initio
calculations, perfectly reproduces the experimental results
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