2,224 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
Trends and Determinants of Rural Poverty: A Logistic Regression Analysis of Selected Districts of Punjab
Poverty is widespread in the rural areas, where the people are in a state of human deprivation with regard to incomes, clothing, housing, health care, education, sanitary facilities and human rights. Nearly 61 percent of the country’s populations live in rural areas. In Pakistan poverty has been increased in rural areas and is higher than urban areas. Of the total rural population 65 percent are directly or indirectly linked with agriculture sector. In Pakistan more than 44.8 percent people generate their income from agriculture sector, and the higher rate of increase in poverty in the rural areas has provoked debate on growth and productivity trends in the agriculture sector. Therefore, it is the need of the hour to determine such factors which affect the poverty status of a rural household. Utilising unique IFPRI (International Food Policy Research Institute) panel data together with sub-sample of PRHS (Pakistan Rural Household Survey) for two districts of Punjab (Attock and Faisalabad) the present study aim at analysing and estimating the rural poverty trends and determinants of rural poverty from the late 1980s to 2002. The data was analysed by using binary logistic model and head count measure. The results show that the chance of a household tripping to poverty increased due to increase in household size, dependency ratio, while, education, value of livestock, remittances and farming decreased the likelihood of being a poor. Moreover, the socio-economic opportunities as represented by the availability of infrastructure in the residential region also play a significant role in the level of poverty faced by a household. This study makes a modest contribution by attempting to analyse the need for focusing on anti-poverty policies, which can nip the evil in the bud.Rural Poverty, Poverty Trends, Agriculture Growth, Determinants
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
An Activating Mutation in sos-1 Identifies Its Dbl Domain as a Critical Inhibitor of the Epidermal Growth Factor Receptor Pathway during Caenorhabditis elegans Vulval Development
Proper regulation of receptor tyrosine kinase (RTK)-Ras-mitogen-activated protein kinase (MAPK) signaling pathways is critical for normal development and the prevention of cancer. SOS is a dual-function guanine nucleotide exchange factor (GEF) that catalyzes exchange on Ras and Rac. Although the physiologic role of SOS and its CDC25 domain in RTK-mediated Ras activation is well established, the in vivo function of its Dbl Rac GEF domain is less clear. We have identified a novel gain-of-function missense mutation in the Dbl domain of Caenorhabditis elegans SOS-1 that promotes epidermal growth factor receptor (EGFR) signaling in vivo. Our data indicate that a major developmental function of the Dbl domain is to inhibit EGF-dependent MAPK activation. The amount of inhibition conferred by the Dbl domain is equal to that of established trans-acting inhibitors of the EGFR pathway, including c-Cbl and RasGAP, and more than that of MAPK phosphatase. In conjunction with molecular modeling, our data suggest that the C. elegans mutation, as well as an equivalent mutation in human SOS1, activates the MAPK pathway by disrupting an autoinhibitory function of the Dbl domain on Ras activation. Our work suggests that functionally similar point mutations in humans could directly contribute to disease
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
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
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
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
A Model for the Topology of Excitatory Amino Acid Transporters Determined by the Extracellular Accessibility of Substituted Cysteines
AbstractExcitatory amino acid transporters (EAATs) function as both substrate transporters and ligand-gated anion channels. Characterization of the transporter's general topology is the first requisite step in defining the structural bases for these distinct activities. While the first six hydrophobic domains can be readily modeled as conventional transmembrane segments, the organization of the C-terminal hydrophobic domains, which have been implicated in both substrate and ion interactions, has been controversial. Here, we report the results of a comprehensive evaluation of the C-terminal topology of EAAT1 determined by the chemical modification of introduced cysteine residues. Our data support a model in which two membrane-spanning domains flank a central region that is highly accessible to the extracellular milieu and contains at least one reentrant loop domain
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