125 research outputs found
A Density Functional Study of Atomic Hydrogen and Oxygen Chemisorption on the Relaxed (0001) Surface of Double Hexagonal Close Packed Americium
Ab initio total energy calculations within the framework of density
functional theory have been performed for atomic hydrogen and oxygen
chemisorption on the (0001) surface of double hexagonal packed americium using
a full-potential all-electron linearized augmented plane wave plus local
orbitals method. Chemisorption energies were optimized with respect to the
distance of the adatom from the relaxed surface for three adsorption sites,
namely top, bridge, and hollow hcp sites, the adlayer structure corresponding
to coverage of a 0.25 monolayer in all cases. Chemisorption energies were
computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at
the fully relativistic level (with spin-orbit coupling SOC). The two-fold
bridge adsorption site was found to be the most stable site for O at both the
NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and
8.368 eV respectively, while the three-fold hollow hcp adsorption site was
found to be the most stable site for H with chemisorption energies of 3.136 eV
at the NSOC level and 3.217 eV at the SOC level. The respective distances of
the H and O adatoms from the surface were found to be 1.196 Ang. and 1.164 Ang.
Overall our calculations indicate that chemisorption energies in cases with SOC
are slightly more stable than the cases with NSOC in the 0.049-0.238 eV range.
The work functions and net magnetic moments respectively increased and
decreased in all cases compared with the corresponding quantities of bare dhcp
Am (0001) surface. The partial charges inside the muffin-tins, difference
charge density distributions, and the local density of states have been used to
analyze the Am-adatom bond interactions in detail. The implications of
chemisorption on Am 5f electron localization-delocalization are also discussed.Comment: 9 Tables, 5 figure
Increased mitogen-activated protein kinase expression and activity in white adipose tissue of ventromedial hypothalamus-lesioned rats
Studies of Vibrational Properties in Ga Stabilized d-Pu by Extended X-ray Absorption Fine Structure
Temperature dependent extended x-ray absorption fine structure (EXAFS)
spectra were measured for a 3.3 at% Ga stabilized Pu alloy over the range T= 20
- 300 K at both the Ga K-edge and the Pu L_III-edge. The temperature dependence
of the pair-distance distribution widths, \sigma(T) was accurately modeled
using a correlated-Debye model for the lattice vibrational properties,
suggesting Debye-like behavior in this material. We obtain pair- specific
correlated-Debye temperatures, \Theta_cD, of 110.7 +/- 1.7 K and 202.6 +/- 3.7
K, for the Pu-Pu and Ga-Pu pairs, respectively. These results represent the
first unambiguous determination of Ga-specific vibrational properties in PuGa
alloys, and indicate the Ga-Pu bonds are significantly stronger than the Pu-Pu
bonds. This effect has important implications for lattice stabilization
mechanisms in these alloys.Comment: 7 pages, 4 figures, Phys. Rev. B in pres
Adsorption and dissociation of molecular oxygen on the (0001) surface of double hexagonal close packed americium
In our continuing attempts to understand theoretically various surface
properties such as corrosion and potential catalytic activity of actinide
surfaces in the presence of environmental gases, we report here the first ab
initio study of molecular adsorption on the double hexagonal packed (dhcp)
americium (0001) surface. Dissociative adsorption is found to be energetically
more favorable compared to molecular adsorption. The most stable configuration
corresponds to a horizontal approach molecular dissociation with the oxygen
atoms occupying neighboring h3 sites, with chemisorption energies at the NSOC
and SOC theoretical levels being 9.395 eV and 9.886 eV, respectively. The
corresponding distances of the oxygen molecule from the surface and
oxygen-oxygen distance were found to be 0.953 Ang. and 3.731 Ang.,
respectively. Overall our calculations indicate that chemisorption energies in
cases with SOC are slightly more stable than the cases with NSOC in the
0.089-0.493 eV range. The work functions and net magnetic moments respectively
increased and decreased in all cases compared with the corresponding quantities
of the bare dhcp Am (0001) surface. The adsorbate-substrate interactions have
been analyzed in detail using the partial charges inside the muffin-tin
spheres, difference charge density distributions, and the local density of
states. The effects, if any, of chemisorption on the Am 5f electron
localization-delocalization characteristics in the vicinity of the Fermi level
are also discussed.Comment: 6 tables, 10 figure
Design and synthesis of imidazoline derivatives active on glucose homeostasis in a rat model of type II diabetes.2. Syntheses and biological activities of 1,4-dialkyl-,1,4-dibenzyl and 1-benzyl-4-alkyl-2-(4',5'-dihydro-1' H-imidazol-2'yl)piperazines and isosteric analogues of imidazoline
International audienc
Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60 tetraphenylphosphoniumiodide
We report high-resolution far-infrared transmission measurements on C(60)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C(60) monoanion and identify low-frequency combination modes. The well-known F(1u)(1) and F(1u)(2) modes are split into doublers at room temperature, indicating a D(5d) or D(3d) distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C(60)(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7]
Probing the charging mechanisms of carbon nanomaterial polyelectrolytes
Chemical charging of single-walled carbon nanotubes (SWCNTs) and graphenes to generate soluble salts shows great promise as a processing route for electronic applications, but raises fundamental questions. The reduction potentials of highly-charged nanocarbon polyelectrolyte ions were investigated by considering their chemical reactivity towards metal salts/complexes in forming metal nanoparticles. The redox activity, degree of functionalisation and charge utilisation were quantified via the relative metal nanoparticle content, established using thermogravimetric analysis (TGA), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS). The fundamental relationship between the intrinsic nanocarbon electronic density of states and Coulombic effects during charging is highlighted as an important area for future research
Relativistic calculations to assess the ability of the generalized gradient approximation to reproduce trends in cohesive properties of solids
Correlation Effects on Stability in Pu Metal and Its Alloys
The existence of six crystallographic allotropes from room temperature up to the solid-liquid transition just above 913 K at atmospheric pressure makes solid Plutonium unique among the elements in the periodic table. Among these phases (labeled {alpha}, {beta}, {gamma}, {delta}{delta}{prime}), and {var_epsilon}, the {delta} phase, stable between 593 K and 736 K, has commanded considerable interest in the metallurgical and solid state communities. In contrast to the low-temperature monoclinic {alpha} phase, which is brittle, the face-centered cubic (fcc) {delta} phase is ductile, a property that makes it convenient for engineering applications. This phase can also be stabilized through alloying with a number of other elements such as Ga, Al, Sc, and Am
Does congenital deafness affect the structural and functional architecture of primary visual cortex?
Deafness results in greater reliance on the remaining senses. It is unknown whether the cortical architecture of the intact senses is optimized to compensate for lost input. Here we performed widefield population receptive field (pRF) mapping of primary visual cortex (V1) with functional magnetic resonance imaging (fMRI) in hearing and congenitally deaf participants, all of whom had learnt sign language after the age of 10 years. We found larger pRFs encoding the peripheral visual field of deaf compared to hearing participants. This was likely driven by larger facilitatory center zones of the pRF profile concentrated in the near and far periphery in the deaf group. pRF density was comparable between groups, indicating pRFs overlapped more in the deaf group. This could suggest that a coarse coding strategy underlies enhanced peripheral visual skills in deaf people. Cortical thickness was also decreased in V1 in the deaf group. These findings suggest deafness causes structural and functional plasticity at the earliest stages of visual cortex
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