7,441 research outputs found
Feasibility of using neutron radiography to inspect the Space Shuttle solid rocket booster aft skirt, forward skirt and frustum. Part 1: Summary report
The space shuttle's solid rocket boosters (SRB) include components made primarily of aluminum that are parachuted back for retrieval from the ocean and refurbished for repeated usage. Nondestructive inspection methods used on these aging parts to reduce the risk of unforeseen problems include x-ray, ultrasonics, and eddy current. Neutron radiography tests on segments of an SRB component show that entrapped moisture and naturally occurring aluminum corrosion can be revealed by neutron radiography even if present in only small amounts. Voids in sealant can also be evaluated. Three alternatives are suggested to follow-up this study: (1) take an SRB component to an existing neutron radiography system; (2) take an existing mobile neutron radiography system to the NASA site; or (3) plan a dedicated system custom designed for NASA applications
Stabilization of the high-spin state of Co in LaCoRhO
The rhodium doping in the LaCoRhO perovskite series
() has been studied by X-ray diffraction, electric transport and
magnetization measurements, complemented by electronic structure GGA+U
calculations in supercell for different concentration regimes. No charge
transfer between Co and Rh is evidenced. The diamagnetic ground
state of LaCoO, based on Co in low-spin (LS) state, is disturbed
even by a small doping of Rh. The driving force is the elastic energy connected
with incorporation of a large Rh cation into the matrix of small LS
Co cations, which is relaxed by formation of large Co in
high-spin (HS) state in the next-nearest sites to the inserted Rh atom. With
increasing temperature, the population of Co in HS state increases
through thermal excitation, and a saturated phase is obtained close to room
temperature, consisting of a nearest-neighbor correlation of small (LS
Co) and large (HS Co and LS Rh) cations in a kind of
double perovskite structure. The stabilizing role of elastic and electronic
energy contributions is demonstrated in supercell calculations for dilute Rh
concentration compared to other dopants with various trivalent ionic radius.Comment: 8 pages, 8 figure
Beyond the local approximation to exchange and correlation: the role of the Laplacian of the density in the energy density of Si
We model the exchange-correlation (XC) energy density of the Si crystal and
atom as calculated by variational Monte Carlo (VMC) methods with a gradient
analysis beyond the local density approximation (LDA). We find the Laplacian of
the density to be an excellent predictor of the discrepancy between VMC and LDA
energy densities in each system. A simple Laplacian-based correction to the LDA
energy density is developed by means of a least square fit to the VMC XC energy
density for the crystal, which fits the homogeneous electron gas and Si atom
without further effort.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
The local electronic structure of alpha-Li3N
New theoretical and experimental investigation of the occupied and unoccupied
local electronic density of states (DOS) are reported for alpha-Li3N. Band
structure and density functional theory calculations confirm the absence of
covalent bonding character. However, real-space full-multiple-scattering
(RSFMS) calculations of the occupied local DOS finds less extreme nominal
valences than have previously been proposed. Nonresonant inelastic x-ray
scattering (NRIXS), RSFMS calculations, and calculations based on the
Bethe-Salpeter equation are used to characterize the unoccupied electronic
final states local to both the Li and N sites. There is good agreement between
experiment and theory. Throughout the Li 1s near-edge region, both experiment
and theory find strong similarities in the s- and p-type components of the
unoccupied local final density of states projected onto an orbital angular
momentum basis (l-DOS). An unexpected, significant correspondence exists
between the near-edge spectra for the Li 1s and N 1s initial states. We argue
that both spectra are sampling essentially the same final density of states due
to the combination of long core-hole lifetimes, long photoelectron lifetimes,
and the fact that orbital angular momentum is the same for all relevant initial
states. Such considerations may be generically applicable for low atomic number
compounds.Comment: 34 pages, 7 figures, 1 tabl
Polylithiated (OLi2) functionalized graphane as a potential hydrogen storage material
Hydrogen storage capacity, stability, bonding mechanism and the electronic
structure of polylithiated molecules (OLi2) functionalized graphane (CH) has
been studied by means of first principle density functional theory (DFT).
Molecular dynamics (MD) have confirmed the stability, while Bader charge
analysis describe the bonding mechanism of OLi2 with CH. The binding energy of
OLi2 on CH sheet has been found to be large enough to ensure its uniform
distribution without any clustering. It has been found that each OLi2 unit can
adsorb up to six H2 molecules resulting into a storage capacity of 12.90 wt%
with adsorption energies within the range of practical H2 storage application.Comment: 11 pages, 4 figures, 1 table, Phys. Chem. Chem. Phys. (submitted
Four-terminal resistance of an interacting quantum wire with weakly invasive contacts
We analyze the behavior of the four-terminal resistance, relative to the
two-terminal resistance of an interacting quantum wire with an impurity, taking
into account the invasiveness of the voltage probes. We consider a
one-dimensional Luttinger model of spinless fermions for the wire. We treat the
coupling to the voltage probes perturbatively, within the framework of
non-equilibrium Green function techniques. Our investigation unveils the
combined effect of impurities, electron-electron interactions and invasiveness
of the probes on the possible occurrence of negative resistance.Comment: 10 pages, 7 figure
The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions
We study the effect of Coulomb interactions in transition metal oxides
junctions. In this paper we analyze charge transfer at the interface of a three
layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose
a charge model considering a few atomic planes within each layer and obtain
results for the magnetic coupling between the ferromagnetic layers. For large
number of planes in the paramagnetic spacer we find that the coupling
oscillates with the same period as in RKKY but the amplitude is sensitive to
the Coulomb energy. At small spacer thickness however, large differences may
appear as function of : the number of electrons per atom in the ferromagnetics
and paramagnetics materials, the dielectric constant at each component, and the
charge defects at the interface plane emphasizing the effects of charge
transfer.Comment: tex file and 7 figure
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