4,547 research outputs found
First-principles Calculations of Engineered Surface Spin Structures
The engineered spin structures recently built and measured in scanning
tunneling microscope experiments are calculated using density functional
theory. By determining the precise local structure around the surface
impurities, we find the Mn atoms can form molecular structures with the binding
surface, behaving like surface molecular magnets. The spin structures are
confirmed to be antiferromagnetic, and the exchange couplings are calculated
within 8% of the experimental values simply by collinear-spin GGA+U
calculations. We can also explain why the exchange couplings significantly
change with different impurity binding sites from the determined local
structure. The bond polarity is studied by calculating the atomic charges with
and without the Mn adatoms
Ideal strengths and bonding properties of PuO2 under tension
We perform a first-principles computational tensile test on PuO based
on density-functional theory within local density approximation (LDA)+\emph{U}
formalism to investigate its structural, mechanical, magnetic, and intrinsic
bonding properties in the four representative directions: [001], [100], [110],
and [111]. The stress-strain relations show that the ideal tensile strengths in
the four directions are 81.2, 80.5, 28.3, and 16.8 GPa at strains of 0.36,
0.36, 0.22, and 0.18, respectively. The [001] and [100] directions are
prominently stronger than other two directions since that more PuO bonds
participate in the pulling process. Through charge and density of states
analysis along the [001] direction, we find that the strong mixed
ionic/covalent character of PuO bond is weakened by tensile strain and
PuO will exhibit an insulator-to-metal transition after tensile stress
exceeds about 79 GPa.Comment: 11 pages, 6 figure
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
Polarization and Charge Transfer in the Hydration of Chloride Ions
A theoretical study of the structural and electronic properties of the
chloride ion and water molecules in the first hydration shell is presented. The
calculations are performed on an ensemble of configurations obtained from
molecular dynamics simulations of a single chloride ion in bulk water. The
simulations utilize the polarizable AMOEBA force field for trajectory
generation, and MP2-level calculations are performed to examine the electronic
structure properties of the ions and surrounding waters in the external field
of more distant waters. The ChelpG method is employed to explore the effective
charges and dipoles on the chloride ions and first-shell waters. The Quantum
Theory of Atoms in Molecules (QTAIM) is further utilized to examine charge
transfer from the anion to surrounding water molecules.
From the QTAIM analysis, 0.2 elementary charges are transferred from the ion
to the first-shell water molecules. The default AMOEBA model overestimates the
average dipole moment magnitude of the ion compared with the estimated quantum
mechanical value. The average magnitude of the dipole moment of the water
molecules in the first shell treated at the MP2 level, with the more distant
waters handled with an AMOEBA effective charge model, is 2.67 D. This value is
close to the AMOEBA result for first-shell waters (2.72 D) and is slightly
reduced from the bulk AMOEBA value (2.78 D). The magnitude of the dipole moment
of the water molecules in the first solvation shell is most strongly affected
by the local water-water interactions and hydrogen bonds with the second
solvation shell, rather than by interactions with the ion.Comment: Slight revision, in press at J. Chem. Phy
Development of a new laser Doppler velocimeter for the Ames High Reynolds Channel No. 2
A new two-channel laser Doppler velocimeter developed for the Ames High Reynolds Channel No. 2 is described. Design features required for the satisfactory operation of the optical system in the channel environment are discussed. Fiber optics are used to transmit the megahertz Doppler signal to the photodetectors located outside the channel pressure vessel, and provision is made to isolate the optical system from pressure and thermal strain effects. Computer-controlled scanning mirrors are used to position the laser beams in the channel flow. Techniques used to seed the flow with 0.5-micron-diam polystyrene spheres avoiding deposition on the test-section windows and porous boundary-layer removal panels are described. Preliminary results are presented with a discussion of several of the factors affecting accuracy
Quantized time correlation function approach to non-adiabatic decay rates in condensed phase: Application to solvated electrons in water and methanol
A new, alternative form of the golden rule formula defining the non-adiabatic transition rate between two quantum states in condensed phase is presented. The formula involves the quantum time correlation function of the energy gap, of the non-adiabatic coupling, and their cross terms. Those quantities can be inferred from their classical counterparts, determined via MD simulations. The formalism is applied to the problem of the non-adiabatic relaxation of an equilibrated p-electron in water and methanol. We find that, in both solvent, the relaxation is induced by the coupling to the vibrational modes and the quantum effects modify the rate by a factor of 2-10 depending on the quantization procedure applied. The resulting p-state lifetime for a hypothetical equilibrium excited state appears extremely short, in the sub-100 fs regime. Although this result is in contrast with all previous theoretical predictions, we also illustrate that the lifetimes computed here are very sensitive to the simulated electronic quantum gap and to the strongly correlated non-adiabatic coupling
A hard metallic material: Osmium Diboride
We calculate the structural and electronic properties of OsB2 using density
functional theory with or without taking into account spin-orbit (SO)
interaction. Our results show that the bulk modulus with and without SO
interaction are 364 and 365 Gpa respectively, both are in good agreement with
experiment (365-395 Gpa). The evidence of covalent bonding of Os-B, which plays
an important role to form a hard material, is indicated both in charge density,
atoms in molecules analysis, and density of states analysis. The good
metallicity and hardness of OsB2 might suggest its potential application as
hard conductors.Comment: Figures improve
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
- …