7,823 research outputs found
The magnetic environment of the Orion-Eridanus superbubble as revealed by Planck
Using the 353-GHz polarization observations by the Planck satellite we
characterize the magnetic field in the Orion-Eridanus superbubble, a nearby
expanding structure that spans more than 1600 square degrees in the sky. We
identify a region of both low dispersion of polarization orientations and high
polarization fraction associated with the outer wall of the superbubble
identified in the most recent models of the large-scale shape of the region. We
use the Davis-Chandrasekhar-Fermi method to derive plane-of-the-sky magnetic
field strengths of tens of microGauss toward the southern edge of the bubble.
The comparison of these values with existing Zeeman splitting observations of
HI in emission suggests that the large-scale magnetic field in the region was
primarily shaped by the expanding superbubble.Comment: 7 pages, 8 figures. Accepted for publication as a Letter in A&A,
section 1. Letters to the Editor (08/12/2017
Reactive Force Field for Proton Diffusion in BaZrO3 using an empirical valence bond approach
A new reactive force field to describe proton diffusion within the
solid-oxide fuel cell material BaZrO3 has been derived. Using a quantum
mechanical potential energy surface, the parameters of an interatomic potential
model to describe hydroxyl groups within both pure and yttrium-doped BaZrO3
have been determined. Reactivity is then incorporated through the use of the
empirical valence bond model. Molecular dynamics simulations (EVB-MD) have been
performed to explore the diffusion of hydrogen using a stochastic thermostat
and barostat whose equations are extended to the isostress-isothermal ensemble.
In the low concentration limit, the presence of yttrium is found not to
significantly influence the diffusivity of hydrogen, despite the proton having
a longer residence time at oxygen adjacent to the dopant. This lack of
influence is due to the fact that trapping occurs infrequently, even when the
proton diffuses through octahedra adjacent to the dopant. The activation energy
for diffusion is found to be 0.42 eV, in good agreement with experimental
values, though the prefactor is slightly underestimated.Comment: Corrected titl
Recovering hidden Bloch character: Unfolding Electrons, Phonons, and Slabs
For a quantum state, or classical harmonic normal mode, of a system of
spatial periodicity "R", Bloch character is encoded in a wavevector "K". One
can ask whether this state has partial Bloch character "k" corresponding to a
finer scale of periodicity "r". Answering this is called "unfolding." A theorem
is proven that yields a mathematically clear prescription for unfolding, by
examining translational properties of the state, requiring no "reference
states" or basis functions with the finer periodicity (r,k). A question then
arises, how should one assign partial Bloch character to a state of a finite
system? A slab, finite in one direction, is used as the example. Perpendicular
components k_z of the wavevector are not explicitly defined, but may be hidden
in the state (and eigenvector |i>.) A prescription for extracting k_z is
offered and tested. An idealized silicon (111) surface is used as the example.
Slab-unfolding reveals surface-localized states and resonances which were not
evident from dispersion curves alone.Comment: 11 pages, 7 figure
Modelling of epitaxial graphene functionalization
A new model for graphene, epitaxially grown on silicon carbide is proposed.
Density functional theory modelling of epitaxial graphene functionalization by
hydrogen, fluorine and phenyl groups has been performed with hydrogen and
fluorine showing a high probability of cluster formation in high adatom
concentration. It has also been shown that the clusterization of fluorine
adatoms provides midgap states in formation due to significant flat distortion
of graphene. The functionalization of epitaxial graphene using larger species
(methyl and phenyl groups) renders cluster formation impossible, due to the
steric effect and results in uniform coverage with the energy gap opening.Comment: 15 pages, 4 figures, to appear in Nanotechnolog
A new route towards uniformly functionalized single-layer graphene
It is shown, by DFT calculations, that the uniform functionalization of upper
layer of graphite by hydrogen or fluorine does not change essentially its
bonding energy with the underlying layers, whereas the functionalization by
phenyl groups decreases the bonding energy by a factor of approximately ten.
This means that the functionalized monolayer in the latter case can be easily
separated by mild sonication. According to our computational results, such
layers can be cleaned up to pure graphene, as well as functionalized further up
to 25% coverage, without essential difficulties. The energy gap within the
interval from 0.5 to 3 eV can be obtained by such one-side funtionalization
using different chemical species.Comment: 15 pages, 3 figures, to appear in J. Phys. D: Applied Physic
Fast computation of the Kohn-Sham susceptibility of large systems
For hybrid systems, such as molecules grafted onto solid surfaces, the
calculation of linear response in time dependent density functional theory is
slowed down by the need to calculate, in N^4 operations, the susceptibility of
N non interacting Kohn-Sham reference electrons. We show how this
susceptibility can be calculated N times faster within finite precision. By
itself or in combination with previous methods, this should facilitate the
calculation of TDDFT response and optical spectra of hybrid systems.Comment: submitted 25/1/200
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