4,384 research outputs found
Acoustic vibrations of anisotropic nanoparticles
Acoustic vibrations of nanoparticles made of materials with anisotropic
elasticity and nanoparticles with non-spherical shapes are theoretically
investigated using a homogeneous continuum model. Cubic, hexagonal and
tetragonal symmetries of the elasticity are discussed, as are spheroidal,
cuboctahedral and truncated cuboctahedral shapes. Tools are described to
classify the different vibrations and for example help identify the modes
having a significant low-frequency Raman scattering cross-section. Continuous
evolutions of the modes starting from those of an isotropic sphere coupled with
the determination of the irreducible representation of the branches permit some
qualitative statements to be made about the nature of various modes. For
spherical nanoparticles, a more accurate picture is obtained through
projections onto the vibrations of an isotropic sphere.Comment: 11 pages, 9 tables, 6 figure
Vibrations of free and embedded anisotropic elastic spheres: Application to low-frequency Raman scattering of silicon nanoparticles in silica
Vibrational mode frequencies and damping are calculated for an elastic sphere
embedded in an infinite, homogeneous, isotropic elastic medium. Anisotropic
elasticity of the sphere significantly shifts the frequencies in comparison to
simplified calculations that assume isotropy. New low frequency Raman light
scattering data are presented for silicon spheres grown in a SiO2 glass matrix.
Principal features of the Raman spectrum are not correctly described by a
simple model of the nanoparticle as a free, isotropic sphere, but require both
matrix effects and the anisotropy of the silicon to be taken into account.
Libration, not vibration, is the dominant mechanism
Delayed Sampling and Automatic Rao-Blackwellization of Probabilistic Programs
We introduce a dynamic mechanism for the solution of analytically-tractable
substructure in probabilistic programs, using conjugate priors and affine
transformations to reduce variance in Monte Carlo estimators. For inference
with Sequential Monte Carlo, this automatically yields improvements such as
locally-optimal proposals and Rao-Blackwellization. The mechanism maintains a
directed graph alongside the running program that evolves dynamically as
operations are triggered upon it. Nodes of the graph represent random
variables, edges the analytically-tractable relationships between them. Random
variables remain in the graph for as long as possible, to be sampled only when
they are used by the program in a way that cannot be resolved analytically. In
the meantime, they are conditioned on as many observations as possible. We
demonstrate the mechanism with a few pedagogical examples, as well as a
linear-nonlinear state-space model with simulated data, and an epidemiological
model with real data of a dengue outbreak in Micronesia. In all cases one or
more variables are automatically marginalized out to significantly reduce
variance in estimates of the marginal likelihood, in the final case
facilitating a random-weight or pseudo-marginal-type importance sampler for
parameter estimation. We have implemented the approach in Anglican and a new
probabilistic programming language called Birch.Comment: 13 pages, 4 figure
Far infrared absorption by acoustic phonons in titanium dioxide nanopowders
We report spectral features of far infrared electromagnetic radiation
absorption in anatase TiO2 nanopowders which we attribute to absorption by
acoustic phonon modes of nanoparticles. The frequency of peak excess absorption
above the background level corresponds to the predicted frequency of the
dipolar acoustic phonon from continuum elastic theory. The intensity of the
absorption cannot be accounted for in a continuum elastic dielectric
description of the nanoparticle material. Quantum mechanical scale dependent
effects must be considered. The absorption cross section is estimated from a
simple mechanical phenomenological model. The results are in plausible
agreement with the absorption being due to a sparse layer of charge on the
nanoparticle surface.Comment: 8 pages, 5 figures, submitted to Journal of Nanoelectronics and
Optoelectronic
Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment
The inelastic scattering of neutrons by nanoparticles due to acoustic
vibrational modes (energy below 10 meV) confined in nanoparticles is calculated
using the Zemach-Glauber formalism. Such vibrational modes are commonly
observed by light scattering techniques (Brillouin or low-frequency Raman
scattering). We also report high resolution inelastic neutron scattering
measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling
the observation of such vibrations are discussed. These include a narrow
nanoparticle size distribution which minimizes inhomogeneous broadening of the
spectrum and the presence of hydrogen atoms oscillating with the nanoparticle
surfaces which enhances the number of scattered neutrons.Comment: 3 figures, 1 tabl
Chimera States for Coupled Oscillators
Arrays of identical oscillators can display a remarkable spatiotemporal
pattern in which phase-locked oscillators coexist with drifting ones.
Discovered two years ago, such "chimera states" are believed to be impossible
for locally or globally coupled systems; they are peculiar to the intermediate
case of nonlocal coupling. Here we present an exact solution for this state,
for a ring of phase oscillators coupled by a cosine kernel. We show that the
stable chimera state bifurcates from a spatially modulated drift state, and
dies in a saddle-node bifurcation with an unstable chimera.Comment: 4 pages, 4 figure
Simulations of spectral lines from an eccentric precessing accretion disc
Two dimensional SPH simulations of a precessing accretion disc in a q=0.1
binary system (such as XTE J1118+480) reveal complex and continuously varying
shape, kinematics, and dissipation. The stream-disc impact region and disc
spiral density waves are prominent sources of energy dissipation.The dissipated
energy is modulated on the period P_{sh} = ({P_{orb}}^{-1}-{P_{prec}}^{-1}^{-1}
with which the orientation of the disc relative to the mass donor repeats. This
superhump modulation in dissipation energy has a variation in amplitude of ~10%
relative to the total dissipation energy and evolves, repeating exactly only
after a full disc precession cycle. A sharp component in the light curve is
associated with centrifugally expelled material falling back and impacting the
disc. Synthetic trailed spectrograms reveal two distinct "S-wave" features,
produced respectively by the stream gas and the disc gas at the stream-disc
impact shock. These S-waves are non-sinusoidal, and evolve with disc precession
phase. We identify the spiral density wave emission in the trailed spectrogram.
Instantaneous Doppler maps show how the stream impact moves in velocity space
during an orbit. In our maximum entropy Doppler tomogram the stream impact
region emission is distorted, and the spiral density wave emission is
uppressed. A significant radial velocity modulation of the whole line profile
occurs on the disc precession period. We compare our SPH simulation with a
simple 3D model: the former is appropriate for comparison with emission lines
while the latter is preferable for skewed absorption lines from precessing
discs.Comment: See http://physics.open.ac.uk/FHMR/ for associated movie (avi) files.
The full paper is in MNRAS press. Limited disk space limit of 650k, hence low
resolution figure file
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