12,585 research outputs found
Depolarization volume and correlation length in the homogenization of anisotropic dielectric composites
In conventional approaches to the homogenization of random particulate
composites, both the distribution and size of the component phase particles are
often inadequately taken into account. Commonly, the spatial distributions are
characterized by volume fraction alone, while the electromagnetic response of
each component particle is represented as a vanishingly small depolarization
volume. The strong-permittivity-fluctuation theory (SPFT) provides an
alternative approach to homogenization wherein a comprehensive description of
distributional statistics of the component phases is accommodated. The
bilocally-approximated SPFT is presented here for the anisotropic homogenized
composite which arises from component phases comprising ellipsoidal particles.
The distribution of the component phases is characterized by a two-point
correlation function and its associated correlation length. Each component
phase particle is represented as an ellipsoidal depolarization region of
nonzero volume. The effects of depolarization volume and correlation length are
investigated through considering representative numerical examples. It is
demonstrated that both the spatial extent of the component phase particles and
their spatial distributions are important factors in estimating coherent
scattering losses of the macroscopic field.Comment: Typographical error in eqn. 16 in WRM version is corrected in arxiv
versio
Depolarization regions of nonzero volume in bianisotropic homogenized composites
In conventional approaches to the homogenization of random particulate
composites, the component phase particles are often treated mathematically as
vanishingly small, point-like entities. The electromagnetic responses of these
component phase particles are provided by depolarization dyadics which derive
from the singularity of the corresponding dyadic Green functions. Through
neglecting the spatial extent of the depolarization region, important
information may be lost, particularly relating to coherent scattering losses.
We present an extension to the strong-property-fluctuation theory in which
depolarization regions of nonzero volume and ellipsoidal geometry are
accommodated. Therein, both the size and spatial distribution of the component
phase particles are taken into account. The analysis is developed within the
most general linear setting of bianisotropic homogenized composite mediums
(HCMs). Numerical studies of the constitutive parameters are presented for
representative examples of HCM; both Lorentz-reciprocal and
Lorentz-nonreciprocal HCMs are considered. These studies reveal that estimates
of the HCM constitutive parameters in relation to volume fraction, particle
eccentricity, particle orientation and correlation length are all significantly
influenced by the size of the component phase particles
Antibound States and Halo Formation in the Gamow Shell Model
The open quantum system formulation of the nuclear shell model, the so-called
Gamow Shell Model (GSM), is a multi-configurational SM that employs a
single-particle basis given by the Berggren ensemble consisting of Gamow states
and the non-resonant continuum of scattering states. The GSM is of particular
importance for weakly bound/unbound nuclear states where both many-body
correlations and the coupling to decay channels are essential. In this context,
we investigate the role of l=0 antibound (virtual) neutron single-particle
states in the shell model description of loosely bound wave functions, such as
the ground state wave function of a halo nucleus 11Li
The CoRoT target HD 49933: 2- Comparison of theoretical mode amplitudes with observations
From the seismic data obtained by CoRoT for the star HD 49933 it is possible,
as for the Sun, to constrain models of the excitation of acoustic modes by
turbulent convection. We compare a stochastic excitation model described in
Paper I (arXiv:0910.4027) with the asteroseismology data for HD 49933, a star
that is rather metal poor and significantly hotter than the Sun. Using the mode
linewidths measured by CoRoT for HD 49933 and the theoretical mode excitation
rates computed in Paper I, we derive the expected surface velocity amplitudes
of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic
approximation relating the mode amplitudes in intensity to those in velocity,
we derive the expected values of the mode amplitude in intensity. Our amplitude
calculations are within 1-sigma error bars of the mode surface velocity
spectrum derived with the HARPS spectrograph. The same is found with the mode
amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other
hand, at high frequency, our calculations significantly depart from the CoRoT
and HARPS measurements. We show that assuming a solar metal abundance rather
than the actual metal abundance of the star would result in a larger
discrepancy with the seismic data. Furthermore, calculations that assume the
``new'' solar chemical mixture are in better agreement with the seismic data
than those that assume the ``old'' solar chemical mixture. These results
validate, in the case of a star significantly hotter than the Sun and Alpha Cen
A, the main assumptions in the model of stochastic excitation. However, the
discrepancies seen at high frequency highlight some deficiencies of the
modelling, whose origin remains to be understood.Comment: 8 pages, 3 figures (B-W and color), accepted for publication in
Astronomy & Astrophysics. Corrected typo in Eq. (4). Updated references.
Language improvement
Phase transitions in nonequilibrium d-dimensional models with q absorbing states
A nonequilibrium Potts-like model with absorbing states is studied using
Monte Carlo simulations. In two dimensions and the model exhibits a
discontinuous transition. For the three-dimensional case and the model
exhibits a continuous, transition with (mean-field). Simulations are
inconclusive, however, in the two-dimensional case for . We suggest that
in this case the model is close to or at the crossing point of lines separating
three different types of phase transitions. The proposed phase diagram in the
plane is very similar to that of the equilibrium Potts model. In
addition, our simulations confirm field-theory prediction that in two
dimensions a branching-annihilating random walk model without parity
conservation belongs to the directed percolation universality class.Comment: 8 pages, figures included, accepted in Phys.Rev.
The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations
By means of high-resolution cosmological hydrodynamical simulations of Milky
Way-like disc galaxies, we conduct an analysis of the associated stellar
metallicity distribution functions (MDFs). After undertaking a kinematic
decomposition of each simulation into spheroid and disc sub-components, we
compare the predicted MDFs to those observed in the solar neighbourhood and the
Galactic bulge. The effects of the star formation density threshold are visible
in the star formation histories, which show a modulation in their behaviour
driven by the threshold. The derived MDFs show median metallicities lower by
0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic
bulge. Possible reasons for this apparent discrepancy include the use of low
stellar yields and/or centrally-concentrated star formation. The dispersions
are larger than the one of the observed MDF; this could be due to simulated
discs being kinematically hotter relative to the Milky Way. The fraction of low
metallicity stars is largely overestimated, visible from the more negatively
skewed MDF with respect to the observational sample. For our fiducial Milky Way
analog, we study the metallicity distribution of the stars born "in situ"
relative to those formed via accretion (from disrupted satellites), and
demonstrate that this low-metallicity tail to the MDF is populated primarily by
accreted stars. Enhanced supernova and stellar radiation energy feedback to the
surrounding interstellar media of these pre-disrupted satellites is suggested
as an important regulator of the MDF skewness.Comment: 20 pages, 14 figures, MNRAS, accepte
Beating quantum limits in interferometers with quantum locking of mirrors
The sensitivity in interferometric measurements such as gravitational-wave
detectors is ultimately limited by quantum noise of light. We discuss the use
of feedback mechanisms to reduce the quantum effects of radiation pressure.
Recent experiments have shown that it is possible to reduce the thermal motion
of a mirror by cold damping. The mirror motion is measured with an
optomechanical sensor based on a high-finesse cavity, and reduced by a feedback
loop. We show that this technique can be extended to lock the mirror at the
quantum level. In gravitational-waves interferometers with Fabry-Perot cavities
in each arms, it is even possible to use a single feedback mechanism to lock
one cavity mirror on the other. This quantum locking greatly improves the
sensitivity of the interferometric measurement. It is furthermore insensitive
to imperfections such as losses in the interferometer
alpha-nucleus potentials for the neutron-deficient p nuclei
alpha-nucleus potentials are one important ingredient for the understanding
of the nucleosynthesis of heavy neutron-deficient p nuclei in the astrophysical
gamma-process where these p nuclei are produced by a series of (gamma,n),
(gamma,p), and (gamma,alpha) reactions. I present an improved alpha-nucleus
potential at the astrophysically relevant sub-Coulomb energies which is derived
from the analysis of alpha decay data and from a previously established
systematic behavior of double-folding potentials.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev.
Cell-Free Synthesis of the Mitochondrial ADP/ATP Carrier Protein of Neurospora crassa
ADP/ATP carrier protein was synthesized in heterologous cell-free systems programmed with Neurospora poly(A)-containing RNA and homologous cell-free systems from Neurospora. The apparent molecular weight of the product obtained in vitro was the same as that of the authentic mitochondrial protein. The primary translation product obtained in reticulocyte lysates starts with formylmethionine when formylated initiator methionyl-tRNA (fMet-tRNAfMet) was present. The product synthesized in vitro was released from the ribosomes into the postribosomal supernatant.
The evidence presented indicates that the ADP/ATP carrier is synthesized as a polypeptide with the same molecular weight as the mature monomeric protein and does not carry an additional sequence
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