3,759 research outputs found
Full-wave electromagnetic modes and hybridization in nanoparticle dimers
The plasmon hybridization theory is based on a quasi-electrostatic approximation of the Maxwellâs equations. It does not take into account magnetic interactions, retardation effects, and radiation losses. Magnetic interactions play a dominant role in the scattering from dielectric nanoparticles. The retardation effects play a fundamental role in the coupling of the modes with the incident radiation and in determining their radiative strength; their exclusion may lead to erroneous predictions of the excited modes and of the scattered power spectra. Radiation losses may lead to a significant broadening of the scattering resonances. We propose a hybridization theory for non-Hermitian composite systems based on the full-Maxwell equations that, overcoming all the limitations of the plasmon hybridization theory, unlocks the description of dielectric dimers. As an example, we decompose the scattered field from silicon and silver dimers, under different excitation conditions and gap-sizes, in terms of dimer modes, pinpointing the hybridizing isolated-sphere modes behind them
The Metallicity Distribution Function of Field Stars in M31's Bulge
We have used Hubble Space Telescope Wide Field Planetary Camera 2
observations to construct a color-magnitude diagram (CMD) for the bulge of M31
at a location ~1.6 kpc from the galaxy's center. Using scaled-solar abundance
theoretical red giant branches with a range of metallicities, we have
translated the observed colors of the stars in the CMD to abundances and
constructed a metallicity distribution function (MDF) for this region. The MDF
shows a peak at [M/H]~0 with a steep decline at higher metallicities and a more
gradual tail to lower metallicities. This is similar in shape to the MDF of the
Milky Way bulge but shifted to higher metallicities by ~0.1 dex. As is the case
with the Milky Way bulge MDF, a pure closed box model of chemical evolution,
even with significant pre-enrichment, appears to be inconsistent with the M31
bulge MDF. However, a scenario in which an initial infall of gas enriched the
bulge to an abundance of [M/H] ~ -1.6 with subsequent evolution proceeding as a
closed box provides a better fit to the observed MDF. The similarity between
the MDF of the M31 bulge and that of the Milky Way stands in stark contrast to
the significant differences in the MDFs of their halo populations. This
suggests that the bulk of the stars in the bulges of both galaxies were in
place before the accretion events that occurred in the halos could influence
them.Comment: 12 pages, 9 figures, accepted for publication in The Astronomical
Journal, October 200
An off-centred bulge or a satellite? Hydrodynamical N-body simulations of the disc galaxy NGC 5474
We present dynamical models of the star-forming galaxy NGC 5474 based on N-body hydrodynamical numerical simulations. We investigate the possible origin of the compact round stellar structure, generally interpreted as the bulge of the galaxy, but unusually off-set by â 1 kpc in projection from the visual and the kinematic centres of both the star and the gas discs. We argue that it is very unlikely that the putative bulge is in a coplanar orbit in the disc plane, showing that such a configuration would be hardly compatible with its smooth and regular spatial distribution, and, in case its mass is above 108 Mâ, also with the regular H I velocity field of NGC 5474. Instead, if the putative bulge is in fact an early-type satellite galaxy orbiting around NGC 5474, not only the off-set can be easily produced by projection effects, but our simulations suggest that the gravitational interaction between the two systems can explain also the warped H I distribution of NGC 5474 and the formation of its loose spiral arms. As a by-product of the simulations, we find that the peculiar overdensity of old stars detected in the south-west region of NGC 5474 may be explained with the interaction between NGC 5474 and a smaller stellar system, unrelated to the putative bulge, accreted in the disc plane
Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte Carlo study
We present polarised-neutron diffraction measurements of the Ising-like
spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature
TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)
structure to a mixture of SDW and commensurate antiferromagnet (CAFM)
structures is observed on cooling. For a rapidly-cooled sample, the majority
phase at low temperature is the SDW, while if the cooling is performed
sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5
and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse
scattering data. We show that both intra- and inter-chain correlations persist
above TN, but are essentially decoupled. Intra-chain correlations resemble the
ferromagnetic Ising model, while inter-chain correlations resemble the
frustrated triangular-lattice antiferromagnet. Using previously-published bulk
property measurements and our neutron diffraction data, we obtain values of the
ferromagnetic and antiferromagnetic exchange interactions and the single-ion
anisotropy.Comment: 10 pages, 7 figure
Polarization Asymmetry In The Photodisintegration Of The Deuteron
The reaction ÂČ(Îł,p)n has been studied using a monochromatic and polarized gamma ray beam at energies E(Îł)=19.8, 29.0, 38.6, and 60.8 MeV. The beam of an intensity âŒ4Ă10â” Îł/sec was obtained by Compton back scattering of mode-locked laser light off electron bunches in the Adone storage ring. Photoneutron yields were measured at nine neutron angles thetanâ15, 30, 45, 60, 90, 120, 135, 150, and 165 deg in the center of mass (c.m.) for E(Îł)=19.8, 29.0, and 38.6 MeV, and at thetanâ30, 60, 90, 120, and 150 deg c.m. for E(Îł)=60.8 MeV. The polarization independent component Iâ(theta) of the differential cross section and the polarization dependent component PIâ(theta) were deduced and the angular distribution of the azimuthal asymmetry factor ÎŁ(theta)=Iâ(theta)/Iâ(theta) was obtained. An extensive comparison with theory has been carried out and the inclusion of corrections due to meson exchange currents and to Î-isobar configurations have been shown to be mandatory at energies E(Îł)âł40 MeV. Theoretical and experimental implications of intermediate energy deuteron photo- disintegration studies are discussed in some detail
Monge's transport problem in the Heisenberg group
We prove the existence of solutions to Monge transport problem between two
compactly supported Borel probability measures in the Heisenberg group equipped
with its Carnot-Caratheodory distance assuming that the initial measure is
absolutely continuous with respect to the Haar measure of the group
The BaR-SPOrt Experiment
BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation Observations) is an
experiment to measure the linearly polarized emission of sky patches at 32 and
90 GHz with sub-degree angular resolution. It is equipped with high sensitivity
correlation polarimeters for simultaneous detection of both the U and Q stokes
parameters of the incident radiation. On-axis telescope is used to observe
angular scales where the expected polarization of the Cosmic Microwave
Background (CMBP) peaks. This project shares most of the know-how and
sophisticated technology developed for the SPOrt experiment onboard the
International Space Station. The payload is designed to flight onboard long
duration stratospheric balloons both in the Northern and Southern hemispheres
where low foreground emission sky patches are accessible. Due to the weakness
of the expected CMBP signal (in the range of microK), much care has been spent
to optimize the instrument design with respect to the systematics generation,
observing time efficiency and long term stability. In this contribution we
present the instrument design, and first tests on some components of the 32 GHz
radiometer.Comment: 12 pages, 10 figures, Astronomical Telescopes and Instrumentation
(Polaimetry in Astronomy) Hawaii August 2002 SPIE Meetin
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