4,005 research outputs found
Localized exciton-polariton modes in dye-doped nanospheres: a quantum approach
We model a dye-doped polymeric nanosphere as an ensemble of quantum emitters
and use it to investigate the localized exciton-polaritons supported by such a
nanosphere. By determining the time evolution of the density matrix of the
collective system, we explore how an incident laser field may cause transient
optical field enhancement close to the surface of such nanoparticles. Our
results provide further evidence that excitonic materials can be used to good
effect in nanophotonics.Comment: 16 pages, 4 figure
Multifunctional Devices and Logic Gates With Undoped Silicon Nanowires
We report on the electronic transport properties of multiple-gate devices
fabricated from undoped silicon nanowires. Understanding and control of the
relevant transport mechanisms was achieved by means of local electrostatic
gating and temperature dependent measurements. The roles of the source/drain
contacts and of the silicon channel could be independently evaluated and tuned.
Wrap gates surrounding the silicide-silicon contact interfaces were proved to
be effective in inducing a full suppression of the contact Schottky barriers,
thereby enabling carrier injection down to liquid-helium temperature. By
independently tuning the effective Schottky barrier heights, a variety of
reconfigurable device functionalities could be obtained. In particular, the
same nanowire device could be configured to work as a Schottky barrier
transistor, a Schottky diode or a p-n diode with tunable polarities. This
versatility was eventually exploited to realize a NAND logic gate with gain
well above one.Comment: 6 pages, 5 figure
A rigorous implementation of the Jeans--Landau--Teller approximation
Rigorous bounds on the rate of energy exchanges between vibrational and
translational degrees of freedom are established in simple classical models of
diatomic molecules. The results are in agreement with an elementary
approximation introduced by Landau and Teller. The method is perturbative
theory ``beyond all orders'', with diagrammatic techniques (tree expansions) to
organize and manipulate terms, and look for compensations, like in recent
studies on KAM theorem homoclinic splitting.Comment: 23 pages, postscrip
Exploring the origin and degree of genetic isolation of Anopheles gambiae from the islands of São Tomé and Príncipe, potential sites for testing transgenic-based vector control
The evolutionary processes at play between island and mainland populations of the malaria mosquito vector Anopheles gambiae sensu stricto are of great interest as islands may be suitable sites for preliminary application of transgenic-based vector control strategies. São Tomé and Príncipe, located off the West African coast, have received such attention in recent years. This study investigates the degree of isolation of An. gambiae s.s. populations between these islands and the mainland based on mitochondrial and ribosomal DNA molecular data. We identify possible continental localities from which these island populations derived. For these purposes, we used FST values, haplotype networks, and nested clade analysis to estimate migration rates and patterns. Haplotypes from both markers are geographically widespread across the African continent. Results indicate that the populations from São Tomé and Príncipe are relatively isolated from continental African populations, suggesting they are promising sites for test releases of transgenic individuals. These island populations are possibly derived from two separate continental migrations. This result is discussed in the context of the history of the African slave trade with respect to São Tomé and Príncipe
Draco, a flawless dwarf galaxy
The Draco dwarf spheroidal galaxy (dSph), with its apparent immense mass to
light ratio and compact size, holds many clues to the nature of the enigmatic
dark matter. Here we present deep photometric studies of this dwarf galaxy,
undertaken with the MegaCam Camera at the Canada-France-hawaii Telescope, the
Wide Field Camera at the Isaac Newton Telescope and the Wide-Field and
Planetary Camera on board the Hubble Space Telescope. The new photometric data
cover the entirety of the galaxy, and reach i=24.5 at 50% completeness,
significantly deeper than previous panoramic studies, allowing searches for
tidal disturbances of much lower surface brightness than has been possible
before. With these improved statistics, we find no evidence for asymmetric
disturbances or tidal tails that possess more than 3% of the stars found within
the centre of the galaxy. We find that the central stellar density, as probed
by the HST data, rises into the central 0.5'. Uncertainties in the position of
the centroid of the galaxy do not allow us to determine whether the apparent
flattening of the profile interior to 0.5' is reliable or not. Draco is
therefore a flawless dwarf galaxy, featureless and apparently unaffected by
Galactic tides.Comment: 13 pages, 16 figures, accepted for publication in MNRA
GREAT3 results I: systematic errors in shear estimation and the impact of real galaxy morphology
We present first results from the third GRavitational lEnsing Accuracy
Testing (GREAT3) challenge, the third in a sequence of challenges for testing
methods of inferring weak gravitational lensing shear distortions from
simulated galaxy images. GREAT3 was divided into experiments to test three
specific questions, and included simulated space- and ground-based data with
constant or cosmologically-varying shear fields. The simplest (control)
experiment included parametric galaxies with a realistic distribution of
signal-to-noise, size, and ellipticity, and a complex point spread function
(PSF). The other experiments tested the additional impact of realistic galaxy
morphology, multiple exposure imaging, and the uncertainty about a
spatially-varying PSF; the last two questions will be explored in Paper II. The
24 participating teams competed to estimate lensing shears to within systematic
error tolerances for upcoming Stage-IV dark energy surveys, making 1525
submissions overall. GREAT3 saw considerable variety and innovation in the
types of methods applied. Several teams now meet or exceed the targets in many
of the tests conducted (to within the statistical errors). We conclude that the
presence of realistic galaxy morphology in simulations changes shear
calibration biases by per cent for a wide range of methods. Other
effects such as truncation biases due to finite galaxy postage stamps, and the
impact of galaxy type as measured by the S\'{e}rsic index, are quantified for
the first time. Our results generalize previous studies regarding sensitivities
to galaxy size and signal-to-noise, and to PSF properties such as seeing and
defocus. Almost all methods' results support the simple model in which additive
shear biases depend linearly on PSF ellipticity.Comment: 32 pages + 15 pages of technical appendices; 28 figures; submitted to
MNRAS; latest version has minor updates in presentation of 4 figures, no
changes in content or conclusion
Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation
(abridged) Predictions of the Concordance Cosmological Model (CCM) of the
structures in the environment of large spiral galaxies are compared with
observed properties of Local Group galaxies. Five new most probably
irreconcilable problems are uncovered. However, the Local Group properties
provide hints that may lead to a solution of the above problems The DoS and
bulge--satellite correlation suggest that dissipational events forming bulges
are related to the processes forming phase-space correlated satellite
populations. Such events are well known to occur since in galaxy encounters
energy and angular momentum are expelled in the form of tidal tails, which can
fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star
clusters. If Local Group satellite galaxies are to be interpreted as TDGs then
the sub-structure predictions of CCM are internally in conflict. All findings
thus suggest that the CCM does not account for the Local Group observations and
that therefore existing as well as new viable alternatives have to be further
explored. These are discussed and natural solutions for the above problems
emerge.Comment: A and A, in press, 25 pages, 9 figures; new version contains minor
text adjustments for conformity with the published version and additional
minor changes resulting from reader's feedback. The speculation on a dark
force has been added. Also, the Fritz Zwicky Paradox is now included to agree
with the published versio
Atomic Resonance and Scattering
Contains reports on six research projects.National Science Foundation (Grant PHY 83-06273)U.S. Navy - Office of Naval Research (Contract N00014-79-C-0183)Joint Services Electronics Program (Contract DAALO03-86-K-0002)National Science Foundation (Grant PHY 84-11483)National Science Foundation (Grant PHY 86-05893)National Science Foundation (Grant ECS 84-21392)U.S. Navy - Office of Naval Research (Contract N00014-83-K-0695)National Science Foundation (Grant CHE 84-21392
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