3,175 research outputs found
Massive creation of entangled exciton states in semiconductor quantum dots
An intense laser pulse propagating in a medium of inhomogeneously broadened
quantum dots massively creates entangled exciton states. After passage of the
pulse all single-exciton states remain unpopulated (self-induced transparency)
whereas biexciton coherence (exciton entanglement) is generated through
two-photon transitions. We propose several experimental techniques for the
observation of such unexpected behavior
Observations of H3+ in the Diffuse Interstellar Medium
Surprisingly large column densities of H3+ have been detected using infrared
absorption spectroscopy in seven diffuse cloud sightlines (Cygnus OB2 12,
Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating
that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard
model of diffuse cloud chemistry, our H3+ column densities imply unreasonably
long path lengths (~1 kpc) and low densities (~3 cm^-3). Complimentary
millimeter-wave, infrared, and visible observations of related species suggest
that the chemical model is incorrect and that the number density of H3+ must be
increased by one to two orders of magnitude. Possible solutions include a
reduced electron fraction, an enhanced rate of H2 ionization, and/or a smaller
value of the H3+ dissociative recombination rate constant than implied by
laboratory experiments.Comment: To be published in Astrophysical Journal, March 200
The Extinction and Distance of Maffei 1
We have obtained low- and high-resolution spectra of the core of the
highly-reddened elliptical galaxy Maffei 1. From these data, we have obtained
the first measurement of the Mg2 index, and have measured the velocity
dispersion and radial velocity with improved accuracy. To evaluate the
extinction, a correlation between the Mg2 index and effective V-I colour has
been established for elliptical galaxies. Using a new method for correcting for
effective wavelength shifts, we find A_V = 4.67 +/- 0.19 mag, which is lower by
0.4 mag than previously thought. To establish the distance, the Fundamental
Plane for elliptical galaxies has been constructed in I. The velocity
dispersion of Maffei 1, measured to be 186.8 +/- 7.4 km/s, in combination with
modern wide-field photometry in I, leads to a distance of 2.92 +/- 0.37 Mpc.
The Dn-sigma relation, which is independently calibrated, gives 3.08 +/- 0.85
Mpc and 3.23 +/- 0.67 Mpc from photometry in B and K`, respectively. The
weighted mean of the three estimates is 3.01 +/- 0.30 Mpc. The distance and
luminosity make Maffei 1 the nearest giant elliptical galaxy. The radial
velocity of Maffei 1 is +66.4 +/- 5.0 km/s, significantly higher than the
accepted value of -10 km/s. The Hubble distance corresponding to the mean
velocity of Maffei 1, Maffei 2 and IC342 is 3.5 Mpc. Thus, it is unlikely that
Maffei 1 has had any influence on Local Group dynamics
Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized -Pu
The effects on the local structure due to self-irradiation damage of Ga
stabilized -Pu stored at cryogenic temperatures have been examined
using extended x-ray absorption fine structure (EXAFS) experiments. Extensive
damage, seen as a loss of local order, was evident after 72 days of storage
below 15 K. The effect was observed from both the Pu and Ga sites, although
less pronounced around Ga. Isochronal annealing was performed on this sample to
study the annealing processes that occur between cryogenic and room temperature
storage conditions, where damage is mostly reversed. Damage fractions at
various points along the annealing curve have been determined using an
amplitude-ratio method, standard EXAFS fitting, and a spherical crystallite
model, and provide information complementary to previous electrical
resistivity- and susceptibility-based isochronal annealing studies. The use of
a spherical crystallite model accounts for the changes in EXAFS spectra using
just two parameters, namely, the crystalline fraction and the particle radius.
Together, these results are discussed in terms of changes to the local
structure around Ga and Pu throughout the annealing process and highlight the
unusual role of Ga in the behavior of the lowest temperature anneals.Comment: 13 pages, 10 figure
Whispering Vortices
Experiments indicating the excitation of whispering gallery type
electromagnetic modes by a vortex moving in an annular Josephson junction are
reported. At relativistic velocities the Josephson vortex interacts with the
modes of the superconducting stripline resonator giving rise to novel
resonances on the current-voltage characteristic of the junction. The
experimental data are in good agreement with analysis and numerical
calculations based on the two-dimensional sine--Gordon model.Comment: 5 pages, 5 figures, text shortened to fit 4 pages, correction of
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Spatiotemporally Localized Multidimensional Solitons in Self-Induced Transparency Media
"Light bullets" are multi-dimensional solitons which are localized in both
space and time. We show that such solitons exist in two- and three-dimensional
self-induced-transparency media and that they are fully stable. Our approximate
analytical calculation, backed and verified by direct numerical simulations,
yields the multi-dimensional generalization of the one-dimensional Sine-Gordon
soliton.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Iso-osmotic regulation of nitrate accumulation in lettuce (Lactuca sativa L.)
Concerns about possible health hazards arising from human consumption of lettuce and other edible vegetable crops with high concentrations of nitrate have generated demands for a greater understanding of processes involved in its uptake and accumulation in order to devise more sustainable strategies for its control. This paper evaluates a proposed iso-osmotic mechanism for the regulation of nitrate accumulation in lettuce (Lactuca sativa L.) heads. This mechanism assumes that changes in the concentrations of nitrate and all other endogenous osmotica (including anions, cations and neutral solutes) are continually adjusted in tandem to minimise differences in osmotic potential of the shoot sap during growth, with these changes occurring independently of any variations in external water potential. The hypothesis was tested using data from six new experiments, each with a single unique treatment comprising a separate combination of light intensity, N source (nitrate with or without ammonium) and nitrate concentration carried out hydroponically in a glasshouse using a butterhead lettuce variety. Repeat measurements of plant weights and estimates of all of the main soluble constituents (nitrate, potassium, calcium, magnesium, organic anions, chloride, phosphate, sulphate and soluble carbohydrates) in the shoot sap were made at intervals from about 2 weeks after transplanting until commercial maturity, and the data used to calculate changes in average osmotic potential in the shoot. Results showed that nitrate concentrations in the sap increased when average light levels were reduced by between 30 and 49 % and (to a lesser extent) when nitrate was supplied at a supra-optimal concentration, and declined with partial replacement of nitrate by ammonium in the external nutrient supply. The associated changes in the proportions of other endogenous osmotica, in combination with the adjustment of shoot water content, maintained the total solute concentrations in shoot sap approximately constant and minimised differences in osmotic potential between treatments at each sampling date. There was, however, a gradual increase in osmotic potential (ie a decline in total solute concentration) over time largely caused by increases in shoot water content associated with the physiological and morphological development of the plants. Regression analysis using normalised data (to correct for these time trends) showed that the results were consistent with a 1:1 exchange between the concentrations of nitrate and the sum of all other endogenous osmotica throughout growth, providing evidence that an iso-osmotic mechanism (incorporating both concentration and volume regulation) was involved in controlling nitrate concentrations in the shoot
Braggoriton--Excitation in Photonic Crystal Infiltrated with Polarizable Medium
Light propagation in a photonic crystal infiltrated with polarizable
molecules is considered. We demonstrate that the interplay between the spatial
dispersion caused by Bragg diffraction and polaritonic frequency dispersion
gives rise to novel propagating excitations, or braggoritons, with intragap
frequencies. We derive the braggoriton dispersion relation and show that it is
governed by two parameters, namely, the strength of light-matter interaction
and detuning between the Bragg frequency and that of the infiltrated molecules.
We also study defect-induced states when the photonic band gap is divided into
two subgaps by the braggoritonic branches and find that each defect creates two
intragap localized states inside each subgap.Comment: LaTeX, 8 pages, 5 figure
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