11,036 research outputs found
Achieving dietary recommendations and reducing greenhouse gas emissions : modelling diets to minimise the change from current intakes
Acknowledgements We are grateful to Mr Hubert Ehlert for extending the work on the coding for the linear programming and to Mrs Jennifer Loe for preparing the dataset matching the food consumption data with the GHGE data. Funding This work was supported as part of the research programme funded by the Scottish Government Rural and Environment Science and Analytical Services Division [RESAS].Peer reviewedPublisher PD
Self-pulsation dynamics in narrow stripe semiconductor lasers
In this paper, we address the physical origin of self-pulsation in narrow stripe edge emitting semiconductor lasers. We present both experimental time-averaged polarization-resolved near-field measurements performed with a charged-coupled device camera and picosecond time resolved near-field measurements performed with a streak camera. These results demonstrate dynamic spatial-hole burning during pulse formation and evolution. We conclude from these experimental results that the dominant process which drives the self-pulsation in this type of laser diode is carrier induced effective refractive index change induced by the spatial-hole burning
Witnessed entanglement and the geometric measure of quantum discord
We establish relations between geometric quantum discord and entanglement
quantifiers obtained by means of optimal witness operators. In particular, we
prove a relation between negativity and geometric discord in the
Hilbert-Schmidt norm, which is slightly different from a previous conjectured
one [Phys. Rev. A 84, 052110 (2011)].We also show that, redefining the
geometric discord with the trace norm, better bounds can be obtained. We
illustrate our results numerically.Comment: 8 pages + 3 figures. Revised version with erratum for PRA 86, 024302
(2012). Simplified proof that discord is bounded by entanglement in any nor
Evolution of Migrating Planets Undergoing Gas Accretion
We analyze the orbital and mass evolution of planets that undergo run-away
gas accretion by means of 2D and 3D hydrodynamic simulations. The disk torque
distribution per unit disk mass as a function of radius provides an important
diagnostic for the nature of the disk-planet interactions. We first consider
torque distributions for nonmigrating planets of fixed mass and show that there
is general agreement with the expectations of resonance theory. We then present
results of simulations for mass-gaining, migrating planets. For planets with an
initial mass of 5 Earth masses, which are embedded in disks with standard
parameters and which undergo run-away gas accretion to one Jupiter mass (Mjup),
the torque distributions per unit disk mass are largely unaffected by migration
and accretion for a given planet mass. The migration rates for these planets
are in agreement with the predictions of the standard theory for planet
migration (Type I and Type II migration). The planet mass growth occurs through
gas capture within the planet's Bondi radius at lower planet masses, the Hill
radius at intermediate planet masses, and through reduced accretion at higher
planet masses due to gap formation. During run-away mass growth, a planet
migrates inwards by only about 20% in radius before achieving a mass of ~1
Mjup. For the above models, we find no evidence of fast migration driven by
coorbital torques, known as Type III migration. We do find evidence of Type III
migration for a fixed mass planet of Saturn's mass that is immersed in a cold
and massive disk. In this case the planet migration is assumed to begin before
gap formation completes. The migration is understood through a model in which
the torque is due to an asymmetry in density between trapped gas on the leading
side of the planet and ambient gas on the trailing side of the planet.Comment: 26 pages, 29 figures. To appear in The Astrophysical Journal vol.684
(September 20, 2008 issue
Fitting EXAFS data using molecular dynamics outputs and a histogram approach
The estimation of metal nanoparticle diameter by analysis of extended x-ray absorption fine structure (EXAFS) data from coordination numbers is nontrivial, particularly for particles <5 nm in diameter, for which the undercoordination of surface atoms becomes an increasingly significant contribution to the average coordination number. These undercoordinated atoms have increased degrees of freedom over those within the core of the particle, which results in an increase in the degree of structural disorder with decreasing particle size. This increase in disorder, however, is not accounted for by the standard means of EXAFS analysis, where each coordination shell is fitted with a single bond length and disorder term. In addition, the surface atoms of nanoparticles have been observed to undergo a greater contraction than those in the core, further increasing the range of bond distances. Failure to account for this structural change results in an increased disorder being measured, and therefore, a lower apparent coordination number and corresponding particle size are found. Here, we employ molecular dynamics (MD) simulations for a range of nanoparticle sizes to determine each of the nearest neighbor bond lengths, which were then binned into a histogram to construct a radial distribution function (RDF). Each bin from the histogram was considered to be a single scattering path and subsequently used in fitting the EXAFS data obtained for a series of carbon-supported platinum nanoparticles. These MD-based fits are compared with those obtained using a standard fitting model using Artemis and the standard model with the inclusion of higher cumulants, which has previously been used to account for the non-Gaussian distribution of neighboring atoms around the absorber. The results from all three fitting methods were converted to particle sizes and compared with those obtained from transmission electron microscopy (TEM) and x-ray diffraction (XRD) measurements. We find that the use of molecular dynamics simulations resulted in an improved fit over both the standard and cumulant models, in terms of both quality of fit and correlation with the known average particle size
Breakdown of large-N quenched reduction in SU(N) lattice gauge theories
We study the validity of the large-N equivalence between four-dimensional
SU(N) lattice gauge theory and its momentum quenched version--the Quenched
Eguchi-Kawai (QEK) model. We find that the assumptions needed for the proofs of
equivalence do not automatically follow from the quenching prescription. We use
weak-coupling arguments to show that large-N equivalence is in fact likely to
break down in the QEK model, and that this is due to dynamically generated
correlations between different Euclidean components of the gauge fields. We
then use Monte-Carlo simulations at intermediate couplings with 20 <= N <= 200
to provide strong evidence for the presence of these correlations and for the
consequent breakdown of reduction. This evidence includes a large discrepancy
between the transition coupling of the "bulk" transition in lattice gauge
theories and the coupling at which the QEK model goes through a strongly
first-order transition. To accurately measure this discrepancy we adapt the
recently introduced Wang-Landau algorithm to gauge theories.Comment: 51 pages, 16 figures, Published verion. Historical inaccuracies in
the review of the quenched Eguchi-Kawai model are corrected, discussion on
reduction at strong-coupling added, references updated, typos corrected. No
changes to results or conclusion
On the Transit Potential of the Planet Orbiting iota Draconis
Most of the known transiting exoplanets are in short-period orbits, largely
due to the bias inherent in detecting planets through the transit technique.
However, the eccentricity distribution of the known radial velocity planets
results in many of those planets having a non-negligible transit probability.
One such case is the massive planet orbiting the giant star iota Draconis, a
situation where both the orientation of the planet's eccentric orbit and the
size of the host star inflate the transit probability to a much higher value
than for a typical hot Jupiter. Here we present a revised fit of the radial
velocity data with new measurements and a photometric analysis of the stellar
variability. We provide a revised transit probability, an improved transit
ephemeris, and discuss the prospects for observing a transit of this planet
from both the ground and space.Comment: 6 pages, 7 figures, accepted for publication in ApJ. Radial
velocities will be made available in the on-line version and through the NASA
Star and Exoplanet Database (NStED). Minor corrections from ApJ proof have
been applie
Live-Cell Imaging of Single Receptor Composition Using Zero-Mode Waveguide Nanostructures
We exploit the optical and spatial features of subwavelength nanostructures to examine individual receptors on the plasma membrane of living cells. Receptors were sequestered in portions of the membrane projected into zero-mode waveguides. Using single-step photobleaching of green fluorescent protein incorporated into individual subunits, the resulting spatial isolation was used to measure subunit stoichiometry in α4ÎČ4 and α4ÎČ2 nicotinic acetylcholine and P2X2 ATP receptors. We also show that nicotine and cytisine have differential effects on α4ÎČ2 stoichiometry
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