9,562 research outputs found
Estimating exploitable stock biomass for the Maine green sea urchin (Strongylocentrotus droebachiensis) fishery using a spatial statistics approach
The objective of this study was to investigate the spatial patterns in green sea urchin (Strongylocentrotus
droebachiensis) density off the coast of Maine, using data from a fishery-independent survey program, to estimate the exploitable biomass of this species. The dependence of sea
urchin variables on the environment, the lack of stationarity, and the presence of discontinuities in the study area made intrinsic geostatistics inappropriate for the study; therefore, we used triangulated irregular
networks (TINs) to characterize the large-scale patterns in sea urchin density. The resulting density surfaces were modified to include only areas of the appropriate substrate
type and depth zone, and were used to calculate total biomass. Exploitable biomass was estimated by using two
different sea urchin density threshold values, which made different assumptions about the fishing industry. We
observed considerable spatial variability on both small and large scales, including large-scale patterns in sea urchin density related to depth and fishing pressure. We conclude that the TIN method provides a reasonable spatial approach for generating biomass estimates for a fishery unsuited
to geostatistics, but we suggest further studies into uncertainty estimation and the selection of threshold
density values
Pinning modes and interlayer correlation in high magnetic field bilayer Wigner solids
We report studies of pinning mode resonances in the low total Landau filling
(\nu) Wigner solid of a series of bilayer hole samples with negligible
interlayer tunneling, and with varying interlayer separation d. Comparison of
states with equal layer densities (p,p) to single layer states (p,0) produced
{in situ} by biasing, indicates that there is interlayer quantum correlation in
the solid at small d. Also, the resonance frequency at small d is decreased
just near \nu=1/2 and 2/3, indicating the importance in the solid of
correlations related to those in the fractional quantum Hall effects
Experimental study on discretely modulated continuous-variable quantum key distribution
We present a discretely modulated continuous-variable quantum key
distribution system in free space by using strong coherent states. The
amplitude noise in the laser source is suppressed to the shot-noise limit by
using a mode cleaner combined with a frequency shift technique. Also, it is
proven that the phase noise in the source has no impact on the final secret key
rate. In order to increase the encoding rate, we use broadband homodyne
detectors and the no-switching protocol. In a realistic model, we establish a
secret key rate of 46.8 kbits/s against collective attacks at an encoding rate
of 10 MHz for a 90% channel loss when the modulation variance is optimal.Comment: 7 pages,6 figure
Localizing Region-Based Level-set Contouring for Common Carotid Artery in Ultrasonography
This work developed a fully-automated and efficient method for detecting contour of common carotid artery in the cross section view of two-dimensional B-mode sonography. First, we applied a preprocessing filter to the ultrasound image for the sake of reducing speckle. An adaptive initial contouring method was then performed to obtain the initial contour for level set segmentation. Finally, the localizing region-based level set segmentation automatically extracted the precise contours of common carotid artery. The proposed method evaluated 130 ultrasound images from three healthy volunteers and the segmentation results were compared to the boundaries outlined by an expert. Preliminary results showed that the method described here could identify the contour of common carotid artery with satisfactory accuracy in this dataset
Time-dependent versus static quantum transport simulations beyond linear response
To explore whether the density-functional theory non-equilibrium Green's
function formalism (DFT-NEGF) provides a rigorous framework for quantum
transport, we carried out time-dependent density functional theory (TDDFT)
calculations of the transient current through two realistic molecular devices,
a carbon chain and a benzenediol molecule inbetween two aluminum electrodes.
The TDDFT simulations for the steady state current exactly reproduce the
results of fully self-consistent DFT-NEGF calculations even beyond linear
response. In contrast, sizable differences are found with respect to an
equilibrium, non-self-consistent treatment which are related here to
differences in the Kohn-Sham and fully interacting susceptibility of the device
region. Moreover, earlier analytical conjectures on the equivalence of static
and time-dependent approaches in the low bias regime are confirmed with high
numerical precision.Comment: 4 pages, 4 figure
The neutron/proton ratio of squeezed-out nucleons and the high density behavior of the nuclear symmetry energy
Within a transport model it is shown that the neutron/proton ratio of squeezed-out nucleons perpendicular to the reaction plane, especially at high transverse momenta, in heavy-ion reactions induced by high energy neutron-rich nuclei can be a useful tool for studying the high density behavior of the nuclear symmetry energy
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