1,362 research outputs found
Observation of ChiralityâInduced RotonâLike Dispersion in a 3D Micropolar Elastic Metamaterial
A theoretical paper based on chiral micropolar effective-medium theory suggested the possibility of unusual roton-like acoustical-phonon dispersion relations in 3D elastic materials. Here, as a first novelty, the corresponding inverse problem is solved, that is, a specific 3D chiral elastic metamaterial structure is designed, the behavior of which follows this effective-medium description. The metamaterial structure is based on a simple-cubic lattice of cubes, each of which not only has three translational but also three rotational degrees of freedom. The additional rotational degrees of freedom are crucial within micropolar elasticity. The cubes and their degrees of freedom are coupled by a chiral network of slender rods. As a second novelty, this complex metamaterial is manufactured in polymer form by 3D laser printing and its behavior is characterized experimentally by phonon-band-structure measurements. The results of these measurements, microstructure finite-element calculations, and solutions of micropolar effective-medium theory are in good agreement. The roton-like dispersion behavior of the lowest phonon branch results from two aspects. First, chirality splits the transverse acoustical branches as well as the transverse optical branches. Second, chirality leads to an ultrastrong coupling and hybridization of chiral acoustical and optical phonons at finite wavevectors
Hierarchical multi-class segmentation of glioma images using networks with multi-level activation function
For many segmentation tasks, especially for the biomedical image, the
topological prior is vital information which is useful to exploit. The
containment/nesting is a typical inter-class geometric relationship. In the
MICCAI Brain tumor segmentation challenge, with its three hierarchically nested
classes 'whole tumor', 'tumor core', 'active tumor', the nested classes
relationship is introduced into the 3D-residual-Unet architecture. The network
comprises a context aggregation pathway and a localization pathway, which
encodes increasingly abstract representation of the input as going deeper into
the network, and then recombines these representations with shallower features
to precisely localize the interest domain via a localization path. The
nested-class-prior is combined by proposing the multi-class activation function
and its corresponding loss function. The model is trained on the training
dataset of Brats2018, and 20% of the dataset is regarded as the validation
dataset to determine parameters. When the parameters are fixed, we retrain the
model on the whole training dataset. The performance achieved on the validation
leaderboard is 86%, 77% and 72% Dice scores for the whole tumor, enhancing
tumor and tumor core classes without relying on ensembles or complicated
post-processing steps. Based on the same start-of-the-art network architecture,
the accuracy of nested-class (enhancing tumor) is reasonably improved from 69%
to 72% compared with the traditional Softmax-based method which blind to
topological prior.Comment: 12pages first versio
The Mean Number of Extra Micro-Image Pairs for Macro-Lensed Quasars
When a gravitationally lensed source crosses a caustic, a pair of images is
created or destroyed. We calculate the mean number of such pairs of
micro-images for a given macro-image of a gravitationally lensed point
source, due to microlensing by the stars of the lensing galaxy. This quantity
was calculated by Wambsganss, Witt & Schneider (1992) for the case of zero
external shear, $\gamma=0$, at the location of the macro-image. Since in
realistic lens models a non-zero shear is expected to be induced by the lensing
galaxy, we extend this calculation to a general value of $\gamma$. We find a
complex behavior of as a function of and the normalized surface
mass density in stars . Specifically, we find that at high
magnifications, where the average total magnification of the macro-image is
, . The ratio is largest near
the line where the magnification
to the results of numerical simulations and find good agreement. We find that
the probability distribution for the number of extra image pairs is reasonably
described by a Poisson distribution with a mean value of , and that the
width of the macro-image magnification distribution tends to be largest for
.Comment: As accepted for publication in ApJ. 11 pages, 4 figures, minor
change
Four problems with global carbon markets: a critical review
This article offers a critique of global carbon markets and trading, with a special focus on the Clean Development Mechanism of the Kyoto Protocol. It explores
problems with the use of tradable permits to address climate change revolving around four areas: homogeneity, justice, gaming, and information. Homogeneity problems arise from the non-linear nature of climate change and sensitivity of emissions, which complicate attempts to calculate carbon offsets. Justice problems involve issues of dependency and the concentration of wealth among the rich, meaning carbon trading often counteracts attempts to reduce poverty. Gaming problems include pressures to promote high-volume, least-cost projects and the
consequences of emissions leakage. Information problems encompass transaction costs related to carbon trading and market participation and the comparatively weak institutional capacity of project evaluators
Co-ordination of brain and heart oscillations during non-rapid eye movement sleep
Oscillatory activities of the brain and heart show a strong variation across wakefulness and sleep. Separate lines of research indicate that nonârapid eye movement (NREM) sleep is characterised by electroencephalographic slow oscillations (SO), sleep spindles, and phaseâamplitude coupling of these oscillations (SOâspindle coupling), as well as an increase in highâfrequency heart rate variability (HFâHRV), reflecting enhanced parasympathetic activity. The present study aimed to investigate further the potential coordination between brain and heart oscillations during NREM sleep. Data were derived from one sleep laboratory night with polysomnographic monitoring in 45 healthy participants (22 male, 23 female; mean age 37 years). The associations between the strength (modulation index [MI]) and phase direction of SOâspindle coupling (circular measure) and HFâHRV during NREM sleep were investigated using linear modelling. First, a significant SOâspindle coupling (MI) was observed for all participants during NREM sleep, with spindle peaks preferentially occurring during the SO upstate (phase direction). Second, linear model analyses of NREM sleep showed a significant relationship between the MI and HFâHRV (F = 20.1, r (2) = 0.30, p < 0.001) and a tentative circularâlinear correlation between phase direction and HFâHRV (F = 3.07, r (2) = 0.12, p = 0.056). We demonstrated a coâordination between SOâspindle phaseâamplitude coupling and HFâHRV during NREM sleep, presumably related to parallel central nervous and peripheral vegetative arousal systems regulation. Further investigating the fineâgraded coâordination of brain and heart oscillations might improve our understanding of the links between sleep and cardiovascular health
The Ensemble Photometric Variability of ~25000 Quasars in the Sloan Digital Sky Survey
Using a sample of over 25000 spectroscopically confirmed quasars from the
Sloan Digital Sky Survey, we show how quasar variability in the rest frame
optical/UV regime depends upon rest frame time lag, luminosity, rest
wavelength, redshift, the presence of radio and X-ray emission, and the
presence of broad absorption line systems. The time dependence of variability
(the structure function) is well-fit by a single power law on timescales from
days to years. There is an anti-correlation of variability amplitude with rest
wavelength, and quasars are systematically bluer when brighter at all
redshifts. There is a strong anti-correlation of variability with quasar
luminosity. There is also a significant positive correlation of variability
amplitude with redshift, indicating evolution of the quasar population or the
variability mechanism. We parameterize all of these relationships. Quasars with
RASS X-ray detections are significantly more variable (at optical/UV
wavelengths) than those without, and radio loud quasars are marginally more
variable than their radio weak counterparts. We find no significant difference
in the variability of quasars with and without broad absorption line troughs.
Models involving multiple discrete events or gravitational microlensing are
unlikely by themselves to account for the data. So-called accretion disk
instability models are promising, but more quantitative predictions are needed.Comment: 41 pages, 21 figures, AASTeX, Accepted for publication in Ap
Broad Absorption Line Variability in Repeat Quasar Observations from the Sloan Digital Sky Survey
We present a time-variability analysis of 29 broad absorption line quasars
(BALQSOs) observed in two epochs by the Sloan Digital Sky Survey (SDSS). These
spectra are selected from a larger sample of BALQSOs with multiple observations
by virtue of exhibiting a broad CIV 1549 absorption trough separated
from the rest frame of the associated emission peak by more than 3600 km
s. Detached troughs facilitate higher precision variability
measurements, since the measurement of the absorption in these objects is not
complicated by variation in the emission line flux. We have undertaken a
statistical analysis of these detached-trough BALQSO spectra to explore the
relationships between BAL features that are seen to vary and the dynamics of
emission from the quasar central engine. We have measured variability within
our sample, which includes three strongly variable BALs. We have also verified
that the statistical behavior of the overall sample agrees with current model
predictions and previous studies of BAL variability. Specifically, we observe
that the strongest BAL variability occurs among the smallest equivalent width
features and at velocities exceeding 12,000 km s, as predicted by recent
disk-wind modeling.Comment: 11 pages, 7 figures. Accepted for publication in Ap
An L Band Spectrum of the Coldest Brown Dwarf
The coldest brown dwarf, WISE 0855, is the closest known planetary-mass,
free-floating object and has a temperature nearly as cold as the solar system
gas giants. Like Jupiter, it is predicted to have an atmosphere rich in
methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint
at near-infrared wavelengths and emits almost all its energy in the
mid-infrared. Skemer et al. 2016 presented a spectrum of WISE 0855 from 4.5-5.1
micron (M band), revealing water vapor features. Here, we present a spectrum of
WISE 0855 in L band, from 3.4-4.14 micron. We present a set of atmosphere
models that include a range of compositions (metallicities and C/O ratios) and
water ice clouds. Methane absorption is clearly present in the spectrum. The
mid-infrared color can be better matched with a methane abundance that is
depleted relative to solar abundance. We find that there is evidence for water
ice clouds in the M band spectrum, and we find a lack of phosphine spectral
features in both the L and M band spectra. We suggest that a deep continuum
opacity source may be obscuring the near-infrared flux, possibly a deep
phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE
0855 provide critical constraints for cold planetary atmospheres, bridging the
temperature range between the long-studied solar system planets and accessible
exoplanets. JWST will soon revolutionize our understanding of cold brown dwarfs
with high-precision spectroscopy across the infrared, allowing us to study
their compositions and cloud properties, and to infer their atmospheric
dynamics and formation processes.Comment: 19 pages, 21 figures. Accepted for publication in Ap
Observations and Theoretical Implications of the Large Separation Lensed Quasar SDSS J1004+4112
We study the recently discovered gravitational lens SDSS J1004+4112, the
first quasar lensed by a cluster of galaxies. It consists of four images with a
maximum separation of 14.62''. The system has been confirmed as a lensed quasar
at z=1.734 on the basis of deep imaging and spectroscopic follow-up
observations. We present color-magnitude relations for galaxies near the lens
plus spectroscopy of three central cluster members, which unambiguously confirm
that a cluster at z=0.68 is responsible for the large image separation. We find
a wide range of lens models consistent with the data, but they suggest four
general conclusions: (1) the brightest cluster galaxy and the center of the
cluster potential well appear to be offset by several kpc; (2) the cluster mass
distribution must be elongated in the North--South direction, which is
consistent with the observed distribution of cluster galaxies; (3) the
inference of a large tidal shear (~0.2) suggests significant substructure in
the cluster; and (4) enormous uncertainty in the predicted time delays between
the images means that measuring the delays would greatly improve constraints on
the models. We also compute the probability of such large separation lensing in
the SDSS quasar sample, on the basis of the CDM model. The lack of large
separation lenses in previous surveys and the discovery of one in SDSS together
imply a mass fluctuation normalization \sigma_8=1.0^{+0.4}_{-0.2} (95% CL), if
cluster dark matter halos have an inner slope -1.5. Shallower profiles would
require higher values of \sigma_8. Although the statistical conclusion might be
somewhat dependent on the degree of the complexity of the lens potential, the
discovery is consistent with the predictions of the abundance of cluster-scale
halos in the CDM scenario. (Abridged)Comment: 21 pages, 24 figures, 5 tables, accepted for publication in Ap
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