26,486 research outputs found
Numerical investigation of gapped edge states in fractional quantum Hall-superconductor heterostructures
Fractional quantum Hall-superconductor heterostructures may provide a
platform towards non-abelian topological modes beyond Majoranas. However their
quantitative theoretical study remains extremely challenging. We propose and
implement a numerical setup for studying edge states of fractional quantum Hall
droplets with a superconducting instability. The fully gapped edges carry a
topological degree of freedom that can encode quantum information protected
against local perturbations. We simulate such a system numerically using exact
diagonalization by restricting the calculation to the quasihole-subspace of a
(time-reversal symmetric) bilayer fractional quantum Hall system of Laughlin
states. We show that the edge ground states are permuted by
spin-dependent flux insertion and demonstrate their fractional Josephson
effect, evidencing their topological nature and the Cooper pairing of
fractionalized quasiparticles.Comment: 12 pages, 9 figure
Excited nucleon electromagnetic form factors from broken spin-flavor symmetry
A group theoretical derivation of a relation between the N --> Delta charge
quadrupole transition and neutron charge form factors is presented.Comment: 4 pages, Proc. of the 12 th Int'l. Workshop on the Physics of Excited
Nucleons, NSTAR 2009, Beijing, April 19-22, 200
A Proper Motion Survey for White Dwarfs with the Wide Field Planetary Camera 2
We have performed a search for halo white dwarfs as high proper motion
objects in a second epoch WFPC2 image of the Groth-Westphal strip. We identify
24 high proper motion objects with mu > 0.014 ''/yr. Five of these high proper
motion objects are identified as strong white dwarf candidates on the basis of
their position in a reduced proper motion diagram. We create a model of the
Milky Way thin disk, thick disk and stellar halo and find that this sample of
white dwarfs is clearly an excess above the < 2 detections expected from these
known stellar populations. The origin of the excess signal is less clear.
Possibly, the excess cannot be explained without invoking a fourth galactic
component: a white dwarf dark halo. We present a statistical separation of our
sample into the four components and estimate the corresponding local white
dwarf densities using only the directly observable variables, V, V-I, and mu.
For all Galactic models explored, our sample separates into about 3 disk white
dwarfs and 2 halo white dwarfs. However, the further subdivision into the thin
and thick disk and the stellar and dark halo, and the subsequent calculation of
the local densities are sensitive to the input parameters of our model for each
Galactic component. Using the lowest mean mass model for the dark halo we find
a 7% white dwarf halo and six times the canonical value for the thin disk white
dwarf density (at marginal statistical significance), but possible systematic
errors due to uncertainty in the model parameters likely dominate these
statistical error bars. The white dwarf halo can be reduced to around 1.5% of
the halo dark matter by changing the initial mass function slightly. The local
thin disk white dwarf density in our solution can be made consistent with the
canonical value by assuming a larger thin disk scaleheight of 500 pc.Comment: revised version, accepted by ApJ, results unchanged, discussion
expande
Development of Uniform CdTe Pixel Detectors Based on Caltech ASIC
We have developed a large CdTe pixel detector with dimensions of 23.7 x 13.0
mm and a pixel size of 448 x 448 um^2. The detector is based on recent
technologies of an uniform CdTe single crystal, a two-dimensional ASIC, and
stud bump-bonding to connect pixel electrodes on the CdTe surface to the ASIC.
Good spectra are obtained from 1051 pixels out of total 1056 pixels. When we
operate the detector at -50 C, the energy resolution is 0.67 keV and 0.99 keV
at 14 keV and 60 keV, respectively. Week-long stability of the detector is
confirmed at operating temperatures of both -50 C and -20 C. The detector also
shows high uniformity: the peak positions for all pixels agree to within 0.82%,
and the average of the energy resolution is 1.04 keV at a temperature of -50 C.
When we normalized the peak area by the total counts detected by each pixel, a
variation of 2.1 % is obtained.Comment: 11pages, 17figures, accepted for publication in Proc. SPIE 200
The Innermost Stable Circular Orbit of Binary Black Holes
We introduce a new method to construct solutions to the constraint equations
of general relativity describing binary black holes in quasicircular orbit.
Black hole pairs with arbitrary momenta can be constructed with a simple method
recently suggested by Brandt and Bruegmann, and quasicircular orbits can then
be found by locating a minimum in the binding energy along sequences of
constant horizon area. This approach produces binary black holes in a
"three-sheeted" manifold structure, as opposed to the "two-sheeted" structure
in the conformal-imaging approach adopted earlier by Cook. We focus on locating
the innermost stable circular orbit and compare with earlier calculations. Our
results confirm those of Cook and imply that the underlying manifold structure
has a very small effect on the location of the innermost stable circular orbit.Comment: 8 pages, 3 figures, RevTex, submitted to PR
Ice and Dust in the Quiescent Medium of Isolated Dense Cores
The relation between ices in the envelopes and disks surrounding YSOs and
those in the quiescent interstellar medium is investigated. For a sample of 31
stars behind isolated dense cores, ground-based and Spitzer spectra and
photometry in the 1-25 um wavelength range are combined. The baseline for the
broad and overlapping ice features is modeled, using calculated spectra of
giants, H2O ice and silicates. The adopted extinction curve is derived
empirically. Its high resolution allows for the separation of continuum and
feature extinction. The extinction between 13-25 um is ~50% relative to that at
2.2 um. The strengths of the 6.0 and 6.85 um absorption bands are in line with
those of YSOs. Thus, their carriers, which, besides H2O and CH3OH, may include
NH4+, HCOOH, H2CO and NH3, are readily formed in the dense core phase, before
stars form. The 3.53 um C-H stretching mode of solid CH3OH was discovered. The
CH3OH/H2O abundance ratios of 5-12% are larger than upper limits in the Taurus
molecular cloud. The initial ice composition, before star formation occurs,
therefore depends on the environment. Signs of thermal and energetic processing
that were found toward some YSOs are absent in the ices toward background
stars. Finally, the peak optical depth of the 9.7 um band of silicates relative
to the continuum extinction at 2.2 um is significantly shallower than in the
diffuse interstellar medium. This extends the results of Chiar et al. (2007) to
a larger sample and higher extinctions.Comment: Accepted for publication in The Astrophysical Journa
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Long-term follow-up and objective physical activity measurements of community-based physical interventions in adults: a systematic review and meta-analysis
Reproducibility of graph metrics of human brain structural networks
Recent interest in human brain connectivity has led to the application of graph theoretical analysis to human brain structural networks, in particular white matter connectivity inferred from diffusion imaging and fiber tractography. While these methods have been used to study a variety of patient populations, there has been less examination of the reproducibility of these methods. A number of tractography algorithms exist and many of these are known to be sensitive to user-selected parameters. The methods used to derive a connectivity matrix from fiber tractography output may also influence the resulting graph metrics. Here we examine how these algorithm and parameter choices influence the reproducibility of proposed graph metrics on a publicly available test-retest dataset consisting of 21 healthy adults. The dice coefficient is used to examine topological similarity of constant density subgraphs both within and between subjects. Seven graph metrics are examined here: mean clustering coefficient, characteristic path length, largest connected component size, assortativity, global efficiency, local efficiency, and rich club coefficient. These reproducibility of these network summary measures is examined using the intraclass correlation coefficient (ICC). Graph curves are created by treating the graph metrics as functions of a parameter such as graph density. Functional data analysis techniques are usedto examine differences in graph measures that result from the choice of fiber tracking algorithm. The graph metrics consistently showed good levels of reproducibility as measured with ICC, with the exception of some instability at low graph density levels. The global and local efficiency measures were the most robust to the choice of fiber tracking algorithm
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