1,568 research outputs found
Bi2Te1.6S1.4 - a Topological Insulator in the Tetradymite Family
We describe the crystal growth, crystal structure, and basic electrical
properties of Bi2Te1.6S1.4, which incorporates both S and Te in its Tetradymite
quintuple layers in the motif -[Te0.8S0.2]-Bi-S-Bi-[Te0.8S0.2]-. This material
differs from other Tetradymites studied as topological insulators due to the
increased ionic character that arises from its significant S content.
Bi2Te1.6S1.4 forms high quality crystals from the melt and is the S-rich limit
of the ternary Bi-Te-S {\gamma}-Tetradymite phase at the melting point. The
native material is n-type with a low resistivity; Sb substitution, with
adjustment of the Te to S ratio, results in a crossover to p-type and resistive
behavior at low temperatures. Angle resolved photoemission study shows that
topological surface states are present, with the Dirac point more exposed than
it is in Bi2Te3 and similar to that seen in Bi2Te2Se. Single crystal structure
determination indicates that the S in the outer chalcogen layers is closer to
the Bi than the Te, and therefore that the layers supporting the surface states
are corrugated on the atomic scale.Comment: To be published in Physical Review B Rapid Communications 16 douuble
spaced pages. 4 figures 1 tabl
Particle transport in evolving protoplanetary disks: Implications for results from Stardust
Samples returned from comet 81P/Wild 2 by Stardust confirm that substantial
quantities of crystalline silicates were incorporated into the comet at
formation. We investigate the constraints that this observation places upon
protoplanetary disk physics, assuming that outward transport of particles
processed at high temperatures occurs via advection and turbulent diffusion in
an evolving disk. We also look for constraints on particle formation locations.
Our results are based upon 1D disk models that evolve with time under the
action of viscosity and photoevaporation, and track solid transport using an
ensemble of individual particle trajectories. We find that two classes of disk
model are consistent with the Stardust findings. One class features a high
particle diffusivity (a Schmidt number Sc < 1), which suffices to diffuse
particles up to 20 microns in size outward against the mean gas flow. For Sc >
1, such models are unlikely to be viable, and significant outward transport
requires that the particles of interest settle into a midplane layer that
experiences an outward gas flow. In either class of models, the mass of inner
disk material that reaches the outer disk is a strong function of the disk's
initial compactness. Hence, models of grain transport within steady-state disks
underestimate the efficiency of outward transport. Neither model results in
sustained outward transport of very large particles exceeding a mm in size. We
show that the transport efficiency generally falls off rapidly with time.
Hence, high-temperature material must be rapidly incorporated into icy bodies
to avoid fallback, and significant radial transport may only occur during the
initial phase of rapid disk evolution. It may also vary substantially between
disks depending upon their initial mass distributions. We discuss implications
for Spitzer observations of crystalline silicates in T Tauri disks.Comment: ApJ, in pres
Development and mapping of SNP assays in allotetraploid cotton
A narrow germplasm base and a complex allotetraploid genome have made the discovery of single nucleotide polymorphism (SNP) markers difficult in cotton (Gossypiumhirsutum). To generate sequence for SNP discovery, we conducted a genome reduction experiment (EcoRI, BafI double digest, followed by adapter ligation, biotin–streptavidin purification, and agarose gel separation) on two accessions of G. hirsutum and two accessions of G. barbadense. From the genome reduction experiment, a total of 2.04 million genomic sequence reads were assembled into contigs with an N50 of 508 bp and analyzed for SNPs. A previously generated assembly of expressed sequence tags (ESTs) provided an additional source for SNP discovery. Using highly conservative parameters (minimum coverage of 8× at each SNP and 20% minor allele frequency), a total of 11,834 and 1,679 non-genic SNPs were identified between accessions of G. hirsutum and G. barbadense in genome reduction assemblies, respectively. An additional 4,327 genic SNPs were also identified between accessions of G. hirsutum in the EST assembly. KBioscience KASPar assays were designed for a portion of the intra-specific G. hirsutum SNPs. From 704 non-genic and 348 genic markers developed, a total of 367 (267 non-genic, 100 genic) mapped in a segregating F2 population (Acala Maxxa × TX2094) using the Fluidigm EP1 system. A G. hirsutum genetic linkage map of 1,688 cM was constructed based entirely on these new SNP markers. Of the genic-based SNPs, we were able to identify within which genome (‘A’ or ‘D’) each SNP resided using diploid species sequence data. Genetic maps generated by these newly identified markers are being used to locate quantitative, economically important regions within the cotton genome
Crystalline Silicate Emission in the Protostellar Binary Serpens--SVS20
We present spatially resolved mid-infrared spectroscopy of the class
I/flat-spectrum protostellar binary system SVS20 in the Serpens cloud core. The
spectra were obtained with the mid-infrared instrument T-ReCS on Gemini-South.
SVS20-South, the more luminous of the two sources, exhibits a mid-infrared
emission spectrum peaking near 11.3 \micron, while SVS20-North exhibits a
shallow amorphous silicate absorption spectrum with a peak optical depth of
. After removal of the the line-of-sight extinction by the
molecular common envelope, the ``protostar-only'' spectra are found to be
dominated by strong amorphous olivine emission peaking near 10 \micron. We also
find evidence for emission from crystalline forsterite and enstatite associated
with both SVS20-S and SVS20-N. The presence of crystalline silicate in such a
young binary system indicates that the grain processing found in more evolved
HAeBe and T Tauri pre-main sequence stars likely begins at a relatively young
evolutionary stage, while mass accretion is still ongoing.Comment: Accepted for publication by The Astrophysical Journa
The DEEP2 Galaxy Redshift Survey: Mean Ages and Metallicities of Red Field Galaxies at z ~ 0.9 from Stacked Keck/DEIMOS Spectra
As part of the DEEP2 galaxy redshift survey, we analyze absorption line
strengths in stacked Keck/DEIMOS spectra of red field galaxies with weak to no
emission lines, at redshifts 0.7 <= z <= 1. Comparison with models of stellar
population synthesis shows that red galaxies at z ~ 0.9 have mean
luminosity-weighted ages of the order of only 1 Gyr and at least solar
metallicities. This result cannot be reconciled with a scenario where all stars
evolved passively after forming at very high z. Rather, a significant fraction
of stars can be no more than 1 Gyr old, which means that star formation
continued to at least z ~ 1.2. Furthermore, a comparison of these distant
galaxies with a local SDSS sample, using stellar populations synthesis models,
shows that the drop in the equivalent width of Hdelta from z ~ 0.9 to 0.1 is
less than predicted by passively evolving models. This admits of two
interpretations: either each individual galaxy experiences continuing low-level
star formation, or the red-sequence galaxy population from z ~ 0.9 to 0.1 is
continually being added to by new galaxies with younger stars.Comment: A few typos were corrected and numbers in Table 1 were revise
Prospects for detecting the 21cm forest from the diffuse intergalactic medium with LOFAR
We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.Comment: 12 pages, 13 figures. MNRAS, in pres
Foregrounds for observations of the cosmological 21 cm line: II. Westerbork observations of the fields around 3C196 and the North Celestial Pole
In the coming years a new insight into galaxy formation and the thermal
history of the Universe is expected to come from the detection of the highly
redshifted cosmological 21 cm line. The cosmological 21 cm line signal is
buried under Galactic and extragalactic foregrounds which are likely to be a
few orders of magnitude brighter. Strategies and techniques for effective
subtraction of these foreground sources require a detailed knowledge of their
structure in both intensity and polarization on the relevant angular scales of
1-30 arcmin. We present results from observations conducted with the Westerbork
telescope in the 140-160 MHz range with 2 arcmin resolution in two fields
located at intermediate Galactic latitude, centred around the bright quasar
3C196 and the North Celestial Pole. They were observed with the purpose of
characterizing the foreground properties in sky areas where actual observations
of the cosmological 21 cm line could be carried out. The polarization data were
analysed through the rotation measure synthesis technique. We have computed
total intensity and polarization angular power spectra. Total intensity maps
were carefully calibrated, reaching a high dynamic range, 150000:1 in the case
of the 3C196 field. [abridged]Comment: 20 pages, 22 figures, accepted for publication in A&A. A version with
full resolution figures is available at
http://www.astro.rug.nl/~bernardi/NCP_3C196/bernardi.pd
Solving variational inequalities defined on a domain with infinitely many linear constraints
We study a variational inequality problem whose domain is defined by infinitely many linear inequalities. A discretization method and an analytic center based inexact cutting plane method are proposed. Under proper assumptions, the convergence results for both methods are given. We also provide numerical examples to illustrate the proposed method
Independent measurement of the Hoyle state feeding from 12B using Gammasphere
Using an array of high-purity Compton-suppressed germanium detectors, we
performed an independent measurement of the -decay branching ratio from
to the second-excited (Hoyle) state in . Our
result is , which is a factor smaller than the previously
established literature value, but is in agreement with another recent
measurement. This could indicate that the Hoyle state is more clustered than
previously believed. The angular correlation of the Hoyle state
cascade has also been measured for the first time. It is consistent with
theoretical predictions
- …