469 research outputs found
Dirac dispersion and non-trivial Berry's phase in three-dimensional semimetal RhSb3
We report observations of magnetoresistance, quantum oscillations and
angle-resolved photoemission in RhSb, a unfilled skutterudite semimetal
with low carrier density. The calculated electronic band structure of RhSb
entails a quantum number in analogy to
strong topological insulators, and inverted linear valence/conduction bands
that touch at discrete points close to the Fermi level, in agreement with
angle-resolved photoemission results. Transport experiments reveal an
unsaturated linear magnetoresistance that approaches a factor of 200 at 60 T
magnetic fields, and quantum oscillations observable up to 150~K that are
consistent with a large Fermi velocity ( ms), high
carrier mobility ( /Vs), and small three dimensional hole pockets
with nontrivial Berry phase. A very small, sample-dependent effective mass that
falls as low as bare masses scales with Fermi velocity, suggesting
RhSb is a new class of zero-gap three-dimensional Dirac semimetal.Comment: 9 pages, 4 figure
The Relationship Between Galaxies and Low Redshift Weak Lyman alpha Absorbers in the Directions of H1821+643 and PG1116+215
To study the nature of low z Lya absorbers in the spectra of QSOs, we have
obtained high signal-to-noise UV spectra of H 1821+643 (z = 0.297) and PG
1116+215 (z = 0.177) with the GHRS on the HST. The spectra have minimum S/N of
70-100 and 3 sigma limiting equivalent widths of 50-75 mA. We detect 26 Lya
lines with Wr > 50 mA toward H1821+643 and 13 toward PG1116+215, which implies
a density of 102+/-16 lines per unit redshift. The two-point correlation
function shows marginal evidence of clustering on ~500 km/s scales, but only if
the weakest lines are excluded. We have also used the WIYN Observatory to
measure galaxy redshifts in the ~1 degree fields centered on each QSO. We find
17 galaxy-absorber pairs within projected distances of 1 Mpc with velocity
separations of 350 km/s or less. Monte Carlo simulations show that if the Lya
lines are randomly distributed, the probability of observing this many close
pairs is 3.6e-5. We find that all galaxies with projected distances of 600 kpc
or less have associated Lya absorbers within 1000 km/s, and the majority of
these galaxies have absorbers within 350 km/s. We also find that the Lya
equivalent width is anticorrelated with the projected distance of the nearest
galaxy out to at least 600 kpc, but this should be interpreted cautiously
because there are potential selection biases. Statistical tests using the
entire sample also indicate that the absorbers are not randomly distributed. We
discuss the nature of the Lya absorbers in light of the new data.Comment: Accepted for publication in ApJ. 17 pages plus 11 tables and 17
figure
The decrotonylase FoSir5 facilitates mitochondrial metabolic state switching in conidial germination of Fusarium oxysporum.
Fusarium oxysporum is one of the most important pathogenic fungi with a broad range of plant and animal hosts. The first key step of its infection cycle is conidial germination, but there is limited information available on the molecular events supporting this process. We show here that germination is accompanied by a sharp decrease in expression of FoSir5, an ortholog of the human lysine deacetylase SIRT5. We observe that FoSir5 decrotonylates a subunit of the fungal pyruvate dehydrogenase complex (FoDLAT) at K148, resulting in inhibition of the activity of the complex in mitochondria. Moreover, FoSir5 decrotonylates histone H3K18, leading to a downregulation of transcripts encoding enzymes of aerobic respiration pathways. Thus, the activity of FoSir5 coordinates regulation in different organelles to steer metabolic flux through respiration. As ATP content is positively related to fungal germination, we propose that FoSir5 negatively modulates conidial germination in F. oxysporum through its metabolic impact. These findings provide insights into the multifaceted roles of decrotonylation, catalyzed by FoSir5, that support conidial germination in F. oxysporum
Variable O VI and N V emission from the X-ray binary LMC X-3 : heating of the black hole companion
Based on high-resolution ultraviolet spectroscopy obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Cosmic Origins Spectrograph, we present new detections of Ovi and Nv emission from the black hole X-ray binary (XRB) system LMCX-3. We also update the ephemeris of the XRB using recent radial velocity measurements obtained with the echelle spectrograph on the Magellan-Clay telescope. We observe significant velocity variability of the UV emission, and we find that the Ovi and Nv emission velocities follow the optical velocity curve of the XRB. Moreover, the Ovi and Nv intensities regularly decrease between binary phase=0.5 and 1.0, which suggests that the source of the UV emission is increasingly occulted as the B star in the XRB moves from superior to inferior conjunction. These trends suggest that illumination of the B star atmosphere by the intense X-ray emission from the accreting black hole creates a hot spot on one side of the B star, and this hot spot is the origin of the Ovi and Nv emission. However, the velocity semiamplitude of the ultraviolet emission, K-UV approximate to 180 km s(-1), is lower than the optical semiamplitude; this difference could be due to rotation of the B star. Comparison of the FUSE observations taken in 2001 November and 2004 April shows a significant change in the Ovi emission characteristics: in the 2001 data, the Ovi region shows both broad and narrow emission features, while in 2004 only the narrow Ovi emission is clearly present. Rossi X-ray Timing Explorer data show that the XRB was in a high/soft state in the 2001 November epoch but was in a transitional state in 2004 April, so the shape of the X-ray spectrum might change the properties of the region illuminated on the B star and thus change the broad versus narrow characteristics of the UV emission. If our hypothesis about the origin of the highly ionized emission is correct, then careful analysis of the emission occultation could, in principle, constrain the inclination of the XRB and the mass of the black hole
Beyond Triplet: Unconventional Superconductivity in a Spin-3/2 Topological Semimetal
In all known fermionic superfluids, Cooper pairs are composed of spin-1/2
quasi-particles that pair to form either spin-singlet or spin-triplet bound
states. The "spin" of a Bloch electron, however, is fixed by the symmetries of
the crystal and the atomic orbitals from which it is derived, and in some cases
can behave as if it were a spin-3/2 particle. The superconducting state of such
a system allows pairing beyond spin-triplet, with higher spin quasi-particles
combining to form quintet or septet pairs. Here, we report evidence of
unconventional superconductivity emerging from a spin-3/2 quasiparticle
electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density
noncentrosymmetric cubic material with a high symmetry that preserves the
-like manifold in the Bi-based band in the presence of
strong spin-orbit coupling. With a striking linear temperature dependence of
the London penetration depth, the existence of line nodes in the
superconducting order parameter is directly explained by a
mixed-parity Cooper pairing model with high total angular momentum, consistent
with a high-spin fermionic superfluid state. We propose a
model of the fermions to explain how a dominant =3 septet pairing
state is the simplest solution that naturally produces nodes in the mixed
even-odd parity gap. Together with the underlying topologically non-trivial
band structure, the unconventional pairing in this system represents a truly
novel form of superfluidity that has strong potential for leading the
development of a new generation of topological superconductors.Comment: 12 pages, 10 figures, supplementary info include
Highly Ionized High Velocity Clouds: Intergalactic Gas in the Local Group or Distant Gas in the Galactic Halo?
We have recently identified several high velocity (V < -100 km/s) clouds in
the directions of Mrk 509 and PKS 2155-304 that have unusual ionization
properties. The clouds exhibit strong C IV absorption with little or no
detectable low ion (C II, Si II) absorption or H I 21cm emission. As the
closest known analog to the outer diffuse halos of damped Ly-alpha absorbers
and the low H I column density metal line absorption systems seen in the
spectra of high redshift quasars, these "C IV-HVCs" present unique
opportunities for relating the conditions within the Milky Way halo and nearby
intergalactic gas to the properties of galactic halos at higher redshift.
The C IV-HVCs have ionization properties consistent with photoionization by
extragalactic background radiation, though some contribution by collisional
ionization within a hot plasma cannot be ruled out. The clouds are probably low
density [n(H) ~ 10^-4 cm^-3], large [greater than several kiloparsecs], and
mostly ionized [n(HI)/n(H) ~ 10^-3] regions located well beyond the neutral gas
layer of the Galaxy. The presence of weak H I-HVCs detected through 21cm
emission near both sight lines indicates that the C IV-HVCs trace the extended,
ionized, low density regions of the H I-HVCs. Several lines of evidence,
including very low thermal pressures (P/k ~ 2 cm^-3 K), favor a location for
the C IV-HVCs in the Local Group or very distant Galactic halo.Comment: AASTEX manuscript and tables, 55 pages, 8 postscript figures.
Astrophysical Journal, in pres
A comparison of user testing and heuristic evaluation methods for identifying website usability problems
This study compared the effectiveness and efficiency of two usability testing methods, user testing and heuristic evaluation. Thirty two participants took part in the study, sixteen for each of the two methods. Four measures were used to compare their performance: number of problems identified, severity of problems, type of problems and time taken to find problems. It was found that heuristic evaluation found nearly 5 times more individual problems than user testing, so could be seen as more effective. However, user testing found on average slightly more severe problems and took less time to complete than heuristic evaluation. Heuristic evaluation had a faster problem identification rate (number of seconds per problem found), so could also be seen as more efficient. While each method had advantages in the test both methods are seen as complementary to each other in practice
Discovery of the 2010 Eruption and the Pre-Eruption Light Curve for Recurrent Nova U Scorpii
We report the discovery by B. G. Harris and S. Dvorak on JD 2455224.9385
(2010 Jan 28.4385 UT) of the predicted eruption of the recurrent nova U Scorpii
(U Sco). We also report on 815 magnitudes (and 16 useful limits) on the
pre-eruption light curve in the UBVRI and Sloan r' and i' bands from 2000.4 up
to 9 hours before the peak of the January 2010 eruption. We found no
significant long-term variations, though we did find frequent fast variations
(flickering) with amplitudes up to 0.4 mag. We show that U Sco did not have any
rises or dips with amplitude greater than 0.2 mag on timescales from one day to
one year before the eruption. We find that the peak of this eruption occurred
at JD 2455224.69+-0.07 and the start of the rise was at JD 2455224.32+-0.12.
From our analysis of the average B-band flux between eruptions, we find that
the total mass accreted between eruptions is consistent with being a constant,
in agreement with a strong prediction of nova trigger theory. The date of the
next eruption can be anticipated with an accuracy of +-5 months by following
the average B-band magnitudes for the next ~10 years, although at this time we
can only predict that the next eruption will be in the year 2020+-2.Comment: Astronomical Journal submitted, 36 pages, 3 figures, full table
Combining Spatial-Temporal and Phylogenetic Analysis Approaches for Improved Understanding on Global H5N1 Transmission
Background
Since late 2003, the highly pathogenic influenza A H5N1 had initiated several outbreak waves that swept across the Eurasia and Africa continents. Getting prepared for reassortment or mutation of H5N1 viruses has become a global priority. Although the spreading mechanism of H5N1 has been studied from different perspectives, its main transmission agents and spread route problems remain unsolved.
Methodology/Principal Findings
Based on a compilation of the time and location of global H5N1 outbreaks from November 2003 to December 2006, we report an interdisciplinary effort that combines the geospatial informatics approach with a bioinformatics approach to form an improved understanding on the transmission mechanisms of H5N1 virus. Through a spherical coordinate based analysis, which is not conventionally done in geographical analyses, we reveal obvious spatial and temporal clusters of global H5N1 cases on different scales, which we consider to be associated with two different transmission modes of H5N1 viruses. Then through an interdisciplinary study of both geographic and phylogenetic analysis, we obtain a H5N1 spreading route map. Our results provide insight on competing hypotheses as to which avian hosts are responsible for the spread of H5N1.
Conclusions/Significance
We found that although South China and Southeast Asia may be the virus pool of avian flu, East Siberia may be the source of the H5N1 epidemic. The concentration of migratory birds from different places increases the possibility of gene mutation. Special attention should be paid to East Siberia, Middle Siberia and South China for improved surveillance of H5N1 viruses and monitoring of migratory birds
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