9,814 research outputs found
Comment on "Two Phase Transitions in the Fully frustrated XY Model"
The conclusions of a recent paper by Olsson (Phys. Rev. Lett. 75, 2758
(1995), cond-mat/9506082) about the fully frustrated XY model in two dimensions
are questioned. In particular, the evidence presented for having two separate
chiral and U(1) phase transitions are critically considered.Comment: One page one table, to Appear in Physical Review Letter
Genetic heterogeneity of residual variance in broiler chickens
Aims were to estimate the extent of genetic
heterogeneity in environmental variance. Data comprised 99 535 records of
35-day body weights from broiler chickens reared in a controlled
environment. Residual variance within dam families was estimated using
ASREML, after fitting fixed effects such as genetic groups and hatches, for
each of 377 genetically contemporary sires with a large number of progeny
(100 males or females each). Residual variance was computed separately
for male and female offspring, and after correction for sampling, strong
evidence for heterogeneity was found, the standard deviation between sires
in within variance amounting to 15–18% of its mean. Reanalysis using
log-transformed data gave similar results, and elimination of 2–3% of
outlier data reduced the heterogeneity but it was still over 10%. The
correlation between estimates for males and females was low, however. The
correlation between sire effects on progeny mean and residual variance for
body weight was small and negative (-0.1). Using a data set bigger than any
yet presented and on a trait measurable in both sexes, this study has shown
evidence for heterogeneity in the residual variance, which could not be
explained by segregation of major genes unless very few determined the
trait
Graphene Squeeze-Film Pressure Sensors
The operating principle of squeeze-film pressure sensors is based on the
pressure dependence of a membrane's resonance frequency, caused by the
compression of the surrounding gas which changes the resonator stiffness. To
realize such sensors, not only strong and flexible membranes are required, but
also minimization of the membrane's mass is essential to maximize responsivity.
Here, we demonstrate the use of a few-layer graphene membrane as a squeeze-film
pressure sensor. A clear pressure dependence of the membrane's resonant
frequency is observed, with a frequency shift of 4 MHz between 8 and 1000 mbar.
The sensor shows a reproducible response and no hysteresis. The measured
responsivity of the device is 9000 Hz/mbar, which is a factor 45 higher than
state-of-the-art MEMS-based squeeze-film pressure sensors while using a 25
times smaller membrane area
X-ray flares on the UV Ceti-type star CC Eridani: a "peculiar" time-evolution of spectral parameters
Context: Weak flares are supposed to be an important heating agent of the
outer layers of stellar atmospheres. However, due to instrumental limitations,
only large X-ray flares have been studied in detail until now.
Aims: We used an XMM-Newton observation of the very active BY-Dra type binary
star CC Eri in order to investigate the properties of two flares that are
weaker than those typically studied in the literature.
Methods: We performed time-resolved spectroscopy of the data taken with the
EPIC-PN CCD camera. A multi-temperature model was used to fit the spectra. We
inferred the size of the flaring loops using the density-temperature diagram.
The loop scaling laws were applied for deriving physical parameters of the
flaring plasma. We also estimated the number of loops involved in the observed
flares.
Results: A large X-ray variability was found. Spectral analysis showed that
all the regions in the light curve, including the flare segments, are
well-described by a 3-T model with variable emission measures but,
surprisingly, with constant temperatures (values of 3, 10 and 22 MK). The
analysed flares lasted ~ 3.4 and 7.1 ks, with flux increases of factors
1.5-1.9. They occurred in arcades made of a few tens of similar coronal loops.
The size of the flaring loops is much smaller than the distance between the
stellar surfaces in the binary system, and even smaller than the radius of each
of the stars. The obtained results are consistent with the following ideas: (i)
the whole X-ray light curve of CC Eri could be the result of a superposition of
multiple low-energy flares, and (ii) stellar flares can be scaled-up versions
of solar flares.Comment: 14 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
Analytic Reconstruction of heavy-quark two-point functions at O(\alpha_s^3)
Using a method previously developed, based on the Mellin-Barnes transform, we
reconstruct the two-point correlators in the vector, axial, scalar and
pseudoscalar channels from the Taylor expansion at q^2=0, the threshold
expansion at q^2=4m^2 and the OPE at q^2\rightarrow -\infty, where m is the
heavy quark mass. The reconstruction is analytic and systematic and is
controlled by an error function which becomes smaller as more terms in those
expansions are known.Comment: 19 pages, 11 figure
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