30,156 research outputs found
Universal features of Thermopower in High Tc systems and Quantum Criticality
In high Tc superconductors a wide ranging connection between the doping
dependence of the transition temperature Tc and the room temperature
thermopower Q has been observed. A "universal correlation" between these two
quantities exists with the thermopower vanishing at optimum doping as noted by
OCTHH (Obertelli, Cooper, Tallon, Honma and Hor). In this work we provide an
interpretation of this OCTHH universality in terms of a possible underlying
quantum critical point (QCP) at Tc. Central to our viewpoint is the recently
noted Kelvin formula relating the thermopower to the density derivative of the
entropy. Perspective on this formula is gained through a model calculation of
the various Kubo formulas in an exactly solved 1-dimensional model with various
limiting procedures of wave vector and frequency.Comment: 12 pages, 8 figure
Minimal weight digit set conversions
Copyright © 2004 IEEEWe consider the problem of recoding a number to minimize the number of nonzero digits in its representation, that is, to minimize the weight of the representation. A general sliding window scheme is described that extends minimal binary sliding window conversion to arbitrary radix and to encompass signed digit sets. This new conversion expresses a number of known recoding techniques as special cases. Proof that this scheme achieves minimal weight for a given digit set is provided and results concerning the theoretical average and worst-case weight are derived.Braden Phillips and Neil Burges
Coplanar Circumbinary Debris Disks
We present resolved Herschel images of circumbinary debris disks in the alpha
CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are
consistent with being aligned with the binary orbital planes. Though secular
perturbations from the binary can align the disk, in both cases the alignment
time at the distances at which the disk is resolved is greater than the stellar
age, so we conclude that the coplanarity was primordial. Neither disk can be
modelled as a narrow ring, requiring extended radial distributions. To satisfy
both the Herschel and mid-IR images of the alpha CrB disk, we construct a model
that extends from 1-300AU, whose radial profile is broadly consistent with a
picture where planetesimal collisions are excited by secular perturbations from
the binary. However, this model is also consistent with stirring by other
mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk
model extends from 50-400AU. A model with depleted (rather than empty) inner
regions also reproduces the observations and is consistent with binary and
other stirring mechanisms. As part of the modelling process, we find that the
Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The
70um variation can therefore hinder image interpretation, particularly for
poorly resolved objects. The number of systems in which circumbinary debris
disk orientations have been compared with the binary plane is now four. More
systems are needed, but a picture in which disks around very close binaries
(alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are
aligned, and disks around wider binaries (99 Her, with a 50 yr period) are
misaligned, may be emerging. This picture is qualitatively consistent with the
expectation that the protoplanetary disks from which the debris emerged are
more likely to be aligned if their binaries have shorter periods.Comment: accepted to MNRA
Statistical Power, the Bispectrum and the Search for Non-Gaussianity in the CMB Anisotropy
We use simulated maps of the cosmic microwave background anisotropy to
quantify the ability of different statistical tests to discriminate between
Gaussian and non-Gaussian models. Despite the central limit theorem on large
angular scales, both the genus and extrema correlation are able to discriminate
between Gaussian models and a semi-analytic texture model selected as a
physically motivated non-Gaussian model. When run on the COBE 4-year CMB maps,
both tests prefer the Gaussian model. Although the bispectrum has comparable
statistical power when computed on the full sky, once a Galactic cut is imposed
on the data the bispectrum loses the ability to discriminate between models.
Off-diagonal elements of the bispectrum are comparable to the diagonal elements
for the non-Gaussian texture model and must be included to obtain maximum
statistical power.Comment: Accepted for publication in ApJ; 20 pages, 6 figures, uses AASTeX
v5.
Antimicrobial susceptibility of recent Australian isolates of Brachyspira hyodysenteriae
Swine dysentery (SD) is an important disease in Australia, causing considerable economic loss through reduced growth rates in grower/ finisher pigs and control costs. Swine dysentery is characterized by mucohaemorrhagic colitis, resulting from infection with the anaerobic intestinal spirochaete Brachyspira hyodysenteriae. The diseases can be controlled, but worldwide there is concern about reduced susceptibility of many strains to the commonly available antimicrobials. Furthermore, antimicrobials are being withdrawn due to fears of transmission of resistance to human pathogenic microorganisms, or the presence of potentially toxic residues. The aim of this study was to examine the susceptibility of recent Australian B. hyodysenteriae isolates to commonly available antimicrobial agents
An atomic clock with instability
Atomic clocks have been transformational in science and technology, leading
to innovations such as global positioning, advanced communications, and tests
of fundamental constant variation. Next-generation optical atomic clocks can
extend the capability of these timekeepers, where researchers have long aspired
toward measurement precision at 1 part in . This milestone will
enable a second revolution of new timing applications such as relativistic
geodesy, enhanced Earth- and space-based navigation and telescopy, and new
tests on physics beyond the Standard Model. Here, we describe the development
and operation of two optical lattice clocks, both utilizing spin-polarized,
ultracold atomic ytterbium. A measurement comparing these systems demonstrates
an unprecedented atomic clock instability of after
only hours of averaging
Constraints On The Topology Of The Universe From The WMAP First-Year Sky Maps
We compute the covariance expected between the spherical harmonic
coefficients of the cosmic microwave temperature anisotropy if the
universe had a compact topology. For fundamental cell size smaller than the
distance to the decoupling surface, off-diagonal components carry more
information than the diagonal components (the power spectrum). We use a maximum
likelihood analysis to compare the Wilkinson Microwave Anisotropy Probe
first-year data to models with a cubic topology. The data are compatible with
finite flat topologies with fundamental domain times the distance to
the decoupling surface at 95% confidence. The WMAP data show reduced power at
the quadrupole and octopole, but do not show the correlations expected for a
compact topology and are indistinguishable from infinite models.Comment: 16 pages, 5 figure
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