41,959 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
Aerodynamic loads on deployed canard surfaces and rocket nose section of the Apollo launch escape vehicle
Aerodynamic loads on deployed canard surfaces and rocket nose section of Apollo launch escape vehicl
Gas morphology and energetics at the surface of PDRs: New insights with Herschel observations of NGC 7023
Context. We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field.
Aims. Using Herschel/HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel.
Methods. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 μm (1901 GHz) [C ii] line measured by HIFI and provide information on the emitting gas.
Results. We show that both the [C ii] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [C ii] intensities.
Conclusions. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [O i] and use of a new PDR model that includes PAH-related species
Ferromagnetic material in the eastern red-spotted newt notophthalmus viridescens
Behavioral results obtained from the eastern red-spotted newt (Notophthalmus viridescens) led to the suggestion of a hybrid homing system involving inputs from both a light-dependent and a non-light-dependent mechanism. To evaluate the possible role of a receptor based on biogenic magnetite in this animal, we performed magnetometry experiments on a set of newts previously used in behavioral assays. The natural remanent magnetization (NRM) carried by these newts was strong enough to be measured easily using a direct-current-biased superconducting quantum interference device functioning as a moment magnetometer. Isothermal remanent magnetizations were two orders of magnitude higher than the NRM, suggesting that ferromagnetic material consistent with magnetite is present in the body of the newt. The NRM has no preferential orientation among the animals when analyzed relative to their body axis, and the demagnetization data show that, overall, the magnetic material grains are not aligned parallel to each other within each newt. Although the precise localization of the particles was not possible, the data indicate that magnetite is not clustered in a limited area. A quantity of single-domain magnetic material is present which would be adequate for use in either a magnetic intensity or direction receptor. Our data, when combined with the functional properties of homing, suggest a link between this behavioral response and the presence of ferromagnetic material, raising the possibility that magnetite is involved at least in the map component of homing of the eastern red-spotted newt
The Renormalization Group Limit Cycle for the 1/r^2 Potential
Previous work has shown that if an attractive 1/r^2 potential is regularized
at short distances by a spherical square-well potential, renormalization allows
multiple solutions for the depth of the square well. The depth can be chosen to
be a continuous function of the short-distance cutoff R, but it can also be a
log-periodic function of R with finite discontinuities, corresponding to a
renormalization group (RG) limit cycle. We consider the regularization with a
delta-shell potential. In this case, the coupling constant is uniquely
determined to be a log-periodic function of R with infinite discontinuities,
and an RG limit cycle is unavoidable. In general, a regularization with an RG
limit cycle is selected as the correct renormalization of the 1/r^2 potential
by the conditions that the cutoff radius R can be made arbitrarily small and
that physical observables are reproduced accurately at all energies much less
than hbar^2/mR^2.Comment: 11 pages, 4 figure
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.
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
‘Fixed-axis’ magnetic orientation by an amphibian: non-shoreward-directed compass orientation, misdirected homing or positioning a magnetite-based map detector in a consistent alignment relative to the magnetic field?
Experiments were carried out to investigate the earlier prediction that prolonged exposure to long-wavelength (>500 nm) light would eliminate homing orientation by male Eastern red-spotted newts Notophthalmus viridescens. As in previous experiments, controls held in outdoor tanks under natural lighting conditions and tested in a visually uniform indoor arena under full-spectrum light were homeward oriented. As predicted, however, newts held under long-wavelength light and tested under either full-spectrum or long-wavelength light (>500 nm) failed to show consistent homeward orientation. The newts also did not orient with respect to the shore directions in the outdoor tanks in which they were held prior to testing. Unexpectedly, however, the newts exhibited bimodal orientation along a more-or-less `fixed' north-northeast—south-southwest magnetic axis. The orientation exhibited by newts tested under full-spectrum light was indistinguishable from that of newts tested under long-wavelength light, although these two wavelength conditions have previously been shown to differentially affect both shoreward compass orientation and homing orientation. To investigate the possibility that the `fixed-axis' response of the newts was mediated by a magnetoreception mechanism involving single-domain particles of magnetite, natural remanent magnetism (NRM) was measured from a subset of the newts. The distribution of NRM alignments with respect to the head—body axis of the newts was indistinguishable from random. Furthermore, there was no consistent relationship between the NRM of individual newts and their directional response in the overall sample. However, under full-spectrum, but not long-wavelength, light, the alignment of the NRM when the newts reached the 20 cm radius criterion circle in the indoor testing arena (estimated by adding the NRM alignment measured from each newt to its magnetic bearing) was non-randomly distributed. These findings are consistent with the earlier suggestion that homing newts use the light-dependent magnetic compass to align a magnetite-based `map detector' when obtaining the precise measurements necessary to derive map information from the magnetic field. However, aligning the putative map detector does not explain the fixed-axis response of newts tested under long-wavelength light. Preliminary evidence suggests that, in the absence of reliable directional information from the magnetic compass (caused by the 90° rotation of the response of the magnetic compass under long-wavelength light), newts may resort to a systematic sampling strategy to identify alignment(s) of the map detector that yields reliable magnetic field measurements
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