4,490 research outputs found
Unconventional superconducting pairing symmetry induced by phonons
The possibility of non-s-wave superconductivity induced by phonons is
investigated using a simple model that is inspired by SrRuO. The model
assumes a two-dimensional electronic structure, a two-dimensional
spin-fluctuation spectrum, and three-dimensional electron-phonon coupling.
Taken separately, each interaction favors formation of spin-singlet pairs (of s
symmetry for the phonon interaction and d symmetry for the spin
interaction), but in combination, a variety of more unusual singlet and triplet
states are found, depending on the interaction parameters. This may have
important implications for SrRuO, providing a plausible explanation of
how the observed spin fluctuations, which clearly favor d pairing,
may still be instrumental in creating a superconducting state with a different
(e.g., p-wave) symmetry. It also suggests an interpretation of the large
isotope effect observed in SrRuO. These results indicate that phonons
could play a key role in establishing the order-parameter symmetry in
SrRuO, and possibly in other unconventional superconductors.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Overcoming Repeated Testing Schedule Bias in Estimates of Disease Prevalence
During the COVID-19 pandemic, many institutions such as universities and
workplaces implemented testing regimens with every member of some population
tested longitudinally, and those testing positive isolated for some time.
Although the primary purpose of such regimens was to suppress disease spread by
identifying and isolating infectious individuals, testing results were often
also used to obtain prevalence and incidence estimates. Such estimates are
helpful in risk assessment and institutional planning and various estimation
procedures have been implemented, ranging from simple test-positive rates to
complex dynamical modeling. Unfortunately, the popular test-positive rate is a
biased estimator of prevalence under many seemingly innocuous longitudinal
testing regimens with isolation. We illustrate how such bias arises and
identify conditions under which the test-positive rate is unbiased. Further, we
identify weaker conditions under which prevalence is identifiable and propose a
new estimator of prevalence under longitudinal testing. We evaluate the
proposed estimation procedure via simulation study and illustrate its use on a
dataset derived by anonymizing testing data from The Ohio State University.Comment: 36 pages, 4 figures, 1 tabl
Limit cycles in the presence of convection, a travelling wave analysis
We consider a diffusion model with limit cycle reaction functions, in the
presence of convection. We select a set of functions derived from a realistic
reaction model: the Schnakenberg equations. This resultant form is
unsymmetrical. We find a transformation which maps the irregular equations into
model form. Next we transform the dependent variables into polar form. From
here, a travelling wave analysis is performed on the radial variable. Results
are complex, but we make some simple estimates.
We carry out numerical experiments to test our analysis. An initial `knock'
starts the propagation of pattern. The speed of the travelling wave is not
quite as expected. We investigate further. The system demonstrates distinctly
different behaviour to the left and the right. We explain how this phenomenon
occurs by examining the underlying behaviour.Comment: 20 pages, 5 figure
Rapid and accurate structure determination of coiled-coil domains using NMR dipolar couplings: Application to cGMP-dependent protein kinase I alpha
Photoelectric search for peculiar stars in open clusters XV. Feinstein 1, NGC 2168, NGC 2323, NGC 2437, NGC 2547, NGC 4103, NGC 6025, NGC 6633, Stock 2, and Trumpler 2
The chemically peculiar (CP) stars of the upper main sequence are mainly characterized by strong overabundances of heavy elements. Two subgroups (CP2 and CP4) have strong local magnetic fields which make them interesting targets for astrophysical studies. This star group, in general, is often used for the analysis of stellar formation and evolution in the context of diffusion as well as meridional circulation. In continuation of a long term study of CP stars (initiated in the 1980s), we present new results based on photoelectric measurements for ten open clusters that are, with one exception, younger than 235 Myr. Observations in star clusters are favourable because they represent samples of stars of constant age and homogeneous chemical composition. The very efficient tool of Δa photometry was applied. It samples the flux depression at 5200 Å typically for CP stars. In addition, it is able to trace emission line Be/Ae and lambda Bootis stars. Virtually all CP2 and CP4 stars can be detected via this tool, and it has been successfully applied even in the Large Magellanic Cloud. For all targets in the cluster areas, we performed a kinematic membership analysis. We obtained new photoelectric Δa photometry of 304 stars from which 207 objects have a membership probability higher than 50%. Our search for chemically peculiar objects results in fifteen detections. The stars have masses between 1.7 Msun and 7.7 Msun and are between the zero- and terminal-age-main-sequence. We discuss the published spectral classifications in the light of our delta-a photometry and identify several misclassified CP stars. We are also able to establish and support the nature of known bona fide CP candidates. It is vital to use kinematic data for the membership determination and also to compare published spectral types with other data, such as Δa photometry. There are no doubts about the accuracy of photoelectric measurements, especially for stars brighter than 10th magnitude. The new and confirmed CP stars are interesting targets for spectroscopic follow-up observations to put constraints on the formation and evolution of CP stars
Estimation of turbulence in fan-rotor wakes for broadband noise prediction during acoustic preliminary design
When calculating broadband fan noise caused by rotor-stator wake interaction analytically, information about the airflow, particularly about the turbulence in the rotor wakes, is necessary. During the pre-design phase, two-dimensional streamline methods are commonly used. These provide only general flow quantities like mean-flow velocities or total-pressure losses. Turbulent parameters such as turbulent kinetic energy and turbulent integral length scale need to be deduced from these quantities. There are several models mentioned in the literature which correlate the wake size with the wake turbulence. But they usually comprise calibration factors that need to be assessed empirically by numerical simulations or measurements. The contribution of the paper is to present an updated semi-empirical model for rotor-wake turbulence quantities, derived on the basis of an extensive comparison of the model with measurements and numerical simulations on four different turbofan stages. A recalibration of the empirical factors improved the noise prediction by 8 dB, reaching an accuracy of 2 dB. In addition, it is shown, that the endwall flow is responsible for large variance in the noise prediction, and may have a contribution of up to 2 dB to the overall sound power
Dielectronic Recombination of Fe XV forming Fe XIV: Laboratory Measurements and Theoretical Calculations
We have measured resonance strengths and energies for dielectronic
recombination (DR) of Mg-like Fe XV forming Al-like Fe XIV via N=3 -> N' = 3
core excitations in the electron-ion collision energy range 0-45 eV. All
measurements were carried out using the heavy-ion Test Storage Ring at the Max
Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also
carried out new multiconfiguration Breit-Pauli (MCBP) calculations using the
AUTOSTRUCTURE code. For electron-ion collision energies < 25 eV we find poor
agreement between our experimental and theoretical resonance energies and
strengths. From 25 to 42 eV we find good agreement between the two for
resonance energies. But in this energy range the theoretical resonance
strengths are ~ 31% larger than the experimental results. This is larger than
our estimated total experimental uncertainty in this energy range of +/- 26%
(at a 90% confidence level). Above 42 eV the difference in the shape between
the calculated and measured 3s3p(^1P_1)nl DR series limit we attribute partly
to the nl dependence of the detection probabilities of high Rydberg states in
the experiment. We have used our measurements, supplemented by our
AUTOSTRUCTURE calculations, to produce a Maxwellian-averaged 3 -> 3 DR rate
coefficient for Fe XV forming Fe XIV. The resulting rate coefficient is
estimated to be accurate to better than +/- 29% (at a 90% confidence level) for
k_BT_e > 1 eV. At temperatures of k_BT_e ~ 2.5-15 eV, where Fe XV is predicted
to form in photoionized plasmas, significant discrepancies are found between
our experimentally-derived rate coefficient and previously published
theoretical results. Our new MCBP plasma rate coefficient is 19-28% smaller
than our experimental results over this temperature range
Bound States and Superconductivity in Dense Fermi Systems
A quantum field theoretical approach to the thermodynamics of dense Fermi
systems is developed for the description of the formation and dissolution of
quantum condensates and bound states in dependence of temperature and density.
As a model system we study the chiral and superconducting phase transitions in
two-flavor quark matter within the NJL model and their interrelation with the
formation of quark-antiquark and diquark bound states. The phase diagram of
quark matter is evaluated as a function of the diquark coupling strength and a
coexistence region of chiral symmetry breaking and color superconductivity is
obtained at very strong coupling. The crossover between Bose-Einstein
condensation (BEC) of diquark bound states and condensation of diquark
resonances (Cooper pairs) in the continuum (BCS) is discussed as a Mott effect.
This effect consists in the transition of bound states into the continuum of
scattering states under the influence of compression and heating. We explain
the physics of the Mott transition with special emphasis on role of the Pauli
principle for the case of the pion in quark matter.Comment: 16 pages, 5 figure
An empirical temperature calibration for the Delta a photometric system. II. The A-type and mid F-type star
With the Delta a photometric system, it is possible to study very distant
galactic and even extragalactic clusters with a high level of accuracy. This
can be done with a classical color-magnitude diagram and appropriate
isochrones. The new calibration presented in this paper is a powerful
extension. For open clusters, the reddening is straightforward for an
estimation via Isochrone fitting and is needed in order to calculate the
reddening-free, temperature sensitive, index (g1-y)0. As a last step, the
calibration can be applied to individual stars. Because no a-priori
reddening-free photometric parameters are available for the investigated
spectral range, we have applied the dereddening calibrations of the Stromgren
uvbybeta system and compared them with extinction models for the Milky Way. As
expected from the sample of bright stars, the extinction is negligible for
almost all objects. As a next step, already established calibrations within the
Stromgren uvbybeta, Geneva 7-color, and Johnson UBV systems were applied to a
sample of 282 normal stars to derive a polynomial fit of the third degree for
the averaged effective temperatures to the individual (g1-y)0 values with a
mean of the error for the whole sample of Delta T(eff) is 134K, which is lower
than the value in Paper I for hotter stars. No statistically significant effect
of the rotational velocity on the precision of the calibration was found.Comment: 5 pages, 2 figures, accepted by A&
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