36,773 research outputs found
The Role of Crystal Symmetry in the Magnetic Instabilities of -YbAlB and -YbAlB
Density functional theory methods are applied to investigate the properties
of the new superconductor -YbAlB and its polymorph
-YbAlB. We utilize the generalized gradient approximation + Hubbard
U (GGA+U) approach with spin-orbit(SO) coupling to approximate the effects of
the strong correlations due to the open shell of Yb. We examine closely
the differences in crystal bonding and symmetry of -YbAlB and
-YbAlB. The in-plane bonding structure amongst the dominant
itinerant electrons in the boron sheets is shown to differ significantly. Our
calculations indicate that, in both polymorphs, the localized 4 electrons
hybridize strongly with the conduction sea when compared to the related
materials YbRhSi and YbB. Comparing -YbAlB to the
electronic structure of related crystal structures indicates a key role of the
7-member boron coordination of the Yb ion in -YbAlB in producing its
enhanced Kondo scale and superconductivity. The Kondo scale is shown to depend
strongly on the angle between the B neighbors and the Yb ion, relative to the
plane, which relates some of the physical behavior to structural
characteristics.Comment: 9 pages, 9 figures, 2 table
High-energy kink in high-temperature superconductors
In conventional metals, electron-phonon coupling, or the phonon-mediated
interaction between electrons, has long been known to be the pairing
interaction responsible for the superconductivity. The strength of this
interaction essentially determines the superconducting transition temperature
TC. One manifestation of electron-phonon coupling is a mass renormalization of
the electronic dispersion at the energy scale associated with the phonons. This
renormalization is directly observable in photoemission experiments. In
contrast, there remains little consensus on the pairing mechanism in cuprate
high temperature superconductors. The recent observation of similar
renormalization effects in cuprates has raised the hope that the mechanism of
high temperature superconductivity may finally be resolved. The focus has been
on the low energy renormalization and associated "kink" in the dispersion at
around 50 meV. However at that energy scale, there are multiple candidates
including phonon branches, structure in the spin-fluctuation spectrum, and the
superconducting gap itself, making the unique identification of the excitation
responsible for the kink difficult. Here we show that the low-energy
renormalization at ~50 meV is only a small component of the total
renormalization, the majority of which occurs at an order of magnitude higher
energy (~350 meV). This high energy kink poses a new challenge for the physics
of the cuprates. Its role in superconductivity and relation to the low-energy
kink remains to be determined.Comment: 13 pages, 4 figure
Prevalence of Chlamydia abortus in Belgian ruminants
Chlamydia (C.) abortus enzootic abortion still remains the most common cause of reproductive failure in sheep-breeding countries all over the world. Chlamydia abortus in cattle is predominantly associated with genital tract disease and mastitis. In this study, Belgian sheep (n=958), goats (n=48) and cattle (n=1849) were examined, using the ID Screen (TM) Chlamydia abortus indirect multi-species antibody ELISA. In the sheep, the highest prevalence rate was found in Limburg (4.05%). The animals of Antwerp, Brabant and Liege tested negative. The prevalence in the remaining five regions was low (0.24% to 2.74%). Of the nine goat herds, only one herd in Luxembourg was seropositive. In cattle, the highest prevalence rate was found in Walloon Brabant (4.23%). The animals of Limburg and Namur tested negative. The prevalence rate in the remaining seven regions ranged between 0.39% and 4.02%
Incentivizing High Quality Crowdwork
We study the causal effects of financial incentives on the quality of
crowdwork. We focus on performance-based payments (PBPs), bonus payments
awarded to workers for producing high quality work. We design and run
randomized behavioral experiments on the popular crowdsourcing platform Amazon
Mechanical Turk with the goal of understanding when, where, and why PBPs help,
identifying properties of the payment, payment structure, and the task itself
that make them most effective. We provide examples of tasks for which PBPs do
improve quality. For such tasks, the effectiveness of PBPs is not too sensitive
to the threshold for quality required to receive the bonus, while the magnitude
of the bonus must be large enough to make the reward salient. We also present
examples of tasks for which PBPs do not improve quality. Our results suggest
that for PBPs to improve quality, the task must be effort-responsive: the task
must allow workers to produce higher quality work by exerting more effort. We
also give a simple method to determine if a task is effort-responsive a priori.
Furthermore, our experiments suggest that all payments on Mechanical Turk are,
to some degree, implicitly performance-based in that workers believe their work
may be rejected if their performance is sufficiently poor. Finally, we propose
a new model of worker behavior that extends the standard principal-agent model
from economics to include a worker's subjective beliefs about his likelihood of
being paid, and show that the predictions of this model are in line with our
experimental findings. This model may be useful as a foundation for theoretical
studies of incentives in crowdsourcing markets.Comment: This is a preprint of an Article accepted for publication in WWW
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