3,035 research outputs found
Phonon-assisted optical absorption in silicon from first principles
The phonon-assisted interband optical absorption spectrum of silicon is
calculated at the quasiparticle level entirely from first principles. We make
use of the Wannier interpolation formalism to determine the quasiparticle
energies, as well as the optical transition and electron-phonon coupling matrix
elements, on fine grids in the Brillouin zone. The calculated spectrum near the
onset of indirect absorption is in very good agreement with experimental
measurements for a range of temperatures. Moreover, our method can accurately
determine the optical absorption spectrum of silicon in the visible range, an
important process for optoelectronic and photovoltaic applications that cannot
be addressed with simple models. The computational formalism is quite general
and can be used to understand the phonon-assisted absorption processes in
general
CORE and the Haldane Conjecture
The Contractor Renormalization group formalism (CORE) is a real-space
renormalization group method which is the Hamiltonian analogue of the Wilson
exact renormalization group equations. In an earlier paper\cite{QGAF} I showed
that the Contractor Renormalization group (CORE) method could be used to map a
theory of free quarks, and quarks interacting with gluons, into a generalized
frustrated Heisenberg antiferromagnet (HAF) and proposed using CORE methods to
study these theories. Since generalizations of HAF's exhibit all sorts of
subtle behavior which, from a continuum point of view, are related to
topological properties of the theory, it is important to know that CORE can be
used to extract this physics. In this paper I show that despite the folklore
which asserts that all real-space renormalization group schemes are necessarily
inaccurate, simple Contractor Renormalization group (CORE) computations can
give highly accurate results even if one only keeps a small number of states
per block and a few terms in the cluster expansion. In addition I argue that
even very simple CORE computations give a much better qualitative understanding
of the physics than naive renormalization group methods. In particular I show
that the simplest CORE computation yields a first principles understanding of
how the famous Haldane conjecture works for the case of the spin-1/2 and spin-1
HAF.Comment: 36 pages, 4 figures, 5 tables, latex; extensive additions to conten
Measurement of Transmission through Printed Circuit Boards : Application to Enclosure Shielding
It is important when considering the shielding effectiveness (SE) of an enclosure to take into account any contents the enclosure may have. Contents such as printed circuit boards (PCBs) will absorb electromagnetic energy and so affect the SE of the enclosure. Previously, it has been shown that the absorption cross section (ACS) of PCBs and the transmission cross section of apertures can be used in the power balance method to predict the SE of simple enclosures. However, in a more realistic enclosure, multiple cavities may be formed by PCBs that cover a large proportion of the enclosure cross section. In this case, the transmission through the PCBs, as well as through the apertures, needs to be considered. In this paper, we describe measuring the transmission through a PCB using a method normally used to measure the SE of planar samples. This measurement is quick and efficient to carry out, as no special preparation of the PCB is required. The results are shown from a selection of PCBs and limitations of the measurement discussed. The data collected can be used in power balance or computational modelling to allow engineers to determine a more accurate estimate of enclosure SE when designing electronic systems
Translational Control of Cyclin B1 mRNA during Meiotic Maturation: Coordinated Repression and Cytoplasmic Polyadenylation
AbstractTranslational control is prominent during meiotic maturation and early development. In this report, we investigate a mode of translational repression in Xenopus laevis oocytes, focusing on the mRNA encoding cyclin B1. Translation of cyclin B1 mRNA is relatively inactive in the oocyte and increases dramatically during meiotic maturation. We show, by injection of synthetic mRNAs, that the cis-acting sequences responsible for repression of cyclin B1 mRNA reside within its 3′UTR. Repression can be saturated by increasing the concentration of reporter mRNA injected, suggesting that the cyclin B1 3′UTR sequences provide a binding site for a trans-acting repressor. The sequences that direct repression overlap and include cytoplasmic polyadenylation elements (CPEs), sequences known to promote cytoplasmic polyadenylation. However, the presence of a CPE per se appears insufficient to cause repression, as other mRNAs that contain CPEs are not translationally repressed. We demonstrate that relief of repression and cytoplasmic polyadenylation are intimately linked. Repressing elements do not override the stimulatory effect of a long poly(A) tail, and polyadenylation of cyclin B1 mRNA is required for its translational recruitment. Our results suggest that translational recruitment of endogenous cyclin B1 mRNA is a collaborative effect of derepression and poly(A) addition. We discuss several molecular mechanisms that might underlie this collaboration
Phonon softening and superconductivity in tellurium under pressure
The phonon dispersion and the electron-phonon interaction for the -Po
and the bcc high pressure phases of tellurium are computed with
density-functional perturbation theory. Our calculations reproduce and explain
the experimentally observed pressure dependence of the superconducting critical
temperature (T) and confirm the connection between the jump in
T and the structural phase transition. The phonon contribution to the
free energy is shown to be responsible for the difference in the structural
transition pressure observed in low and room temperature experiments.Comment: Revtex, 4 Postscript figures, to appear in Phys. Rev. Let
Criminal Justice and Suicide Outcomes with Indiana's Risk-Based Gun Seizure Law
This article examines the application and effectiveness of a 2006 Indiana law designed to prevent gun violence by authorizing police officers to separate firearms from persons who present imminent or future risk of injury to self or others, or display a propensity for violent or emotionally unstable conduct. A court hearing is held to determine ongoing risk in these cases; a judge decides whether to return the seized firearms or retain them for up to five years. The study examines the frequency of criminal arrest as well as suicide outcomes for 395 gun-removal actions in Indiana. Fourteen individuals (3.5%) died from suicide, seven (1.8%) using a firearm. The study population's annualized suicide rate was about 31 times higher than that of the general adult population in Indiana, demonstrating that the law is being applied to a population genuinely at high risk. By extrapolating information on the case fatality rate for different methods of suicide, we calculated that one life was saved for every 10 gun-removal actions, similar to results of a previous study in Connecticut. Perspectives from key stakeholders are also presented along with implications for gun policy reform and implementation
A Cognitive Ethology Study of First- and Third-Person Perspectives
The present investigation was funded by a grant awarded to AK by the Natural Sciences and Engineering Council of Canada. The funders had no role in study design, data collection, and analysis, decision to publish, or preparation of the manuscript.Peer reviewedPublisher PD
Education and older adults at the University of the Third Age
This article reports a critical analysis of older adult education in Malta. In educational gerontology, a critical perspective demands the exposure of how relations of power and inequality, in their myriad forms, combinations, and complexities, are manifest in late-life learning initiatives. Fieldwork conducted at the University of the Third Age (UTA) in Malta uncovered the political nature of elder-learning, especially with respect to three intersecting lines of inequality - namely, positive aging, elitism, and gender. A cautionary note is, therefore, warranted at the dominant positive interpretations of UTAs since late-life learning, as any other education activity, is not politically neutral.peer-reviewe
- …