10,398 research outputs found
Negative Group Velocity
The group velocity for pulses in an optical medium can be negative at
frequencies between those of a pair of laser-pumped spectral lines. The gain
medium then can amplify the leading edge of a pulse resulting in a time advance
of the pulse when it exits the medium, as has been recently demonstrated in the
laboratory. This effect has been called superluminal, but, as a classical
analysis shows, it cannot result in signal propgation at speeds greater than
that of light in vacuum.Comment: v3 adds discussion of "rephasing", and adds a figure. v4 adds
references to the early history of negative group velocity, and adds a
figure; thanks to Alex Grani
Locally Optimally-emitting Clouds and the Narrow Emission Lines in Seyfert Galaxies
The narrow emission line spectra of active galactic nuclei are not accurately
described by simple photoionization models of single clouds. Recent Hubble
Space Telescope images of Seyfert 2 galaxies show that these objects are rich
with ionization cones, knots, filaments, and strands of ionized gas. Here we
extend to the narrow line region the ``locally optimally emitting cloud'' (LOC)
model, in which the observed spectra are predominantly determined by powerful
selection effects. We present a large grid of photoionization models covering a
wide range of physical conditions and show the optimal conditions for producing
many of the strongest emission lines. We show that the integrated narrow line
spectrum can be predicted by an integration of an ensemble of clouds, and we
present these results in the form of diagnostic line ratio diagrams making
comparisons with observations. We also predict key diagnostic line ratios as a
function of distance from the ionizing source, and compare these to
observations. The predicted radial dependence of the [O III]/[O II] ratio may
be matched to the observed one in NGC4151, if the narrow line clouds see a more
intense continuum than we see. The LOC scenario when coupled with a simple
Keplerian gravitational velocity field will quite naturally predict the
observed line width versus critical density relationship. The influence of dust
within the ionized portion of the clouds is discussed and we show that the more
neutral gas is likely to be dusty, although a high ionization dust-free region
is most likely present too. This argues for a variety of NLR cloud origins.Comment: 29 pages plus 16 figures, accepted for publication in Ap
Electronic and optical properties of beryllium chalcogenides/silicon heterostructures
We have calculated electronic and optical properties of
Si/BeSeTe heterostructures by a semiempirical
tight-binding method. Tight-binding parameters and band bowing of
BeSeTe are considered through a recent model for highly
mismatched semiconductor alloys. The band bowing and the measurements of
conduction band offset lead to a type II heterostucture for
Si/BeSeTe with conduction band minimum in the Si layer and
valence band maximum in the BeSeTe layer. The electronic
structure and optical properties of various (Si/(BeSeTe [001] superlattices have been considered. Two
bands of interface states were found within the bandgap of bulk Si. Our
calculations indicate that the optical edges are below the fundamental bandgap
of bulk Si and the transitions are optically allowed.Comment: 16 pager, 7 figure
LRG-BEASTS III: Ground-based transmission spectrum of the gas giant orbiting the cool dwarf WASP-80
We have performed ground-based transmission spectroscopy of the hot Jupiter
orbiting the cool dwarf WASP-80 using the ACAM instrument on the William
Herschel Telescope (WHT) as part of the LRG-BEASTS programme. This is the third
paper of a ground-based transmission spectroscopy survey of hot Jupiters using
low-resolution grism spectrographs. We observed two transits of the planet and
have constructed transmission spectra spanning a wavelength range of
4640-8840A. Our transmission spectrum is inconsistent with a previously claimed
detection of potassium in WASP-80b's atmosphere, and is instead most consistent
with a haze. We also do not see evidence for sodium absorption at a resolution
of 100A.Comment: 11 pages, 9 figures. Accepted for publication in MNRA
Understanding the effect resonant magnetic perturbations have on ELMs
All current estimations of the energy released by type I ELMs indicate that,
in order to ensure an adequate lifetime of the divertor targets on ITER, a
mechanism is required to decrease the amount of energy released by an ELM, or
to eliminate ELMs altogether. One such amelioration mechanism relies on
perturbing the magnetic field in the edge plasma region, either leading to more
frequent, smaller ELMs (ELM mitigation) or ELM suppression. This technique of
Resonant Magnetic Perturbations (RMPs) has been employed to suppress type I
ELMs at high collisionality/density on DIII-D, ASDEX Upgrade, KSTAR and JET and
at low collisionality on DIII-D. At ITER-like collisionality the RMPs enhance
the transport of particles or energy and keep the edge pressure gradient below
the 2D linear ideal MHD critical value that would trigger an ELM, whereas at
high collisionality/density the type I ELMs are replaced by small type II ELMs.
Although ELM suppression only occurs within limitied operational ranges, ELM
mitigation is much more easily achieved. The exact parameters that determine
the onset of ELM suppression are unknown but in all cases the magnetic
perturbations produce 3D distortions to the plasma and enhanced particle
transport. The incorporation of these 3D effects in codes will be essential in
order to make quantitative predictions for future devices.Comment: 32 pages, 9 figure
Targeting Mr Average: Participation, gender equity and school sport partnerships
The School Sport Partnership Programme (SSPP) is one strand of the national strategy for physical education and school sport in England, the physical education and school sport Club Links Strategy (PESSCL). The SSPP aims to make links between school physical education (PE) and out of school sports participation, and has a particular remit to raise the participation levels of several identified under-represented groups, of which girls and young women are one. National evaluations of the SSPP show that it is beginning to have positive impacts on young people's activity levels by increasing the range and provision of extra curricular activities (Office for Standards in Education (OFSTED), 2003, 2004, 2005; Loughborough Partnership, 2005, 2006). This paper contributes to the developing picture of the phased implementation of the programme by providing qualitative insights into the work of one school sport partnership with a particular focus on gender equity. The paper explores the ways in which gender equity issues have been explicitly addressed within the 'official texts' of the SSPP; how these have shifted over time and how teachers are responding to and making sense of these in their daily practice. Using participation observation, interview and questionnaire data, the paper explores how the coordinators are addressing the challenge of increasing the participation of girls and young women. The paper draws on Walby's (2000) conceptualisation of different kinds of feminist praxis to highlight the limitations of the coordinators' work. Two key themes from the data and their implications are addressed: the dominance of competitive sport practices and the PE professionals' views of targeting as a strategy for increasing the participation of under-represented groups. The paper concludes that coordinators work within an equality or difference discourse with little evidence of the transformative praxis needed for the programme to be truly inclusive. © 2008 Taylor & Francis
Electrical current distribution across a metal-insulator-metal structure during bistable switching
Combining scanning electron microscopy (SEM) and electron-beam-induced
current (EBIC) imaging with transport measurements, it is shown that the
current flowing across a two-terminal oxide-based capacitor-like structure is
preferentially confined in areas localized at defects. As the thin-film device
switches between two different resistance states, the distribution and
intensity of the current paths, appearing as bright spots, change. This implies
that switching and memory effects are mainly determined by the conducting
properties along such paths. A model based on the storage and release of charge
carriers within the insulator seems adequate to explain the observed memory
effect.Comment: 8 pages, 7 figures, submitted to J. Appl. Phy
Adapting Water Services to Urban Growth: A Case Study of Salt Lake County
Urbanizing areas thorughout the nation are considering governmental reorganization or consolidation to coordinate planning and improve the cost effectiveness of the delivery of public services. More efficient water supply and wastewater services may become important in the political debate over reorganization. However water factors figure politically, accomplished reorganizations must carefully plan for efficent provision of water services. This study profiles the structure and interactions of municipalities and water service agencies in Utah\u27s Salt Lake County during the 1970s. Both 1975 and 1978 attempts to consolidate Salt Lake City and the unincorporated areas of the county failed. The voting patterns, interest ground positions, and issues are examined. One major water issue surfaced in a concern that service jurisdictions and financial obligations were not sufficiently defined to protect the communities previously bound through water service agreements but excluded from the consolidated government. Additionally, the proposed dissolution of the County Water Conservancy District raised doubts on the division of equity in water rights and distribution facilities. Any large water development stabilizes institutional arrangements to a degree which may become a financial and legal contrainst to desired change. Overall, nonwater issues dominated the decision in this water sensitive area. This implies that water service jurisdictional alignments are set by political decisions based on nonwater considerations. Water utilities must do their best to be effective in the resulting context
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