1,667 research outputs found
Muonium as a hydrogen analogue in silicon and germanium; quantum effects and hyperfine parameters
We report a first-principles theoretical study of hyperfine interactions,
zero-point effects and defect energetics of muonium and hydrogen impurities in
silicon and germanium. The spin-polarized density functional method is used,
with the crystalline orbitals expanded in all-electron Gaussian basis sets. The
behaviour of hydrogen and muonium impurities at both the tetrahedral and
bond-centred sites is investigated within a supercell approximation. To
describe the zero-point motion of the impurities, a double adiabatic
approximation is employed in which the electron, muon/proton and host lattice
degrees of freedom are decoupled. Within this approximation the relaxation of
the atoms of the host lattice may differ for the muon and proton, although in
practice the difference is found to be slight. With the inclusion of zero-point
motion the tetrahedral site is energetically preferred over the bond-centred
site in both silicon and germanium. The hyperfine and superhyperfine
parameters, calculated as averages over the motion of the muon, agree
reasonably well with the available data from muon spin resonance experiments.Comment: 20 pages, including 9 figures. To appear in Phys. Rev.
First Principles Phase Diagram Calculations for the Octahedral-Interstitial System ZrO,
First principles based phase diagram calculations were performed for the
octahedral-interstitial solid solution system \alpha ZrOX (\alpha Zr[
]_(1-X)OX; [ ]=Vacancy; 0 \leq X \leq 1/2). The cluster expansion method was
used to do a ground state analysis, and to calculate the phase diagram. The
predicted diagram has four ordered ground-states in the range 0 \leq X \leq
1/2, but one of these, at X=5/12, is predicted to disproportionate at T \approx
20K, well below the experimentally investigated range T \approx 420K. Thus, at
T \succeq 420K, the first-principles based calculation predicts three ordered
phases rather than the four that have been reported by experimentalists
Role of defects in the electronic properties of amorphous/crystalline Si interface
The mechanism determining the band alignment of the amorphous/crystalline
Si heterostructures is addressed with direct atomistic simulations of the
interface performed using a hierarchical combination of various computational
schemes ranging from classical model-potential molecular dynamics to ab-initio
methods. We found that in coordination defect-free samples the band alignment
is almost vanishing and independent on interface details. In defect-rich
samples, instead, the band alignment is sizeably different with respect to the
defect-free case, but, remarkably, almost independent on the concentration of
defects. We rationalize these findings within the theory of semiconductor
interfaces.Comment: 4 pages in two-column format, 2 postscript figures include
Shape Isomerism at N = 40: Discovery of a Proton Intruder in 67Co
The nuclear structure of 67Co has been investigated through 67Fe beta-decay.
The 67Fe isotopes were produced at the LISOL facility in proton-induced fission
of 238U and selected using resonant laser ionization combined with mass
separation. The application of a new correlation technique unambiguously
revealed a 496(33) ms isomeric state in 67Co at an unexpected low energy of 492
keV. A 67Co level scheme has been deduced. Proposed spin and parities suggest a
spherical (7/2-) 67Co ground state and a deformed first excited (1/2-) state at
492 keV, interpreted as a proton 1p-2h prolate intruder state.Comment: 4 pages, 5 figures, preprint submitted to Physical Review Letter
DATA ASSIMILATION BASED NUMERICAL SIMULATIONS TO ASSIST REAL-TIME SMOKE CONTROL MANAGEMENT IN LARGE SPACES
Abstract The work presented in this paper illustrates the concept of numerical simulations for real-time 'Numerical Fire Forecast' (NFF), applied to smoke control management in large spaces. The numerical calculations performed within the Inverse Zone Modelling framework are based on a series of full scale experiments conducted using the Japanese Building Research Institute (BRI) fire test facility. The experimental set-up consists of a large space of 720m 2 floor area and 26.3m ceiling height, equipped with shafts and fans to study different smoke control options. The measurements include the smoke layer interface (from thermocouples, photometers and visual observation), the smoke layer temperature at different heights (using thermocouples) and the mass flows of air and hot smoke through mechanical and natural vents. In the case of natural filling (i.e. no mechanical or natural venting), the assimilation of smoke layer height data within a 30 s window results in more than 4 minutes lead time of the forecast, with a good level of confidence. Predictions are given in terms of smoke layer height and upper layer temperature. The steady-state value of the methanol fire (Q c = 1300 kW) has been estimated after 30 s with less than 10% error. Widening the assimilation window does not improve the forecast. When mechanical ventilation is activated after the assimilation process with a sufficiently high exhaust rate, the forecast shows with a relatively substantial positive lead time, safe levels of smoke interface height. Introduction Fire Safety Engineering is a multi-disciplinary science, which aims at designing fire-safe buildings with appropriate solutions to preserve property and, most importantly, human life. Therefore, before the construction (or the renovation) of a building, architects, fire engineers and regulators need to consider a given set of fire scenarios in order to examine different options and choose the most appropriate one(s). For this purpose, a large amount of tools have been developed across the years in order to provide an answer to various questions that arise when studying the complex phenomenon of a fire. These questions are often related to the integrity of the building (fire resistance of the structure), fire and smoke spread, and evacuation. The tools used range from simple engineering hand-calculations to the more sophisticated computational fluid and solid mechanics. Many fire simulation tools have been developed to provide guidance in a priori studies. The level of complexity already reached in these tools and the required computational resources render their use for real time predictions impossible. Subsequently, fire fighters have to rely on their intuition and experience as to the decisions and actions to take in real fire situations. It is in this context that the concept of sensor assisted fire fighting has emerge
A multi-modal video analysis approach for car park fire detection
In this paper a novel multi-modal flame and smoke detector is proposed for the detection of fire in large open spaces such as car parks. The flame detector is based on the visual and amplitude image of a time-of-flight camera. Using this multi-modal information, flames can be detected very accurately by visual flame feature analysis and amplitude disorder detection. In order to detect the low-cost flame related features, moving objects in visual images are analyzed over time. If an object possesses high probability for each of the flame characteristics, it is labeled as candidate flame region. Simultaneously, the amplitude disorder is also investigated. Also labeled as candidate flame regions are regions with high accumulative amplitude differences and high values in all detail images of the amplitude image's discrete wavelet transform. Finally, when there is overlap of at least one of the visual and amplitude candidate flame regions, fire alarm is raised. The smoke detector, on the other hand, focuses on global changes in the depth images of the time-of-flight camera, which do not have significant impact on the amplitude images. It was found that this behavior is unique for smoke. Experiments show that the proposed detectors improve the accuracy of fire detection in car parks. The flame detector has an average flame detection rate of 93%, with hardly any false positive detection, and the smoke detection rate of the TOF based smoke detector is 88%. © 2012 Elsevier Ltd
Ferromagnetism in Mn doped GaAs due to substitutional-interstitial complexes
While most calculations on the properties of the ferromagnetic semiconductor
GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn), we
investigate here whether alternate lattice sites are favored and what the
magnetic consequences of this might be. Under As-rich (Ga-poor) conditions
prevalent at growth, we find that the formation energies are lower for
Mn over interstitial Mn (Mn).As the Fermi energy is shifted towards
the valence band maximum via external -doping, the formation energy of
Mn is reduced relative to Mn. Furthermore, under epitaxial growth
conditions, the solubility of both substitutional and interstitial Mn are
strongly enhanced over what is possible under bulk growth conditions. The high
concentration of Mn attained under epitaxial growth of p-type material opens
the possibility of Mn atoms forming small clusters. We consider various types
of clusters, including the Coulomb-stabilized clusters involving two Mn
and one Mn. While isolated Mn are hole killers (donors), and therefore
destroy ferromagnetism,complexes such as Mn-Mn-Mn) are found
to be more stable than complexes involving Mn-Mn-Mn. The
former complexes exhibit partial or total quenching of holes, yet Mn in
these complexes provide a channel for a ferromagnetic arrangement of the spins
on the two Mn within the complex. This suggests that ferromagnetism in
Mn doped GaAs arises both from holes due to isolated Mn as well as from
strongly Coulomb stabilized Mn-Mn-Mn clusters.Comment: 7 figure
Trust in the public sector: Is there any evidence for a long-term decline?
Concerns with declining public trust in government have become a permanent element of the contemporary political discourse. This concern also extends to levels of citizens’ trust in the public administration and public services. Trust is said to be declining, and this decline is generally seen as detrimental to public service delivery. In this article, we examine the main elements in this discussion, review the existing international survey data and summarise the main findings for OECD countries. Citizens’ trust in the public sector is found to fluctuate, and the data generally do not show consistently declining levels of trust. Furthermore, in some countries there simply is insufficient data to come to any conclusions at all about time trends in citizen trust in the public sector.
Points for practitioners
This article summarises some of the survey material on citizens’ trust in the public administration. It allows practitioners to compare trends in public trust in their country across time and space. The findings lead us to reject the hypothesis of a universal decline of trust in the public sector. The article warns against using opinion poll results without considering context. The long-term and comparative perspective on citizens’ trust in the public sector is all too often absent from the policy discourse that is frequently based on assumptions and ad-hoc approaches
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