2,218 research outputs found
Inadequacies in the conventional treatment of the radiation field of moving sources
There is a fundamental difference between the classical expression for the
retarded electromagnetic potential and the corresponding retarded solution of
the wave equation that governs the electromagnetic field. While the boundary
contribution to the retarded solution for the {\em potential} can always be
rendered equal to zero by means of a gauge transformation that preserves the
Lorenz condition, the boundary contribution to the retarded solution of the
wave equation governing the {\em field} may be neglected only if it diminishes
with distance faster than the contribution of the source density in the far
zone. In the case of a source whose distribution pattern both rotates and
travels faster than light {\em in vacuo}, as realized in recent experiments,
the boundary term in the retarded solution governing the field is by a factor
of the order of {\em larger} than the source term of this solution in
the limit that the distance of the boundary from the source tends to
infinity. This result is consistent with the prediction of the retarded
potential that part of the radiation field generated by a rotating superluminal
source decays as , instead of , a prediction that is
confirmed experimentally. More importantly, it pinpoints the reason why an
argument based on a solution of the wave equation governing the field in which
the boundary term is neglected (such as appears in the published literature)
misses the nonspherical decay of the field
The monoclinic crystal structure of -RuCl and the zigzag antiferromagnetic ground state
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to
realize a Kitaev spin model with strongly frustrated, bond-dependent,
anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic
moments. Here we report a detailed study of the three-dimensional crystal
structure using x-ray diffraction on untwinned crystals combined with
structural relaxation calculations. We consider several models for the stacking
of honeycomb layers and find evidence for a crystal structure with a monoclinic
unit cell corresponding to a stacking of layers with a unidirectional in-plane
offset, with occasional in-plane sliding stacking faults, in contrast with the
currently-assumed trigonal 3-layer stacking periodicity. We report electronic
band structure calculations for the monoclinic structure, which find support
for the applicability of the jeff=1/2 picture once spin orbit coupling and
electron correlations are included. We propose that differences in the
magnitude of anisotropic exchange along symmetry inequivalent bonds in the
monoclinic cell could provide a natural mechanism to explain the spin gap
observed in powder inelastic neutron scattering, in contrast to spin models
based on the three-fold symmetric trigonal structure, which predict a gapless
spectrum within linear spin wave theory. Our susceptibility measurements on
both powders and stacked crystals, as well as neutron powder diffraction show a
single magnetic transition at TN ~ 13K. The analysis of the neutron data
provides evidence for zigzag magnetic order in the honeycomb layers with an
antiferromagnetic stacking between layers. Magnetization measurements on
stacked single crystals in pulsed field up to 60T show a single transition
around 8T for in-plane fields followed by a gradual, asymptotic approach to
magnetization saturation, as characteristic of strongly anisotropic exchange
interactions.Comment: 13 pages, 9 figures, published in Physical Review
Compact Pulsed-Power System for Transient Plasma Ignition
The article of record as published may be found at http://dx.doi.org10.1109/TPS.2009.2024672The use of a compact solid-state pulse generator and compact igniters for transient plasma ignition in a pulse detonation engine (PDE) is reported and compared with previous results using a pseudospark pulse generator and threaded rod electrode. Transient plasma is attractive as a technology for the ignition of PDEs and other engine applications because it results in reductions in ignition delay and has been shown to ignite leaner mixtures which allows for lower specific fuel consumption, high-repetition rates, high-altitude operation, and reduced NOx emissions. It has been applied effectively to the ignition of PDEs as well as internal combustion engines. Nonequilibrium transient plasma discharges are produced by applying high-voltage nanosecond pulses that generate streamers, which generate radicals and other electronically excited species over a volume. The pulse generator used is in this experiment is capable of delivering 180 mJ into a 200-Ω load, in the form of a 60-kV 12-ns pulse. Combined with transient plasma igniters comparable with traditional spark plugs, the system was successfully tested in a PDE, resulting in similar ignition delays to those previously reported while using a smaller electrode geometry and delivering an order of magnitude less energy.Office of Naval Researc
Spherical Shells of Classical Gauge Field and their Topological Charge as a Perturbative Expansion
We consider the classical equations of motion of gauge theory,
without a Higgs field, in Minkowski space. We work in the spherical ansatz and
develop a perturbative expansion in the coupling constant for solutions
which in the far past look like freely propagating spherical shells. The
topological charge of these solutions is typically non-integer. We then
show that can be expressed as a power series expansion in which can be
nonzero at finite order. We give an explicit analytic calculation of the order
contribution to for specific initial pulses. We discuss the relation
between our findings and anomalous fermion number violation, and speculate on
the physical implications of our results.Comment: 18 pages in REVTE
Spin-zero anomaly in the magnetic quantum oscillations of a two-dimensional metal
We report on an anomalous behavior of the spin-splitting zeros in the de
Haas-van Alphen (dHvA) signal of a quasi-two-dimensional organic
superconductor. The zeros as well as the angular dependence of the amplitude of
the second harmonic deviate remarkably from the standard Lifshitz-Kosevich (LK)
prediction. In contrast, the angular dependence of the fundamental dHvA
amplitude as well as the spin-splitting zeros of the Shubnikov-de Haas signal
follow the LK theory. We can explain this behavior by small chemical-potential
oscillations and find a very good agreement between theory and experiment. A
detailed wave-shape analysis of the dHvA signal corroborates the existence of
an oscillating chemical potential
Experimental electronic heat capacities of and Plutonium; heavy-fermion physics in an element
We have measured the heat capacities of PuAl and
Pu over the temperature range 2-303 K. The availability of data below
10 K plus an estimate of the phonon contribution to the heat capacity based on
recent neutron-scattering experiments on the same sample enable us to make a
reliable deduction of the electronic contribution to the heat capacity of
PuAl; we find
mJKmol as . This is a factor larger than that
of any element, and large enough for PuAl to be
classed as a heavy-fermion system. By contrast,
mJKmol in Pu. Two distinct anomalies are seen in the
electronic contribution to the heat capacity of
PuAl, one or both of which may be associated with
the formation of the martensitic phase. We suggest that the large
-value of PuAl may be caused by proximity to
a quantum-critical point.Comment: 4 pages, 4 figure
Thick brane in 7D and 8D spacetimes
We consider a thick brane model supported by two interacting scalar fields in
7D and 8D general relativity. Using the special type of a potential energy, we
obtain numerically the regular asymptotically flat vacuum solutions. A
possibility of obtaining the similar solutions for an arbitrary number of the
extra spatial dimensions is estimated.Comment: 3 Ref's are adde
Influenza surveillance among children with pneumonia admitted to a district hospital in coastal Kenya, 2007-2010
Background: Influenza data gaps in sub-Saharan Africa include incidence, case fatality, seasonal patterns, and associations with prevalent disorders.
Methods: Nasopharyngeal samples from children aged <12 years who were admitted to Kilifi District Hospital during 2007–2010 with severe or very severe pneumonia and resided in the local demographic surveillance system were screened for influenza A, B, and C viruses by molecular methods. Outpatient children provided comparative data.
Results: Of 2002 admissions, influenza A virus infection was diagnosed in 3.5% (71), influenza B virus infection, in 0.9% (19); and influenza C virus infection, in 0.8% (11 of 1404 tested). Four patients with influenza died. Among outpatients, 13 of 331 (3.9%) with acute respiratory infection and 1 of 196 without acute respiratory infection were influenza positive. The annual incidence of severe or very severe pneumonia, of influenza (any type), and of influenza A, was 1321, 60, and 43 cases per 100 000 <5 years of age, respectively. Peak occurrence was in quarters 3–4 each year, and approximately 50% of cases involved infants: temporal association with bacteremia was absent. Hypoxia was more frequent among pneumonia cases involving influenza (odds ratio, 1.78; 95% confidence interval, 1.04–1.96). Influenza A virus subtypes were seasonal H3N2 (57%), seasonal H1N1 (12%), and 2009 pandemic H1N1 (7%).
Conclusions: The burden of influenza was small during 2007–2010 in this pediatric hospital in Kenya. Influenza A virus subtype H3N2 predominated, and 2009 pandemic influenza A virus subtype H1N1 had little impact
Small-q phonon-mediated superconductivity in organic \kappa-BEDT-TTF compounds
We propose a mew picture for superconductivity in
salts arguing that small-{\bf q} electron-phonon scattering dominates the
pairing. We reproduce the distinct X-shaped d-wave gap reported recently by
magnetooptic measurements and we argue that the softness of the momentum
structure of the gap and the near degeneracy of s- and d-wave gap states may be
at the origin of the experimental controversy about the gap symmetry. We show
that a magnetic field applied parallel to the planes may induce extended
gapless-regions on the FS accounting for the experimental signatures of a
Fulde-Ferrel-Larkin-Ovchinikov state and it may induce gap symmetry transitions
as well.Comment: 4 pages and 7 figure
Developing an Individual-level Geodemographic Classification
Geodemographics is a spatially explicit classification of socio-economic data, which can be used to describe and analyse individuals by where they live. Geodemographic information is used by the public sector for planning and resource allocation but it also has considerable use within commercial sector applications. Early geodemographic systems, such as the UK’s ACORN (A Classification of Residential Neighbourhoods), used only area-based census data, but more recent systems have added supplementary layers of information, e.g. credit details and survey data, to provide better discrimination between classes. Although much more data has now become available, geodemographic systems are still fundamentally built from area-based census information. This is partly because privacy laws require release of census data at an aggregate level but mostly because much of the research remains proprietary. Household level classifications do exist but they are often based on regressions between area and household data sets. This paper presents a different approach for creating a geodemographic classification at the individual level using only census data. A generic framework is presented, which classifies data from the UK Census Small Area Microdata and then allocates the resulting clusters to a synthetic population created via microsimulation. The framework is then applied to the creation of an individual-based system for the city of Leeds, demonstrated using data from the 2001 census, and is further validated using individual and household survey data from the British Household Panel Survey
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