754 research outputs found
Neutral Plasma Oscillations at Zero Temperature
We use cold plasma theory to calculate the response of an ultracold neutral
plasma to an applied rf field. The free oscillation of the system has a
continuous spectrum and an associated damped quasimode. We show that this
quasimode dominates the driven response. We use this model to simulate plasma
oscillations in an expanding ultracold neutral plasma, providing insights into
the assumptions used to interpret experimental data [Phys. Rev. Lett. 85, 318
(2000)].Comment: 4.3 pages, including 3 figure
Prepyramid-to-pyramid transition of SiGe islands on Si(001)
The morphology of the first three-dimensional islands appearing during
strained growth of SiGe alloys on Si(001) was investigated by scanning
tunneling microscopy. High resolution images of individual islands and a
statistical analysis of island shapes were used to reconstruct the evolution of
the island shape as a function of size. As they grow, islands undergo a
transition from completely unfacetted rough mounds (prepyramids) to partially
{105} facetted islands and then they gradually evolve to {105} facetted
pyramids. The results are in good agreement with the predictions of a recently
proposed theoretical model
Identification of volatiles generated by potato tubers (Solanum tuberosum CV : Maris Piper) infected by erwinia carotovora, bacillus polymyxa and arthrobacter sp
Bacteria were isolated from internal tissues of surface sterilized healthy tubers of Solanum tuberosum cv. Maris Piper (8 different isolates) and from tubers inoculated with Erwinia carotovora ssp. carotovora showing soft-rot symptoms (3 different isolates), and identified by fatty acid profiling. Bacillus polymyxa and an Arthrobacter sp. were isolated from both sources, E. carotovora only from the soft-rotted tubers. The volatile organic compounds (VOCs) generated by tubers inoculated with E. carotovora, B. polymyxa and the Arthrobacter sp. were identified. Inoculated tubers of cv. Maris Piper were incubated under controlled humidity (95% relative humidity) and temperature (10°C) to simulate typical storage conditions. B. polymyxa and Arthrobacter sp. did not cause symptoms, whilst E. carotovora caused limited soft-rot infections after 4 weeks at the low temperatures typically associated with potatoes in storage. The VOCs released to the headspace around these tubers were collected using an adsorbent system and analysed by Gas Chromatography-Mass Spectrometry (GC-MS). Twenty-two volatiles unique to E. carotovora infection of potato tubers were found,) including 10 alkanes, four alkenes, two aldehydes, one sulphide, one ketone, one alcohol, one aromatic, one acid and one heterocyclic compound. B. polymyxa generated three unique volatiles: N,N-dimethylformamide, 1-pentadecene and 1-hexadecane. Only one volatile, 2,3-dihydrofuran, was unique to the Arthrobacter infection. Production of volatile nitrogen species from E. carotovora-infected tubers increased with time, whereas none were detected in the headspace above uninfected tubers. Further analysis using a modified GC-MS method established that ammonia, trimethylamine and several volatile sulphides were evolved from tubers infected by E. carotovora. No specific volatile was useful as a marker associated with any of the three bacterial species but in the case of E. carotovora-infected potato tubers a significant increase in the volume of compounds evolved was clearly observed. The results are discussed in relation to the use of sensors to detect VOCs evolved from infected tubers in order to provide an early warning system for the control of soft rot in potato store
Boxicity of graphs on surfaces
The boxicity of a graph is the least integer for which there
exist interval graphs , , such that . Scheinerman proved in 1984 that outerplanar graphs have boxicity
at most two and Thomassen proved in 1986 that planar graphs have boxicity at
most three. In this note we prove that the boxicity of toroidal graphs is at
most 7, and that the boxicity of graphs embeddable in a surface of
genus is at most . This result yields improved bounds on the
dimension of the adjacency poset of graphs on surfaces.Comment: 9 pages, 2 figure
Extended scaling relations for planar lattice models
It is widely believed that the critical properties of several planar lattice
models, like the Eight Vertex or the Ashkin-Teller models, are well described
by an effective Quantum Field Theory obtained as formal scaling limit. On the
basis of this assumption several extended scaling relations among their indices
were conjectured. We prove the validity of some of them, among which the ones
by Kadanoff, [K], and by Luther and Peschel, [LP].Comment: 32 pages, 7 fi
Theorising Disability: Beyond Common Sense
This article seeks to introduce the topic of disability to political theory via a discussion of some of the literature produced by disability theorists. The author argues that these more radical approaches conceptualise disability in ways that conflict with âcommon-senseâ notions of disability that tend to underpin political theoretical considerations of the topic. Furthermore, the author suggests that these more radical conceptualisations have profound implications for current debates on social justice, equality and citizenship that highlight the extent to which these notions are also currently underpinned by âcommon-senseâ notions of ânormalityâ
The core of Rodinia formed by the juxtaposition of opposed retreating and advancing accretionary orogens
Long-lived (800âŻMa) Paleoâ to Mesoproterozoic accretionary orogens on the margins of Laurentia, Baltica, Amazonia, and Kalahari collided to form the core of the supercontinent, Rodinia. Accretionary orogens in Laurentia and Baltica record predominately radiogenic zircon ΔHf(t) and whole-rock Pb isotopic compositions, short crustal residence times (ca. 0.5âŻGa), and the development of arc-backarc complexes. The accretionary orogenic record of Laurentia and Baltica is consistent with a retreating accretionary orogen and analogous to the Phanerozoic western Pacific orogenic system. In contrast, the Mesoproterozoic orogens of Amazon and Kalahari cratons record unradiogenic zircon ΔHf(t) values, ca. 0.8âŻGa crustal residence times, and more ancient whole-rock Pb isotopic signatures. The accretionary orogenic record of Amazonia and Kalahari indicates the preferential incorporation of cratonic material in continental arcs of advancing accretionary orogens comparable to the Phanerozoic eastern Pacific orogenic system. Based on similarities in the geodynamic evolution of the Phanerozoic circum-Pacific orogens peripheral to Gondwana/Pangea, we suggest that the Mesoproterozoic accretionary orogens formed as peripheral subduction zones along the margin of the supercontinent Nuna (ca. 1.8â1.6âŻGa). The eventual collapse of this peripheral subduction zone onto itself and closure of the external ocean around Nuna to form Rodinia is equivalent to the projected future collapse of the circum-Pacific subduction system and juxtaposition of Australia-Asia with South America. The juxtaposition of advancing and retreating accretionary orogens at the core of the supercontinent Rodinia demonstrates that supercontinent assembly can occur by the closure of external oceans and indicates that future closure of the Pacific Ocean is plausible
Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy
We consider the effect of inhomogeneous neutrino degeneracy on Big Bang
nucleosynthesis for the case where the distribution of neutrino chemical
potentials is given by a Gaussian. The chemical potential fluctuations are
taken to be isocurvature, so that only inhomogeneities in the electron chemical
potential are relevant. Then the final element abundances are a function only
of the baryon-photon ratio , the effective number of additional neutrinos
, the mean electron neutrino degeneracy parameter , and
the rms fluctuation of the degeneracy parameter, . We find that for
fixed , , and , the abundances of helium-4,
deuterium, and lithium-7 are, in general, increasing functions of .
Hence, the effect of adding a Gaussian distribution for the electron neutrino
degeneracy parameter is to decrease the allowed range for . We show that
this result can be generalized to a wide variety of distributions for .Comment: 9 pages, 3 figures, added discussion of neutrino oscillations,
altered presentation of figure
Prospects for radio detection of ultra-high energy cosmic rays and neutrinos
The origin and nature of the highest energy cosmic ray events is currently
the subject of intense investigation by giant air shower arrays and fluorescent
detectors. These particles reach energies well beyond what can be achieved in
ground-based particle accelerators and hence they are fundamental probes for
particle physics as well as astrophysics. Because of the scarcity of these
high-energy particles, larger and larger ground-based detectors have been
built. The new generation of digital radio telescopes may play an important
role in this, if properly designed. Radio detection of cosmic ray showers has a
long history but was abandoned in the 1970's. Recent experimental developments
together with sophisticated air shower simulations incorporating radio emission
give a clearer understanding of the relationship between the air shower
parameters and the radio signal, and have led to resurgence in its use.
Observations of air showers by the SKA could, because of its large collecting
area, contribute significantly to measuring the cosmic ray spectrum at the
highest energies. Because of the large surface area of the moon, and the
expected excellent angular resolution of the SKA, using the SKA to detect radio
Cherenkov emission from neutrino-induced cascades in lunar regolith will be
potentially the most important technique for investigating cosmic ray origin at
energies above the photoproduction cut-off. (abridged)Comment: latex, 26 pages, 17 figures, to appear in: "Science with the Square
Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews,
(Elsevier: Amsterdam
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