4,934 research outputs found
Internal Stress in a Model Elasto-Plastic Fluid
Plastic materials can carry memory of past mechanical treatment in the form
of internal stress. We introduce a natural definition of the vorticity of
internal stress in a simple two-dimensional model of elasto-plastic fluids,
which generates the internal stress. We demonstrate how the internal stress is
induced under external loading, and how the presence of the internal stress
modifies the plastic behavior.Comment: 4 pages, 3 figure
The origin of the red luminescence in Mg-doped GaN
Optically-detected magnetic resonance (ODMR) and positron annihilation
spectroscopy (PAS) experiments have been employed to study magnesium-doped GaN
layers grown by metal-organic vapor phase epitaxy. As the Mg doping level is
changed, the combined experiments reveal a strong correlation between the
vacancy concentrations and the intensity of the red photoluminescence band at
1.8 eV. The analysis provides strong evidence that the emission is due to
recombination in which electrons both from effective mass donors and from
deeper donors recombine with deep centers, the deep centers being
vacancy-related defects.Comment: 4 pages, 3 figure
Amplification and generation of ultra-intense twisted laser pulses via stimulated Raman scattering
Twisted Laguerre-Gaussian lasers, with orbital angular momentum and
characterised by doughnut shaped intensity profiles, provide a transformative
set of tools and research directions in a growing range of fields and
applications, from super-resolution microcopy and ultra-fast optical
communications to quantum computing and astrophysics. The impact of twisted
light is widening as recent numerical calculations provided solutions to
long-standing challenges in plasma-based acceleration by allowing for high
gradient positron acceleration. The production of ultrahigh intensity twisted
laser pulses could then also have a broad influence on relativistic
laser-matter interactions. Here we show theoretically and with ab-initio
three-dimensional particle-in-cell simulations, that stimulated Raman
backscattering can generate and amplify twisted lasers to Petawatt intensities
in plasmas. This work may open new research directions in non-linear optics and
high energy density science, compact plasma based accelerators and light
sources.Comment: 18 pages, 4 figures, 1 tabl
New Marker of Colon Cancer Risk Associated with Heme Intake: 1,4-Dihydroxynonane Mercapturic Acid
Background: Red meat consumption is associated with an increased risk of colon cancer. Animal studies show that heme, found in red meat, promotes preneoplastic lesions in the colon, probably due to the oxidative properties of this compound. End products of lipid peroxidation, such as 4-hydroxynonenal metabolites or 8-iso-prostaglandin-F2 (8-iso-PGF2), could reflect this oxidative process and could be used as biomarkers of colon cancer risk associated with heme intake.
Methods: We measured urinary excretion of 8-iso-PGF2 and 1,4-dihydroxynonane mercapturic acid (DHN-MA), the major urinary metabolite of 4-hydroxynonenal, in three studies. In a short-term and a carcinogenesis long-term animal study, we fed rats four different diets (control, chicken, beef, and blood sausage as a high heme diet). In a randomized crossover human study, four different diets were fed (a 60 g/d red meat baseline diet, 120 g/d red meat, baseline diet supplemented with heme iron, and baseline diet supplemented with non-heme iron).
Results: DHN-MA excretion increased dramatically in rats fed high heme diets, and the excretion paralleled the number of preneoplastic lesions in azoxymethane initiated rats (P < 0.0001). In the human study, the heme supplemented diet resulted in a 2-fold increase in DHN-MA (P < 0.001). Urinary 8-iso-PGF2 increased moderately in rats fed a high heme diet (P < 0.0001), but not in humans.
Conclusion: Urinary DHN-MA is a useful noninvasive biomarker for determining the risk of preneoplastic lesions associated with heme iron consumption and should be further investigated as a potential biomarker of colon cancer risk. (Cancer Epidemiol Biomarkers Prev 2006;15(11):2274–9
An Optimal Linear Time Algorithm for Quasi-Monotonic Segmentation
Monotonicity is a simple yet significant qualitative characteristic. We
consider the problem of segmenting a sequence in up to K segments. We want
segments to be as monotonic as possible and to alternate signs. We propose a
quality metric for this problem using the l_inf norm, and we present an optimal
linear time algorithm based on novel formalism. Moreover, given a
precomputation in time O(n log n) consisting of a labeling of all extrema, we
compute any optimal segmentation in constant time. We compare experimentally
its performance to two piecewise linear segmentation heuristics (top-down and
bottom-up). We show that our algorithm is faster and more accurate.
Applications include pattern recognition and qualitative modeling.Comment: This is the extended version of our ICDM'05 paper (arXiv:cs/0702142
Homomorphisms from functional equations: the Goldie equation
The theory of regular variation, in its Karamata and Bojani´c-Karamata/de Haan forms, is long established and makes essential use of the Cauchy functional equation. Both forms are subsumed within the recent theory of Beurling regular variation, developed elsewhere. Various generalizations of the Cauchy equation, including the Gołab–Schinzel functional equation (GS) and Goldie's equation (GBE) below, are prominent there. Here we unify their treatment by algebraicization: extensive use of group structures introduced by Popa and Javor in the 1960s turn all the various (known) solutions into homomorphisms, in fact identifying them 'en passant', and show that (GS) is present everywhere, even if in a thick disguise
Convergence of the all-time supremum of a L\'evy process in the heavy-traffic regime
In this paper we derive a technique of obtaining limit theorems for suprema
of L\'evy processes from their random walk counterparts. For each , let
be a sequence of independent and identically distributed
random variables and be a L\'evy processes such that
, and as . Let .
Then, under some mild assumptions, , for some random variable and some function
. We utilize this result to present a number of limit theorems
for suprema of L\'evy processes in the heavy-traffic regime
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