442 research outputs found
A dynamical approach to the spatiotemporal aspects of the Portevin-Le Chatelier effect: Chaos,turbulence and band propagation
Experimental time series obtained from single and poly-crystals subjected to
a constant strain rate tests report an intriguing dynamical crossover from a
low dimensional chaotic state at medium strain rates to an infinite dimensional
power law state of stress drops at high strain rates. We present results of an
extensive study of all aspects of the PLC effect within the context a model
that reproduces this crossover. A study of the distribution of the Lyapunov
exponents as a function of strain rate shows that it changes from a small set
of positive exponents in the chaotic regime to a dense set of null exponents in
the scaling regime. As the latter feature is similar to the GOY shell model for
turbulence, we compare our results with the GOY model. Interestingly, the null
exponents in our model themselves obey a power law. The configuration of
dislocations is visualized through the slow manifold analysis. This shows that
while a large proportion of dislocations are in the pinned state in the chaotic
regime, most of them are at the threshold of unpinning in the scaling regime.
The model qualitatively reproduces the different types of deformation bands
seen in experiments. At high strain rates where propagating bands are seen, the
model equations are reduced to the Fisher-Kolmogorov equation for propagative
fronts. This shows that the velocity of the bands varies linearly with the
strain rate and inversely with the dislocation density, consistent with the
known experimental results. Thus, this simple dynamical model captures the
complex spatio-temporal features of the PLC effect.Comment: 17 pages, 18 figure
Genetic Variability and Correlation Analysis in Global Composite Collection of Finger Millet (Eleusine coracana (L.) Gaertn)
A global composite collection of finger millet germplasm which included ICRISAT core
collection was evaluated for fifteen quantitative characters in augmented design with four
control cultivars during 2007 rainy season. The objectives were to assess the variability and
association between the characters related to yield. The genotypic variances were highly
significant for all characters except plot yield. Plot yield exhibited the highest range (0.02 – 2.6
t ha-1). The phenotypic and genotypic coefficients of variations varied from 9.59 to 57.13% and
11.14 to 64.96% respectively. Heritability estimates ranged from 37.01% for single panicle
weight to 98.34% for days to 50 percent flowering. Plot yield was strongly associated positively
with productive tillers, number of fingers and days to 50 percent flowering. Generally, the
result revealed the existence of variability for the characters studied in finger millet genotypes
Genetic Diversity of Global Finger Millet Composite Collection Based on Qualitative Traits and Biological Races
The global composite collection of finger millet (1000 accessions) was evaluated to study the
variability for six qualitative traits under field condition in three environments. Wider variation
for all the qualitative traits was observed and racial wise wild races africana and spontanea
had more diversity for all qualitative traits. However the cultivated races showed difference
in inflorescence shape. Most of the cultivated race accessions were predominated by the
erect plant habit, green pigmented, top curved, low glume prominence and light brown seed
colour. Genetic diversity in these traits can be used by broadening gene pool through national
and international sources, wide hybridization, mutation or other genetic modification
Multifractal burst in the spatio-temporal dynamics of jerky flow
The collective behavior of dislocations in jerky flow is studied in Al-Mg
polycrystalline samples subjected to constant strain rate tests. Complementary
dynamical, statistical and multifractal analyses are carried out on the
stress-time series recorded during jerky flow to characterize the distinct
spatio-temporal dynamical regimes. It is shown that the hopping type B and the
propagating type A bands correspond to chaotic and self-organized critical
states respectively. The crossover between these types of bands is identified
by a large spread in the multifractal spectrum. These results are interpreted
on the basis of competing scales and mechanisms.Comment: 4 pages, 6 figures To be published in Phys. Rev. Lett. (2001
Recurrence analysis of the Portevin-Le Chatelier effect
Tensile tests were carried out by deforming polycrystalline samples of
Al-2.5%Mg alloy at room temperature in a wide range of strain rates where the
Portevin-Le Chatelier (PLC) effect was observed. The experimental stress-time
series data have been analyzed using the recurrence analysis technique based on
the Recurrence Plot (RP) and the Recurrence Quantification Analysis (RQA) to
study the change in the dynamical behavior of the PLC effect with the imposed
strain rate. Our study revealed that the RQA is able to detect the unique
crossover phenomenon in the PLC dynamics.Comment: 17 pages, 3 figure
Retention of Two-Band Superconductivity in Highly Carbon-Doped MgB2
Tunneling data on MgB_{1.8}C_{0.2} show a reduction in the energy gap of the
pi-bands by a factor of two from undoped MgB2 that is consistent with the Tc
reduction, but inconsistent with the expectations of the dirty limit.
Dirty-limit theory for undoped MgB2 predicts a single gap about three times
larger than measured and a reduced Tc comparable to that measured. Our
heavily-doped samples exhibit a uniform dispersion of C suggestive of
significantly enhanced scattering, and we conclude that the retention of
two-band superconductivity in these samples is caused by a selective
suppression of interband scattering.Comment: 4 pages, 4 figures; added one figure, added one reference, minor
changes to the text, manuscript accepted for publication as a Phys. Rev. B
Rapid Communicatio
Hypoxia-induced responses by endothelial colony-forming cells are modulated by placental growth factor
BACKGROUND: Endothelial colony-forming cells (ECFCs), also termed late outgrowth endothelial cells, are a well-defined circulating endothelial progenitor cell type with an established role in vascular repair. ECFCs have clear potential for cell therapy to treat ischaemic disease, although the precise mechanism(s) underlying their response to hypoxia remains ill-defined. METHODS: In this study, we isolated ECFCs from umbilical cord blood and cultured them on collagen. We defined the response of ECFCs to 1% O(2) exposure at acute and chronic time points. RESULTS: In response to low oxygen, changes in ECFC cell shape, proliferation, size and cytoskeleton phenotype were detected. An increase in the number of senescent ECFCs also occurred as a result of long-term culture in 1% O(2). Low oxygen exposure altered ECFC migration and tube formation in Matrigel®. Increases in angiogenic factors secreted from ECFCs exposed to hypoxia were also detected, in particular, after treatment with placental growth factor (PlGF). Exposure of cells to agents that stabilise hypoxia-inducible factors such as dimethyloxalylglycine (DMOG) also increased PlGF levels. Conditioned medium from both hypoxia-treated and DMOG-treated cells inhibited ECFC tube formation. This effect was reversed by the addition of PlGF neutralising antibody to the conditioned medium, confirming the direct role of PlGF in this effect. CONCLUSIONS: This study deepens our understanding of the response of ECFCs to hypoxia and also identifies a novel and important role for PlGF in regulating the vasculogenic potential of ECFCs. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13287-016-0430-0) contains supplementary material, which is available to authorized users
B NMR study of pure and lightly carbon doped MgB superconductors
We report a B NMR line shape and spin-lattice relaxation rate
() study of pure and lightly carbon doped MgBC for
, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe.
We show that while pure MgB exhibits the magnetic field distribution from
superposition of the normal and the Abrikosov state, slight replacement of
boron with carbon unveils the magnetic field distribution of the pure Abrikosov
state. This indicates a considerable increase of with carbon doping
with respect to pure MgB. The spin-lattice relaxation rate
demonstrates clearly the presence of a coherence peak right below in pure
MgB, followed by a typical BCS decrease on cooling. However, at
temperatures lower than K strong deviation from the BCS behavior is
observed, probably from residual contribution of the vortex dynamics. In the
carbon doped systems both the coherence peak and the BCS temperature dependence
of weaken, an effect attributed to the gradual shrinking of the
hole cylinders of the Fermi surface with electron doping.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
75As NMR probe of antiferromagnetic fluctuations in Ba(Fe1-xRux)2As2
The evolution of 75As NMR parameters with composition and temperature was
probed in the Ba(Fe1-xRux)2As2 system where Fe is replaced by isovalent Ru.
While the Ru-end member was found to be a conventional Fermi liquid, the
composition (x=0.5) corresponding to the highest Tc (20K) in this system shows
an upturn in 75As 1/T1T below about 80 K evidencing the presence of
antiferromagnetic (AFM) fluctuations. These results are similar to those
obtained in another system with isovalent substitution BaFe2(As1-xPx)2 [Y.
Nakai, T. Iye, S. Kitagawa, K. Ishida, H. Ikeda, S. Kasahara, H. Shishido, T.
Shibauchi, Y. Matsuda, and T. Terashima, Phys. Rev. Lett. 105, 107003 (2010)]
and point to the possible role of AFM fluctuations in driving
superconductivity.Comment: 5 pages, 4 figure
Temperature Dependence of the Dynamics of Portevin-Le Chatelier Effect in Al-2.5%Mg alloy
Tensile tests were carried out by deforming polycrystalline samples of
Al-2.5%Mg alloy at four different temperatures in an intermediate strain rate
regime of 2x10-4s-1 to 2x10-3s-1. The Portevin-Le Chatelier (PLC) effect was
observed throughout the strain rate and temperature region. The mean cumulative
stress drop magnitude and the mean reloading time exhibit an increasing trend
with temperature which is attributed to the enhanced solute diffusion at higher
temperature. The observed stress-time series data were analyzed using the
nonlinear dynamical methods. From the analyses, we could establish the presence
of deterministic chaos in the PLC effect throughout the temperature regime. The
dynamics goes to higher dimension at a sufficiently high temperature of 425K
but the complexity of the dynamics is not affected by the temperature.Comment: 18 pages, 8 figures; accepted in Met. Mater. Trans.
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