20,109 research outputs found
Dynamic model for failures in biological systems
A dynamic model for failures in biological organisms is proposed and studied
both analytically and numerically. Each cell in the organism becomes dead under
sufficiently strong stress, and is then allowed to be healed with some
probability. It is found that unlike the case of no healing, the organism in
general does not completely break down even in the presence of noise. Revealed
is the characteristic time evolution that the system tends to resist the stress
longer than the system without healing, followed by sudden breakdown with some
fraction of cells surviving. When the noise is weak, the critical stress beyond
which the system breaks down increases rapidly as the healing parameter is
raised from zero, indicative of the importance of healing in biological
systems.Comment: To appear in Europhys. Let
Dynamic model of fiber bundles
A realistic continuous-time dynamics for fiber bundles is introduced and
studied both analytically and numerically. The equation of motion reproduces
known stationary-state results in the deterministic limit while the system
under non-vanishing stress always breaks down in the presence of noise.
Revealed in particular is the characteristic time evolution that the system
tends to resist the stress for considerable time, followed by sudden complete
rupture. The critical stress beyond which the complete rupture emerges is also
obtained
Nano granular metallic Fe - oxygen deficient TiO composite films: A room temperature, highly carrier polarized magnetic semiconductor
Nano granular metallic iron (Fe) and titanium dioxide (TiO) were
co-deposited on (100) lanthanum aluminate (LaAlO) substrates in a low
oxygen chamber pressure using a pulsed laser ablation deposition (PLD)
technique. The co-deposition of Fe and TiO resulted in 10 nm
metallic Fe spherical grains suspended within a TiO matrix. The
films show ferromagnetic behavior with a saturation magnetization of 3100 Gauss
at room temperature. Our estimate of the saturation magnetization based on the
size and distribution of the Fe spheres agreed well with the measured value.
The film composite structure was characterized as p-type magnetic semiconductor
at 300 K with a carrier density of the order of . The
hole carriers were excited at the interface between the nano granular Fe and
TiO matrix similar to holes excited in the metal/n-type
semiconductor interface commonly observed in Metal-Oxide-Semiconductor (MOS)
devices. From the large anomalous Hall effect directly observed in these films
it follows that the holes at the interface were strongly spin polarized.
Structure and magneto transport properties suggested that these PLD films have
potential nano spintronics applications.Comment: 6 pages in Latex including 8 figure
Uric acid enhances longevity and endurance and protects the brain against ischemia
Among mammals, there is a positive correlation between serum uric acid (UA) levels and life span. Humans have high levels of UA because they lack a functional urate oxidase (UOX) enzyme that is present in shorter lived mammals. Here, we show that male and female mice with UOX haploinsufficiency exhibit an age-related elevation of UA levels, and that the life span of female but not male UOX+/− mice is significantly increased compared to wild-type mice. Serum UA levels are elevated in response to treadmill exercise in UOX+/− mice, but not wild-type mice, and the endurance of the UOX+/− mice is significantly greater than wild-type mice. UOX+/− mice exhibit elevated levels of brain-derived neurotrophic factor, reduced brain damage and improved functional outcome in a model of focal ischemic stroke. Levels of oxidative protein nitration and lipid peroxidation are reduced in muscle and brain tissues of UOX+/− mice under conditions of metabolic and oxidative stress (running in the case of muscle and ischemia in the case of the brain), consistent with prior evidence that UA can scavenge peroxynitrite and hydroxyl radical. Our findings reveal roles for UA in life span determination, endurance and adaptive responses to brain injury, and suggest novel approaches for protecting cells against injury and for optimizing physical performance.España, Ministerio de Educación, Cultura y Deporte EX2009–091
Quantum Opacity, the RHIC HBT Puzzle, and the Chiral Phase Transition
We present a relativistic quantum mechanical treatment of opacity and
refractive effects that allows reproduction of observables measured in two-pion
(HBT) interferometry and pion spectra at RHIC. The inferred emission duration
is substantial. The results are consistent with the emission of pions from a
system that has a restored chiral symmetry.Comment: 4 pages, 2 figures. This version mentions shows the STAR 200 GeV
data, and includes some technical improvements. The agreement with experiment
is improved slightly, and the parameters of the model are changed slightl
Wigner crystallization in a polarizable medium
We present a variational study of the 2D and 3D Wigner crystal phase of large
polarons. The method generalizes that introduced by S. Fratini,P.\
Qu{\'{e}}merais [Mod. Phys. Lett. B {\bf 12} 1003 (1998)]. We take into account
the Wigner crystal normal modes rather than a single mean frequency in the
minimization procedure of the variational free energy. We calculate the
renormalized modes of the crystal as well as the charge polarization
correlation function and polaron radius. The solid phase boundaries are
determined via a Lindemann criterion, suitably generalized to take into account
the classical-to-quantum cross-over.
In the weak electron-phonon coupling limit, the Wigner crystal parameters are
renormalized by the electron-phonon interaction leading to a stabilization of
the solid phase for low polarizability of the medium. Conversely, at
intermediate and strong coupling, the behavior of the system depends strongly
on the polarizability of the medium.
For weakly polarizable media, a density crossover occurs inside the solid
phase when the renormalized plasma frequency approaches the phonon frequency.
At low density, we have a renormalized polaron Wigner crystal, while at higher
densities the electron-phonon interaction is weakened irrespective of the {\it
bare} electron-phonon coupling.
For strongly polarizable media, the system behaves as a Lorentz lattice of
dipoles. The abrupt softening of the internal polaronic frequency predicted by
Fratini and Quemerais is observed near the actual melting point only at very
strong coupling, leading to a possible liquid polaronic phase for a wider range
of parameters.Comment: 24 pages, 13 figures v1.
Orientational correlations in confined DNA
We study how the orientational correlations of DNA confined to nanochannels
depend on the channel diameter D by means of Monte Carlo simulations and a
mean-field theory. This theory describes DNA conformations in the
experimentally relevant regime where the Flory-de Gennes theory does not apply.
We show how local correlations determine the dependence of the end-to-end
distance of the DNA molecule upon D. Tapered nanochannels provide the necessary
resolution in D to study experimentally how the extension of confined DNA
molecules depends upon D. Our experimental and theoretical results are in
qualitative agreement.Comment: Revised version including supplemental material, 7 pages, 8 figure
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