3,817 research outputs found
Phase Ordering Dynamics of Theory with Hamiltonian Equations of Motion
Phase ordering dynamics of the (2+1)- and (3+1)-dimensional theory
with Hamiltonian equations of motion is investigated numerically. Dynamic
scaling is confirmed. The dynamic exponent is different from that of the
Ising model with dynamics of model A, while the exponent is the same.Comment: to appear in Int. J. Mod. Phys.
Identification of a lineage of multipotent hematopoietic progenitors
All multipotent hematopoietic progenitors in C57BL-Thy-1.1 bone marrow are divided among three subpopulations of Thy-1.1^(lo) Sca-1^+ Lin^(-/lo) c-kit^+ cells: long-term reconstituting Mac-1^-CD4^-c-kit^+ cells and transiently reconstituting Mac-1^(lo)CD4^-or Mac-1^(lo) CD4^(lo) cells. This study shows that the same populations, with similar functional activities, exist in mice whose hematopoietic systems were reconstituted by hematopoietic stem cells after lethal irradiation. We demonstrate that these populations form a lineage of multipotent progenitors from long-term self-renewing stem cells to the most mature multipotent progenitor population. In reconstituted mice, Mac-1- CD4^-c-kit^+ cells gave rise to Mac-1^(lo)CD4^- cells, which gave rise to Mac-1^(lo)CD4^(lo) cells. Mac-1^- CD4^-c-kit^+ cells had long-term self-renewal potential, with each cell being capable of giving rise to more than 10^4 functionally similar Mac-1^-CD4^-c-kit^+ cells. At least half of Mac-1^(lo)CD4^- cells had transient self-renewal potential, detected in the spleen 7 days after reconstitution. Mac-1^(lo)CD4^(lo) cells did not have detectable self-renewal potential. The identification of a lineage of multipotent progenitors provides an important tool for identifying genes that regulate self-renewal and lineage commitment
Scaling laws for the largest Lyapunov exponent in long-range systems: A random matrix approach
We investigate the laws that rule the behavior of the largest Lyapunov
exponent (LLE) in many particle systems with long range interactions. We
consider as a representative system the so-called Hamiltonian alpha-XY model
where the adjustable parameter alpha controls the range of the interactions of
N ferromagnetic spins in a lattice of dimension d. In previous work the
dependence of the LLE with the system size N, for sufficiently high energies,
was established through numerical simulations. In the thermodynamic limit, the
LLE becomes constant for alpha greater than d whereas it decays as an inverse
power law of N for alpha smaller than d. A recent theoretical calculation based
on Pettini's geometrization of the dynamics is consistent with these numerical
results (M.-C. Firpo and S. Ruffo, cond-mat/0108158). Here we show that the
scaling behavior can also be explained by a random matrix approach, in which
the tangent mappings that define the Lyapunov exponents are modeled by random
simplectic matrices drawn from a suitable ensemble.Comment: 5 pages, no figure
Thermal Excitation of Broadband and Long-range Surface Waves on SiO 2 Submicron Films
We detect thermally excited surfaces waves on a submicron SiO 2 layer,
including Zenneck and guided modes in addition to Surface Phonon Polaritons.
The measurements show the existence of these hybrid thermal-electromagnetic
waves from near-(2.7 m) to far-(11.2 m) infrared. Their propagation
distances reach values on the order of the millimeter, several orders of
magnitude larger than on semi-infinite systems. These two features, spectral
broadness and long range propagation, make these waves good candidates for
near-field applications both in optics and thermics due to their dual nature.Comment: Applied Physics Letters, American Institute of Physics, 201
Topological origin of the phase transition in a mean-field model
We argue that the phase transition in the mean-field XY model is related to a
particular change in the topology of its configuration space. The nature of
this topological transition can be discussed on the basis of elementary Morse
theory using the potential energy per particle V as a Morse function. The value
of V where such a topological transition occurs equals the thermodynamic value
of V at the phase transition and the number of (Morse) critical points grows
very fast with the number of particles N. Furthermore, as in statistical
mechanics, also in topology the way the thermodynamic limit is taken is
crucial.Comment: REVTeX, 5 pages, with 1 eps figure included. Some changes in the
text. To appear in Physical Review Letter
Energy Storage in a Hamiltonian System in Partial Contact with a Heat Bath
To understand the mechanism allowing for the long-term storage of excess
energy in proteins, we study a Hamiltonian system consisting of several coupled
pendula in partial contact with a heat bath. It is found that energy storage is
possible when the motion of each pendulum switches between oscillatory
(vibrational) and rotational (phase-slip) modes. The storage time increases
almost exponentially to the square root of the injected energy. The relevance
of our mechanism to protein motors is discussed.Comment: 8 pages, 4 figures, to appear in J.Phys.Soc.Jp
Phase transitions as topology changes in configuration space: an exact result
The phase transition in the mean-field XY model is shown analytically to be
related to a topological change in its configuration space. Such a topology
change is completely described by means of Morse theory allowing a computation
of the Euler characteristic--of suitable submanifolds of configuration
space--which shows a sharp discontinuity at the phase transition point, also at
finite N. The present analytic result provides, with previous work, a new key
to a possible connection of topological changes in configuration space as the
origin of phase transitions in a variety of systems.Comment: REVTeX file, 5 pages, 1 PostScript figur
Safety and efficacy of short-term intrapulmonary percussive ventilation in patients with bronchiectasis
Background. Treatment of bronchiectasis includes drugs, oxygen therapy and bronchial clearance maneuvers. The aim of the current study was to assess safety and efficacy of IntrapulmonaryPercussive Ventilation when compared with usual Chest Physical Therapy in patients with bronchiectasis Methods. In two consecutive days, 22 patients underwent both Intrapulmonary Percussive Ventilation and Chest Physical Therapy following a randomized cross-over design. At inclusion (T0), at the end of 30-min session (T1), and after 30 min (T2) and 4 hrs (T3), side effects, heart rate, oxygen saturation rate, respiratory rate, sensation of phlegm encumbrance and dyspneameasured by visual analogue scales, were recorded. At T1, discomfort measured by visual analogue scales was also recorded. At T3, we evaluated efficacy in terms of volume (ml), and wet and dry weight (g) of sputum. Results. Side effects were not so severe as to determine study discontinuation and were similar (27%) between the two treatments. Heart rate (p<.001) and respiratory rate (p=0.047) decreased over time while sensation of phlegm encumbrance improved (p=0.026) withboth treatments. Only Intrapulmonary Percussive Ventilation improved (p=0.004) sensation of dyspnea and resulted more comfortable than Chest Physical Therapy (p=0.032). The two treatments caused important phlegm production without differences in total volume, and both wet and dry weight. Conclusions. In patients with bronchiectasis and productive cough, short-term application of Intrapulmonary Percussive Ventilation is similarly safe and effective than traditional chestPhysical Therapy with less discomfort. Further studies on cost-effectiveness of using IPV is recommended
Mutant glycyl-tRNA synthetase (Gars) ameliorates SOD1G93A motor neuron degeneration phenotype but has little affect on Loa dynein heavy chain mutant mice
Background:
In humans, mutations in the enzyme glycyl-tRNA synthetase (GARS) cause motor and sensory axon loss in the peripheral nervous system, and clinical phenotypes ranging from Charcot-Marie-Tooth neuropathy to a severe infantile form of spinal muscular atrophy. GARS is ubiquitously expressed and may have functions in addition to its canonical role in protein synthesis through catalyzing the addition of glycine to cognate tRNAs.
Methodology/Principal findings:
We have recently described a new mouse model with a point mutation in the Gars gene resulting in a cysteine to arginine change at residue 201. Heterozygous Gars^{C201R/+} mice have locomotor and sensory deficits. In an investigation of genetic mutations that lead to death of motor and sensory neurons, we have crossed the Gars^{C201R/+} mice to two other mutants: the TgSOD1^{G93A} model of human amyotrophic lateral sclerosis and the Legs at odd angles mouse (Dync1h1^{Loa}) which has a defect in the heavy chain of the dynein complex. We found the Dync1h1^{Loa/+}; Gars^{C201R/+} double heterozygous mice are more impaired than either parent, and this is may be an additive effect of both mutations. Surprisingly, the Gars^{C201R} mutation significantly delayed disease onset in the SOD1^{G93A}; Gars^{C201R/+} double heterozygous mutant mice and increased lifespan by 29% on the genetic background investigated.
Conclusions/Significance:
These findings raise intriguing possibilities for the study of pathogenetic mechanisms in all three mouse mutant strains
Hamiltonian dynamics and geometry of phase transitions in classical XY models
The Hamiltonian dynamics associated to classical, planar, Heisenberg XY
models is investigated for two- and three-dimensional lattices. Besides the
conventional signatures of phase transitions, here obtained through time
averages of thermodynamical observables in place of ensemble averages,
qualitatively new information is derived from the temperature dependence of
Lyapunov exponents. A Riemannian geometrization of newtonian dynamics suggests
to consider other observables of geometric meaning tightly related with the
largest Lyapunov exponent. The numerical computation of these observables -
unusual in the study of phase transitions - sheds a new light on the
microscopic dynamical counterpart of thermodynamics also pointing to the
existence of some major change in the geometry of the mechanical manifolds at
the thermodynamical transition. Through the microcanonical definition of the
entropy, a relationship between thermodynamics and the extrinsic geometry of
the constant energy surfaces of phase space can be naturally
established. In this framework, an approximate formula is worked out,
determining a highly non-trivial relationship between temperature and topology
of the . Whence it can be understood that the appearance of a phase
transition must be tightly related to a suitable major topology change of the
. This contributes to the understanding of the origin of phase
transitions in the microcanonical ensemble.Comment: in press on Physical Review E, 43 pages, LaTeX (uses revtex), 22
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