557 research outputs found
Thermal Fluctuations in a Lamellar Phase of a Binary Amphiphile-Solvent Mixture: A Molecular Dynamics Study
We investigate thermal fluctuations in a smectic A phase of an
amphiphile-solvent mixture with molecular dynamics simulations. We use an
idealized model system, where solvent particles are represented by simple
beads, and amphiphiles by bead-and-spring tetramers. At a solvent bead fraction
of 20 % and sufficiently low temperature, the amphiphiles self-assemble into a
highly oriented lamellar phase. Our study aims at comparing the structure of
this phase with the predictions of the elastic theory of thermally fluctuating
fluid membrane stacks [Lei et al., J. Phys. II 5, 1155 (1995)]. We suggest a
method which permits to calculate the bending rigidity and compressibility
modulus of the lamellar stack from the simulation data. The simulation results
are in reasonable agreement with the theory
Critical behavior of frustrated systems: Monte Carlo simulations versus Renormalization Group
We study the critical behavior of frustrated systems by means of Pade-Borel
resummed three-loop renormalization-group expansions and numerical Monte Carlo
simulations. Amazingly, for six-component spins where the transition is second
order, both approaches disagree. This unusual situation is analyzed both from
the point of view of the convergence of the resummed series and from the
possible relevance of non perturbative effects.Comment: RevTex, 10 pages, 3 Postscript figure
Assessing senescence patterns in populations of large mammals
Theoretical models such as those of Gompertz and Weibull are commonly used to study senescence in survival for humans and laboratory or captive animals. For wild populations of vertebrates, senescence in survival has more commonly been assessed by fitting simple linear or quadratic relationships between survival and age. By using appropriate constraints on survival parameters in Capture-Mark-Recapture (CMR) models, we propose a first analysis of the suitability of the Gompertz and the two-parameter Weibull models for describing aging-related mortality in free-ranging populations of ungulates. We first show how to handle the Gompertz and the two-parameter Weibull models in the context of CMR analyses. Then we perform a comparative analysis of senescence patterns in both sexes of two ungulate species highly contrasted according to the intensity of sexual selection. Our analyses provide support to the Gompertz model for describing senescence patterns in ungulates. Evolutionary implications of our results are discusse
The critical behavior of frustrated spin models with noncollinear order
We study the critical behavior of frustrated spin models with noncollinear
order, including stacked triangular antiferromagnets and helimagnets. For this
purpose we compute the field-theoretic expansions at fixed dimension to six
loops and determine their large-order behavior. For the physically relevant
cases of two and three components, we show the existence of a new stable fixed
point that corresponds to the conjectured chiral universality class. This
contradicts previous three-loop field-theoretical results but is in agreement
with experiments.Comment: 4 pages, RevTe
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
Monte Carlo renormalization group study of the Heisenberg and XY antiferromagnet on the stacked triangular lattice and the chiral model
With the help of the improved Monte Carlo renormalization-group scheme, we
numerically investigate the renormalization group flow of the antiferromagnetic
Heisenberg and XY spin model on the stacked triangular lattice (STA-model) and
its effective Hamiltonian, 2N-component chiral model which is used in
the field-theoretical studies. We find that the XY-STA model with the lattice
size exhibits clear first-order behavior. We also
find that the renormalization-group flow of STA model is well reproduced by the
chiral model, and that there are no chiral fixed point of
renormalization-group flow for N=2 and 3 cases. This result indicates that the
Heisenberg-STA model also undergoes first-order transition.Comment: v1:15 pages, 15 figures v2:updated references v3:added comments on
the higher order irrelevant scaling variables v4:added results of larger
sizes v5:final version to appear in J.Phys.Soc.Jpn Vol.72, No.
Testing for Features in the Primordial Power Spectrum
Well-known causality arguments show that events occurring during or at the
end of inflation, associated with reheating or preheating, could contribute a
blue component to the spectrum of primordial curvature perturbations, with the
dependence k^3. We explore the possibility that they could be observably large
in CMB, LSS, and Lyman-alpha data. We find that a k^3 component with a cutoff
at some maximum k can modestly improve the fits (Delta chi^2=2.0, 5.4) of the
low multipoles (l ~ 10 - 50) or the second peak (l ~ 540) of the CMB angular
spectrum when the three-year WMAP data are used. Moreover, the results from
WMAP are consistent with the CBI, ACBAR, 2dFGRS, and SDSS data when they are
included in the analysis. Including the SDSS galaxy clustering power spectrum,
we find weak positive evidence for the k^3 component at the level of Delta chi'
= 2.4, with the caveat that the nonlinear evolution of the power spectrum may
not be properly treated in the presence of the k^3 distortion. To investigate
the high-k regime, we use the Lyman-alpha forest data (LUQAS, Croft et al., and
SDSS Lyman-alpha); here we find evidence at the level Delta chi^2' = 3.8.
Considering that there are two additional free parameters in the model, the
above results do not give a strong evidence for features; however, they show
that surprisingly large bumps are not ruled out. We give constraints on the
ratio between the k^3 component and the nearly scale-invariant component, r_3 <
1.5, over the range of wave numbers 0.0023/Mpc < k < 8.2/Mpc. We also discuss
theoretical models which could lead to the k^3 effect, including ordinary
hybrid inflation and double D-term inflation models. We show that the
well-motivated k^3 component is also a good representative of the generic
spikelike feature in the primordial perturbation power spectrum.Comment: 23 pages, 6 figures; added new section on theoretical motivation for
k^3 term, and discussion of double D-term hybrid inflation models; title
changed, added a new section discussing the generic spikelike features,
published in IJMP
Evaluación de pautas de senescencia en poblaciones de grandes mamÃferos
Theoretical models such as those of Gompertz and Weibull are commonly used to study senescence in survival for humans and laboratory or captive animals. For wild populations of vertebrates, senescence in survival has more commonly been assessed by fitting simple linear or quadratic relationships between survival and age. By using appropriate constraints on survival parameters in Capture–Mark–Recapture (CMR) models, we propose a first analysis of the suitability of the Gompertz and the two-parameter Weibull models for describing aging–related mortality in free–ranging populations of ungulates. We first show how to handle the Gompertz and the two–parameter Weibull models in the context of CMR analyses. Then we perform a comparative analysis of senescence patterns in both sexes of two ungulate species highly contrasted according to the intensity of sexual selection. Our analyses provide support to the Gompertz model for describing senescence patterns in ungulates. Evolutionary implications of our results are discussed.Por lo general, para estudiar el papel que desempeña la senescencia en la supervivencia, ya sea en humanos, en animales de laboratorio o en animales cautivos. Se emplean modelos teóricos, como los de Gompertz y Weibull. En el caso de las poblaciones silvestres de vertebrados, dicho papel tiende a evaluarse ajustando relaciones lineales o cuadráticas simples entre la supervivencia y la edad. En el presente estudio proponemos —a partir de la aplicación de constricciones apropiadas en los parámetros de supervivencia empleados en los modelos de captura–marcaje–recaptura (CMR)— un primer análisis de la idoneidad del modelo de Gompertz y del modelo de dos parámetros de Weibull para describir la mortalidad relacionada con el envejecimiento en poblaciones de ungulados criadas en régimen de pasto libre. En primer lugar indicamos cómo emplear el modelo de Gompertz y el modelo de dos parámetros de Weibull en el contexto de los análisis de CMR, para seguidamente llevar a cabo un análisis comparativo de las pautas de senescencia en dos especies de ungulados de sexos opuestos, altamente contrastadas según la intensidad de la selección sexual. Nuestro análisis apoya el modelo de Gompertz para la descripción de pautas de senescencia en ungulados. Se discuten las implicaciones evolutivas de los resultados obtenidos
Translation and validation of the French Movement Imagery Questionnaire. Revised Second Version (MIQ-RS)
Introduction
Motor imagery can be defined as a dynamic state during which the representation of a movement is internally rehearsed in the absence of voluntary movements. There are two strategies to mentally simulate the movements, either a visual representation of the movements (visual imagery), or kinesthetic feeling of the movement (kinetic imagery). In stroke rehabilitation, studies indicate that motor imagery associated with physical therapy results in cortical reorganization and correlative functional improvements.
Aim
The aim of this study is to provide to the French-speaking community a valid and reliable version of the Movement Imagery Questionnaire – Revised Second Version (MIQ-RS).
Method
We examined the test-retest, inter-rate reliability and the internal consistency of the visual and kinesthetic items of our French version of MIQ-RS in 153 healthy subjects.
Results
Results showed the internal consistency (Cronbach α = 0.90) and test-retest reliability (intraclass correlation coefficient for visual items = 0.68 and for kinesthetic items = 0.78) of the French version of MIQ-RS were satisfactory; the two-factor structure was supported by confirmatory factor analysis.
Conclusion
The French version of MIQ-RS is a valid and reliable instrument in French-speaking population and therefore useful as a measure for motor imagery ability
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