2,937 research outputs found
The effect of the range of interaction on the phase diagram of a globular protein
Thermodynamic perturbation theory is applied to the model of globular
proteins studied by ten Wolde and Frenkel (Science 277, pg. 1976) using
computer simulation. It is found that the reported phase diagrams are
accurately reproduced. The calculations show how the phase diagram can be tuned
as a function of the lengthscale of the potential.Comment: 20 pages, 5 figure
Onset of collective and cohesive motion
We study the onset of collective motion, with and without cohesion, of groups
of noisy self-propelled particles interacting locally. We find that this phase
transition, in two space dimensions, is always discontinuous, including for the
minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for
which a non-trivial critical point was previously advocated. We also show that
cohesion is always lost near onset, as a result of the interplay of density,
velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let
Dilaton constraints and LHC prospects
The Standard Model Higgs searches using the first 1-2 fb-1 of LHC data can be
used to put interesting constraints on new scalar particles other than the
Higgs. We investigate one such scenario in which electroweak symmetry is broken
via strongly coupled conformal dynamics. This scenario contains a neutral
scalar dilaton---the Goldstone boson associated with spontaneously broken scale
invariance---with a mass below the conformal symmetry breaking scale and
couplings to Standard Model particles similar (but not identical) to those of
the Standard Model Higgs boson. We translate the LEP and LHC Higgs limits to
constrain the dilaton mass and conformal breaking scale. The conformal breaking
scale f is constrained to be above 1 TeV for dilaton masses between 145 and 600
GeV, though it can be as low as 400 GeV for dilaton masses below 110 GeV. We
also show that (i) a dilaton chi with mass below 110 GeV and consistent with
the LEP constraints can appear in gg --> chi --> gamma gamma with a rate up to
~10 times the corresponding Standard Model Higgs rate, and (ii) a dilaton with
mass of several hundred GeV is much narrower than the corresponding Standard
Model Higgs, leading to improved search sensitivity in chi --> ZZ --> 4l.Comment: 15 pages, 12 figures, References added, Figure 10 modified, Figure 11
adde
Hydrodynamic equations for self-propelled particles: microscopic derivation and stability analysis
Considering a gas of self-propelled particles with binary interactions, we
derive the hydrodynamic equations governing the density and velocity fields
from the microscopic dynamics, in the framework of the associated Boltzmann
equation. Explicit expressions for the transport coefficients are given, as a
function of the microscopic parameters of the model. We show that the
homogeneous state with zero hydrodynamic velocity is unstable above a critical
density (which depends on the microscopic parameters), signaling the onset of a
collective motion. Comparison with numerical simulations on a standard model of
self-propelled particles shows that the phase diagram we obtain is robust, in
the sense that it depends only slightly on the precise definition of the model.
While the homogeneous flow is found to be stable far from the transition line,
it becomes unstable with respect to finite-wavelength perturbations close to
the transition, implying a non trivial spatio-temporal structure for the
resulting flow. We find solitary wave solutions of the hydrodynamic equations,
quite similar to the stripes reported in direct numerical simulations of
self-propelled particles.Comment: 33 pages, 11 figures, submitted to J. Phys.
Permutation-invariant distance between atomic configurations
We present a permutation-invariant distance between atomic configurations,
defined through a functional representation of atomic positions. This distance
enables to directly compare different atomic environments with an arbitrary
number of particles, without going through a space of reduced dimensionality
(i.e. fingerprints) as an intermediate step. Moreover, this distance is
naturally invariant through permutations of atoms, avoiding the time consuming
associated minimization required by other common criteria (like the Root Mean
Square Distance). Finally, the invariance through global rotations is accounted
for by a minimization procedure in the space of rotations solved by Monte Carlo
simulated annealing. A formal framework is also introduced, showing that the
distance we propose verifies the property of a metric on the space of atomic
configurations. Two examples of applications are proposed. The first one
consists in evaluating faithfulness of some fingerprints (or descriptors), i.e.
their capacity to represent the structural information of a configuration. The
second application concerns structural analysis, where our distance proves to
be efficient in discriminating different local structures and even classifying
their degree of similarity
Criterion for purely elastic Taylor-Couette instability in the flows of shear-banding fluids
In the past twenty years, shear-banding flows have been probed by various
techniques, such as rheometry, velocimetry and flow birefringence. In micellar
solutions, many of the data collected exhibit unexplained spatio-temporal
fluctuations. Recently, it has been suggested that those fluctuations originate
from a purely elastic instability of the flow. In cylindrical Couette geometry,
the instability is reminiscent of the Taylor-like instability observed in
viscoelastic polymer solutions. In this letter, we describe how the criterion
for purely elastic Taylor-Couette instability should be adapted to
shear-banding flows. We derive three categories of shear-banding flows with
curved streamlines, depending on their stability.Comment: 6 pages, 3 figure
Comparative Measurements of Inverse Spin Hall and Magnetoresistance in YIG|Pt and YIG|Ta
We report on a comparative study of spin Hall related effects and
magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements
allow to estimate the characteristic transport parameters of both Pt and Ta
layers juxtaposed to YIG: the spin mixing conductance
at the YIGnormal metal interface, the spin Hall angle , and the
spin diffusion length in the normal metal. The inverse spin Hall
voltages generated in Pt and Ta by the pure spin current pumped from YIG
excited at resonance confirm the opposite signs of spin Hall angles in these
two materials. Moreover, from the dependence of the inverse spin Hall voltage
on the Ta thickness, we extract the spin diffusion length in Ta, found to be
nm. Both the YIG|Pt and YIG|Ta systems
display a similar variation of resistance upon magnetic field orientation,
which can be explained in the recently developed framework of spin Hall
magnetoresistance.Comment: 8 pages, 5 figures, 1 tabl
Coordination of Foliar and Wood Anatomical Traits Contributes to Tropical Tree Distributions and Productivity along the Malay-Thai Peninsula
Drought is a critical factor in plant species distributions. Much research points to its relevance even in moist tropical regions. Recent studies have begun to elucidate mechanisms underlying the distributions of tropical tree species with respect to drought; however, how such desiccation tolerance mechanisms correspond with the coordination of hydraulic and photosynthetic traits in determining species distributions with respect to rainfall seasonality deserves attention. In the present study, we used a common garden approach to quantify inherent differences in wood anatomical and foliar physiological traits in 21 tropical tree species with either widespread (occupying both seasonal and aseasonal climates) or southern (restricted to aseasonal forests) distributions with respect to rainfall seasonality. Use of congeneric species pairs and phylogenetically independent contrast analyses allowed examination of this question in a phylogenetic framework. Widespread species opted for wood traits that provide biomechanical support and prevent xylem cavitation and showed associated reductions in canopy productivity and consequently growth rates compared with southern species. These data support the hypothesis that species having broader distributions with respect to climatic variability will be characterized by traits conducive to abiotic stress tolerance. This study highlights the importance of the well-established performance vs. stress tolerance trade-off as a contributor to species distributions at larger scales
Potential "ways of thinking" about the shear-banding phenomenon
Shear-banding is a curious but ubiquitous phenomenon occurring in soft
matter. The phenomenological similarities between the shear-banding transition
and phase transitions has pushed some researchers to adopt a 'thermodynamical'
approach, in opposition to the more classical 'mechanical' approach to fluid
flows. In this heuristic review, we describe why the apparent dichotomy between
those approaches has slowly faded away over the years. To support our
discussion, we give an overview of different interpretations of a single
equation, the diffusive Johnson-Segalman (dJS) equation, in the context of
shear-banding. We restrict ourselves to dJS, but we show that the equation can
be written in various equivalent forms usually associated with opposite
approaches. We first review briefly the origin of the dJS model and its initial
rheological interpretation in the context of shear-banding. Then we describe
the analogy between dJS and reaction-diffusion equations. In the case of
anisotropic diffusion, we show how the dJS governing equations for steady shear
flow are analogous to the equations of the dynamics of a particle in a quartic
potential. Going beyond the existing literature, we then draw on the Lagrangian
formalism to describe how the boundary conditions can have a key impact on the
banding state. Finally, we reinterpret the dJS equation again and we show that
a rigorous effective free energy can be constructed, in the spirit of early
thermodynamic interpretations or in terms of more recent approaches exploiting
the language of irreversible thermodynamics.Comment: 14 pages, 6 figures, tutorial revie
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