8,231 research outputs found
Field-theoretical approach to a dense polymer with an ideal binary mixture of clustering centers
We propose a field-theoretical approach to a polymer system immersed in an
ideal mixture of clustering centers. The system contains several species of
these clustering centers with different functionality, each of which connects a
fixed number segments of the chain to each other. The field-theory is solved
using the saddle point approximation and evaluated for dense polymer melts
using the Random Phase Approximation. We find a short-ranged effective
inter-segment interaction with strength dependent on the average segment
density and discuss the structure factor within this approximation. We also
determine the fractions of linkers of the different functionalities.Comment: 27 pages, 9 figures, accepted on Phys. Rev.
Enhanced diffusion and ordering of self-propelled rods
Starting from a minimal physical model of self propelled hard rods on a
substrate in two dimensions, we derive a modified Smoluchowski equation for the
system. Self -propulsion enhances longitudinal diffusion and modifies the mean
field excluded volume interaction. From the Smoluchowski equation we obtain
hydrodynamic equations for rod concentration, polarization and nematic order
parameter. New results at large scales are a lowering of the density of the
isotropic-nematic transition and a strong enhancement of boundary effects in
confined self-propelled systems.Comment: 4 pages, 2 figure
The Concentration-Density Relation of Galaxies in Las Campanas Redshift Survey
We report the results of the evaluation of the ``concentration-density''
relation of galaxies in the local universe, taking advantage of the very large
and homogeneous data set available from the Las Campanas Redshift Survey
(Shectman et al. 1996). This data set consists of galaxies inhabiting the
entire range of galactic environments, from the sparsest field to the densest
clusters, thus allowing us to study environmental variations without combining
multiple data sets with inhomogeneous characteristics. Concentration is
quantified by the automatically-measured concentration index , which is a
good measure of a galaxy's bulge-to-disk ratio. The environment of the sample
galaxies is characterized both by the three-space local galaxy density and by
membership in groups and clusters. We find that the distribution of C in galaxy
populations varies both with local density and with cluster/group membership:
the fraction of centrally-concentrated galaxies increases with local galaxy
density, and is higher in clusters than in the field. A comparison of the
concentration-local density relation in clusters and the field shows that the
two connect rather smoothly at the intermediate density regime, implying that
the apparent cluster/field difference is only a manifestation of the variation
with the local density. We conclude that the structure of galaxies is
predominantly influenced by the local density and not by the broader
environments characterized by cluster/field memberships.Comment: 11 pages, 4 figures, ApJ in press, uses psfig.st
Entanglement reduction induced by geometrical confinement in polymer thin films
We report simulation results on melts of entangled linear polymers confined
in a free-standing thin film. We study how the geometric constraints imposed by
the confinement alter the entanglement state of the system compared to the
equivalent bulk system using various observables. We find that the confinement
compresses the chain conformation uniaxially, decreasing the volume pervaded by
the chain, which in turn reduces the number of the accessible inter-chain
contact that could lead to entanglements. This local and non-uniform effect
depends on the position of the chain within the film. We also test a recently
presented theory that predicts how the number of entanglements decreases with
geometrical confinement.Comment: 28 pages, 10 figure
Classification of graph C*-algebras with no more than four primitive ideals
We describe the status quo of the classification problem of graph C*-algebras
with four primitive ideals or less
A field theoretic approach to master equations and a variational method beyond the Poisson ansatz
We develop a variational scheme in a field theoretic approach to a stochastic
process. While various stochastic processes can be expressed using master
equations, in general it is difficult to solve the master equations exactly,
and it is also hard to solve the master equations numerically because of the
curse of dimensionality. The field theoretic approach has been used in order to
study such complicated master equations, and the variational scheme achieves
tremendous reduction in the dimensionality of master equations. For the
variational method, only the Poisson ansatz has been used, in which one
restricts the variational function to a Poisson distribution. Hence, one has
dealt with only restricted fluctuation effects. We develop the variational
method further, which enables us to treat an arbitrary variational function. It
is shown that the variational scheme developed gives a quantitatively good
approximation for master equations which describe a stochastic gene regulatory
network.Comment: 13 pages, 2 figure
Topological versus rheological entanglement length in primitive path analysis protocols
Primitive path analysis algorithms are now routinely employed to analyze
entanglements in computer simulations of polymeric systems, but different
analysis protocols result in different estimates of the entanglement length,
N_e. Here we argue that standard PPA measures the rheological entanglement
length, typically employed by tube models and relevant to quantitative
comparisons with experiment, while codes like Z or CReTA also determine the
topological entanglement length. For loosely entangled systems, a simple
analogy between between phantom networks and the mesh of entangled primitive
paths suggests a factor of two between the two numbers. This result is in
excellent agreement with reported values for poly-ethylene, poly-butadiene and
bead-spring polymer melts.Comment: 3 pages, no figure
Nonlinear waves in a model for silicate layers
Some layered silicates are composed of positive ions, surrounded by layers of ions with opposite sign. Mica muscovite is a particularly interesting material, because there exist fossil and experimental evidence for nonlinear excitations transporting localized energy and charge along the cation rows within the potassium layers. This evidence suggest that there are different kinds of excitations with different energies and properties. Some of the authors proposed recently a one-dimensional model based in physical principles and the silicate structure. The main characteristic of the model is that it has a hard substrate potential and two different repulsion terms, between ions and nuclei. In a previous work with this model, it was found the propagation of crowdions, i.e., lattice kinks in a lattice with substrate potential that transport mass and charge. They have a single specific velocity and energy coherent with the experimental data. In the present work we perform a much more thorough search for nonlinear excitations in the same model using the pseudospectral method to obtain exact nanopteron solutions, which are single kinks with tails, crowdions and bi-crowdions. We analyze their velocities, energies and stability or instability and the possible reasons for the latter. We relate the different excitations with their possible origin from recoils from different beta decays and with the fossil tracks. We explore the consequences of some variation of the physical parameters because their values are not perfectly known. Through a different method, we also have found stationary and moving breathers, that is, localized nonlinear excitations with an internal vibration. Moving breathers have small amplitude and energy, which is also coherent with the fossil evidence.MINECO (Spain) FIS2015-65998-C2-2-PJunta de Andalucía 2017/FQM-280Universidad de Sevilla (España) grants VI PPIT-US-201
Polymer drift in a solvent by force acting on one polymer end
We investigate the effect of hydrodynamic interactions on the non-equilibrium
drift dynamics of an ideal flexible polymer pulled by a constant force applied
at one end of the polymer using the perturbation theory and the renormalization
group method. For moderate force, if the polymer elongation is small, the
hydrodynamic interactions are not screened and the velocity and the
longitudinal elongation of the polymer are computed using the renormalization
group method. Both the velocity and elongation are nonlinear functions of the
driving force in this regime. For large elongation we found two regimes. For
large force but finite chain length the hydrodynamic interactions are
screened. For large chain lengths and a finite force the hydrodynamic
interactions are only partially screened, which in three dimensions results in
unusual logarithmic corrections to the velocity and the longitudinal
elongation.Comment: 6 page
Global cross-over dynamics of single semiflexible polymers
We present a mean-field dynamical theory for single semiflexible polymers
which can precisely capture, without fitting parameters, recent fluorescence
correlation spectroscopy results on single monomer kinetics of DNA strands in
solution. Our approach works globally, covering three decades of strand length
and five decades of time: it includes the complex cross-overs occurring between
stiffness-dominated and flexible bending modes, along with larger-scale
rotational and center-of-mass motion. The accuracy of the theory stems in part
from long-range hydrodynamic coupling between the monomers, which makes a
mean-field description more realistic. Its validity extends even to short,
stiff fragments, where we also test the theory through Brownian hydrodynamics
simulations.Comment: 6 pages, 5 figures; updated with minor changes to reflect published
versio
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