967 research outputs found
Simulation of a Single Polymer Chain in Solution by Combining Lattice Boltzmann and Molecular Dynamics
In this paper we establish a new efficient method for simulating
polymer-solvent systems which combines a lattice Boltzmann approach for the
fluid with a continuum molecular dynamics (MD) model for the polymer chain. The
two parts are coupled by a simple dissipative force while the system is driven
by stochastic forces added to both the fluid and the polymer. Extensive tests
of the new method for the case of a single polymer chain in a solvent are
performed. The dynamic and static scaling properties predicted by analytical
theory are validated. In this context, the influence of the finite size of the
simulation box is discussed. While usually the finite size corrections scale as
L^{-1} (L denoting the linear dimension of the box), the decay rate of the
Rouse modes is only subject to an L^{-3} finite size effect. Furthermore, the
mapping to an existing MD simulation of the same system is done so that all
physical input values for the new method can be derived from pure MD
simulation. Both methods can thus be compared quantitatively, showing that the
new method allows for much larger time steps. Comparison of the results for
both methods indicates systematic deviations due to non-perfect match of the
static chain conformations.Comment: 17 pages, 12 figures, submitted to J. Chem. Phy
A Drop of Active Matter
We study theoretically the hydrodynamics of a fluid drop containing oriented
filaments endowed with active contractile or extensile stresses and placed on a
solid surface. The active stresses alter qualitatively the wetting properties
of the drop, leading to new spreading laws and novel static drop shapes.
Candidate systems for testing our predictions include cytoskeletal extracts
with motors and ATP, suspensions of bacteria or pulsatile cells, or fluids
laden with artificial self-propelled colloids.Comment: submitted to J Fluid Mec
Universal low-temperature tricritical point in metallic ferromagnets and ferrimagnets
An earlier theory of the quantum phase transition in metallic ferromagnets is
revisited and generalized in three ways. It is shown that the mechanism that
leads to a fluctuation-induced first-order transition in metallic ferromagnets
with a low Curie temperature is valid, (1) irrespective of whether the magnetic
moments are supplied by the conduction electrons or by electrons in another
band, (2) for ferromagnets in the XY and Ising universality classes as well as
for Heisenberg ferromagnets, and (3) for ferrimagnets as well as for
ferromagnets. This vastly expands the class of materials for which a
first-order transition at low temperatures is expected, and it explains why
strongly anisotropic ferromagnets, such as UGe2, display a first-order
transition as well as Heisenberg magnets.Comment: 11pp, 2 fig
Metastable tight knots in a worm-like polymer
Based on an estimate of the knot entropy of a worm-like chain we predict that
the interplay of bending energy and confinement entropy will result in a
compact metastable configuration of the knot that will diffuse, without
spreading, along the contour of the semi-flexible polymer until it reaches one
of the chain ends. Our estimate of the size of the knot as a function of its
topological invariant (ideal aspect ratio) agrees with recent experimental
results of knotted dsDNA. Further experimental tests of our ideas are proposed.Comment: 4 pages, 3 figure
Columnar Fluctuations as a Source of Non-Fermi-Liquid Behavior in Weak Metallic Magnets
It is shown that columnar fluctuations, in conjunction with weak quenched
disorder, lead to a T^{3/2} temperature dependence of the electrical
resistivity. This is proposed as an explanation of the observed
non-Fermi-liquid behavior in the helimagnet MnSi, with one possible realization
of the columnar fluctuations provided by skyrmion lines that have independently
been proposed to be present in this material.Comment: 4pp, 4 figure
Markov Chain Modeling of Polymer Translocation Through Pores
We solve the Chapman-Kolmogorov equation and study the exact splitting
probabilities of the general stochastic process which describes polymer
translocation through membrane pores within the broad class of Markov chains.
Transition probabilities which satisfy a specific balance constraint provide a
refinement of the Chuang-Kantor-Kardar relaxation picture of translocation,
allowing us to investigate finite size effects in the evaluation of dynamical
scaling exponents. We find that (i) previous Langevin simulation results can be
recovered only if corrections to the polymer mobility exponent are taken into
account and that (ii) the dynamical scaling exponents have a slow approach to
their predicted asymptotic values as the polymer's length increases. We also
address, along with strong support from additional numerical simulations, a
critical discussion which points in a clear way the viability of the Markov
chain approach put forward in this work.Comment: 17 pages, 5 figure
Nematic cells with defect-patterned alignment layers
Using Monte Carlo simulations of the Lebwohl--Lasher model we study the
director ordering in a nematic cell where the top and bottom surfaces are
patterned with a lattice of point topological defects of lattice
spacing . We find that the nematic order depends crucially on the ratio of
the height of the cell to . When the system is very
well--ordered and the frustration induced by the lattice of defects is relieved
by a network of half--integer defect lines which emerge from the point defects
and hug the top and bottom surfaces of the cell. When the
system is disordered and the half--integer defect lines thread through the cell
joining point defects on the top and bottom surfaces. We present a simple
physical argument in terms of the length of the defect lines to explain these
results. To facilitate eventual comparison with experimental systems we also
simulate optical textures and study the switching behavior in the presence of
an electric field
Non-ideal behavior of intramolecular structure factor of dilute polymers in a theta solvent
We study the configurational properties of single polymers in a theta solvent
by Monte Carlo simulation of the bond fluctuation model. The intramolecular
structure factor at the theta point is found to be distinctively different from
that of the ideal chain. The structure factor shows a hump around
and a dip around in the Kratky plot with being the radius
of gyration. This feature is apparently similar to that in a melt. The
theoretical expression by the simple perturbation expansion to the first order
in terms of the Mayer function can be fitted to the obtained structure factor
quite well, but the second virial coefficient cannot be set to zero.Comment: 8 pages, 7figure
Dynamic charge density correlation function in weakly charged polyampholyte globules
We study solutions of statistically neutral polyampholyte chains containing a
large fraction of neutral monomers. It is known that, even if the quality of
the solvent with respect to the neutral monomers is good, a long chain will
collapse into a globule. For weakly charged chains, the interior of this
globule is semi-dilute. This paper considers mainly theta-solvents, and we
calculate the dynamic charge density correlation function g(k,t) in the
interior of the globules, using the quadratic approximation to the
Martin-Siggia-Rose generating functional. It is convenient to express the
results in terms of dimensionless space and time variables. Let R be the blob
size, and let T be the characteristic time scale at the blob level. Define the
dimensionless wave vector q = R k, and the dimensionless time s = t/T. We find
that for q<1, corresponding to length scales larger than the blob size, the
charge density fluctuations relax according to g(q,s) = q^2(1-s^(1/2)) at short
times s < 1, and according to g(q,s) = q^2 s^(-1/2) at intermediate times 1 < s
0.1, where
entanglements are unimportant.Comment: 12 pages RevTex, 1 figure ps, PACS 61.25.Hq, reason replacement:
Expression for dynamic corr. function g(k,t) in old version was incorrect
(though expression for Fourier transform g(k,w) was correct, so the major
part of the calculation remains.) Also major textual chang
Suppression of Spontaneous Supercurrents in a Chiral p-Wave Superconductor
The superconducting state of SRO is widely believed to have chiral p-wave
order that breaks time reversal symmetry. Such a state is expected to have a
spontaneous magnetization, both at sample edges and at domain walls between
regions of different chirality. Indeed, muon spin resonance experiments are
interpreted as evidence of spontaneous magnetization due to domain walls or
defects in the bulk. However, recent magnetic microscopy experiments place
upper limits on the magentic fields at the sample edge and surface which are as
much as two orders of magnitude smaller than the fields predicted theoretically
for a somewhat idealized chiral p-wave superconductor. We investigate the
effects on the spontaneous supercurrents and magnetization of rough and pair
breaking surfaces for a range of parameters within a Ginzburg-Landau formalism.
The effects of competing orders nucleated at the surface are also considered.
We find the conditions under which the edge currents are significantly reduced
while leaving the bulk domain wall currents intact, are quite limited. The
implications for interpreting the existing body of experimental results on
superconducting SRO within a chiral p-wave model are discussed.Comment: Changes to section 3, typos remove
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