2,577 research outputs found
Quantum critical behavior in heavily doped LaFeAsOH pnictide superconductors analyzed using nuclear magnetic resonance
We studied the quantum critical behavior of the second antiferromagnetic (AF)
phase in the heavily electron-doped high- pnictide, LaFeAsOH
by using As and H nuclear-magnetic-resonance (NMR) technique. In
the second AF phase, we observed a spatially modulated spin-density-wave-like
state up to =0.6 from the NMR spectral lineshape and detected a low-energy
excitation gap from the nuclear relaxation time of As. The
excitation gap closes at the AF quantum critical point (QCP) at . The superconducting (SC) phase in a lower-doping regime contacts the
second AF phase only at the AF QCP, and both phases are segregated from each
other. The absence of AF critical fluctuations and the enhancement of the
in-plane electric anisotropy are key factors for the development of
superconductivity.Comment: accepted in Phys. Rev.
Gaussian approximation for finitely extensible bead-spring chains with hydrodynamic interaction
The Gaussian Approximation, proposed originally by Ottinger [J. Chem. Phys.,
90 (1) : 463-473, 1989] to account for the influence of fluctuations in
hydrodynamic interactions in Rouse chains, is adapted here to derive a new
mean-field approximation for the FENE spring force. This "FENE-PG" force law
approximately accounts for spring-force fluctuations, which are neglected in
the widely used FENE-P approximation. The Gaussian Approximation for
hydrodynamic interactions is combined with the FENE-P and FENE-PG spring force
approximations to obtain approximate models for finitely-extensible bead-spring
chains with hydrodynamic interactions. The closed set of ODE's governing the
evolution of the second-moments of the configurational probability distribution
in the approximate models are used to generate predictions of rheological
properties in steady and unsteady shear and uniaxial extensional flows, which
are found to be in good agreement with the exact results obtained with Brownian
dynamics simulations. In particular, predictions of coil-stretch hysteresis are
in quantitative agreement with simulations' results. Additional simplifying
diagonalization-of-normal-modes assumptions are found to lead to considerable
savings in computation time, without significant loss in accuracy.Comment: 26 pages, 17 figures, 2 tables, 75 numbered equations, 1 appendix
with 10 numbered equations Submitted to J. Chem. Phys. on 6 February 200
A general theory of DNA-mediated and other valence-limited interactions
We present a general theory for predicting the interaction potentials between
DNA-coated colloids, and more broadly, any particles that interact via
valence-limited ligand-receptor binding. Our theory correctly incorporates the
configurational and combinatorial entropic factors that play a key role in
valence-limited interactions. By rigorously enforcing self-consistency, it
achieves near-quantitative accuracy with respect to detailed Monte Carlo
calculations. With suitable approximations and in particular geometries, our
theory reduces to previous successful treatments, which are now united in a
common and extensible framework. We expect our tools to be useful to other
researchers investigating ligand-mediated interactions. A complete and
well-documented Python implementation is freely available at
http://github.com/patvarilly/DNACC .Comment: 18 pages, 10 figure
Orientational order and glassy states in networks of semiflexible polymers
Motivated by the structure of networks of cross-linked cytoskeletal
biopolymers, we study the orientationally ordered phases in two-dimensional
networks of randomly cross-linked semiflexible polymers. We consider permanent
cross-links which prescribe a finite angle and treat them as quenched disorder
in a semi-microscopic replica field theory. Starting from a fluid of
un-cross-linked polymers and small polymer clusters (sol) and increasing the
cross-link density, a continuous gelation transition occurs. In the resulting
gel, the semiflexible chains either display long range orientational order or
are frozen in random directions depending on the value of the crossing angle,
the crosslink concentration and the stiffness of the polymers. A crossing angle
leads to long range -fold orientational order, e.g.,
"hexatic" or "tetratic" for or , respectively.
The transition is discontinuous and the critical cross-link density depends on
the bending stiffness of the polymers and the cross-link geometry: the higher
the stiffness and the lower , the lower the critical number of cross-links.
In between the sol and the long range ordered state, we always observe a gel
which is a statistically isotropic amorphous solid (SIAS) with random
positional and random orientational localization of the participating polymers.Comment: 20 pages, added references, minor changes, final version as published
in PR
Elasticity of Stiff Biopolymers
We present a statistical mechanical study of stiff polymers, motivated by
experiments on actin filaments and the considerable current interest in polymer
networks. We obtain simple, approximate analytical forms for the
force-extension relations and compare these with numerical treatments. We note
the important role of boundary conditions in determining force-extension
relations. The theoretical predictions presented here can be tested against
single molecule experiments on neurofilaments and cytoskeletal filaments like
actin and microtubules. Our work is motivated by the buckling of the
cytoskeleton of a cell under compression, a phenomenon of interest to biology.Comment: Submitted for publication, five pages, three figure
Structure and dynamics of colloidal depletion gels: coincidence of transitions and heterogeneity
Transitions in structural heterogeneity of colloidal depletion gels formed
through short-range attractive interactions are correlated with their dynamical
arrest. The system is a density and refractive index matched suspension of 0.20
volume fraction poly(methyl methacyrlate) colloids with the non-adsorbing
depletant polystyrene added at a size ratio of depletant to colloid of 0.043.
As the strength of the short-range attractive interaction is increased,
clusters become increasingly structurally heterogeneous, as characterized by
number-density fluctuations, and dynamically immobilized, as characterized by
the single-particle mean-squared displacement. The number of free colloids in
the suspension also progressively declines. As an immobile cluster to gel
transition is traversed, structural heterogeneity abruptly decreases.
Simultaneously, the mean single-particle dynamics saturates at a localization
length on the order of the short-range attractive potential range. Both
immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian
distributions of single particle displacements reveal enhanced populations of
dynamical trajectories localized on two different length scales. Similar
dependencies of number density fluctuations, free particle number and dynamical
length scales on the order of the range of short-range attraction suggests a
collective structural origin of dynamic heterogeneity in colloidal gels.Comment: 14 pages, 10 figure
Static and dynamic properties of large polymer melts in equilibrium
We present a detailed study of the static and dynamic behavior of long
semiflexible polymer chains in a melt. Starting from previously obtained fully
equilibrated high molecular weight polymer melts [{\it Zhang et al.} ACS Macro
Lett. 3, 198 (2014)] we investigate their static and dynamic scaling behavior
as predicted by theory. We find that for semiflexible chains in a melt, results
of the mean square internal distance, the probability distributions of the
end-to-end distance, and the chain structure factor are well described by
theoretical predictions for ideal chains. We examine the motion of monomers and
chains by molecular dynamics simulations using the ESPResSo++ package. The
scaling predictions of the mean squared displacement of inner monomers, center
of mass, and relations between them based on the Rouse and the reptation theory
are verified, and related characteristic relaxation times are determined.
Finally we give evidence that the entanglement length as determined
by a primitive path analysis (PPA) predicts a plateau modulus,
, consistent with stresses obtained from the
Green-Kubo relation. These comprehensively characterized equilibrium
structures, which offer a good compromise between flexibility, small ,
computational efficiency, and small deviations from ideality provide ideal
starting states for future non-equilibrium studies.Comment: 13 pages, 10 figures, to be published in J. Chem. Phys. (2016
Formation of helical states in wormlike polymer chains
We propose a potential for wormlike polymer chains which can be used to model
the low-temperature conformational structures. We successfully reproduced helix
ground states up to 6.5 helical loops, using the multicanonical Monte Carlo
simulation method. We demonstrate that the coil-helix transition involves four
distinct phases: coil(gaslike), collapsed globular(liquidlike), and two helical
phases I and II (both solidlike). The helix I phase is characterized by a
helical structure with dangling loose ends, and the helix II phase corresponds
to a near perfect helix ordering in the entire crystallized chain.Comment: 5 pages, 2 figures, Submitted to PR
The distribution function of a semiflexible polymer and random walks with constraints
In studying the end-to-end distribution function of a worm like
chain by using the propagator method we have established that the combinatorial
problem of counting the paths contributing to can be mapped onto the
problem of random walks with constraints, which is closely related to the
representation theory of the Temperley-Lieb algebra. By using this mapping we
derive an exact expression of the Fourier-Laplace transform of the distribution
function, , as a matrix element of an inverse of an infinite rank
matrix. Using this result we also derived a recursion relation permitting to
compute directly. We present the results of the computation of
and its moments. The moments of can be
calculated \emph{exactly} by calculating the (1,1) matrix element of -th
power of a truncated matrix of rank .Comment: 6 pages, 2 figures, added a referenc
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