546 research outputs found
Test of J-matrix inverse scattering potentials on electromagnetic reactions of few-nucleon systems
The J-matrix inverse scattering nucleon-nucleon potentials (JISP), describing
both two-nucleon data and bound and resonant states of light nuclei to high
accuracy, are tested on the total photoabsorption cross sections of Deuteron,
Triton, 3He and 4He. The calculations in the three- and four-body systems are
carried out via the Lorentz integral transform method and the hyperspherical
harmonics (HH) technique. To this end the HH formalism has been adapted to
accommodate non-local potentials. The cross sections calculated with the JISP
are compared to those obtained with more traditional realistic interactions,
which include two- and three-nucleon forces. While the results of the two kinds
of potential models do not differ significantly at lower energies, beyond the
resonance peak they show fairly large discrepancies, which increase with the
nuclear mass. We argue that these discrepancies may be due to a probably
incorrect long range behavior of the JISP, since the one pion exchange is not
manifestly implemented there.Comment: 18 pages, 4 figures, 1 tabl
Lattice Boltzmann simulations of lamellar and droplet phases
Lattice Boltzmann simulations are used to investigate spinodal decomposition
in a two-dimensional binary fluid with equilibrium lamellar and droplet phases.
We emphasise the importance of hydrodynamic flow to the phase separation
kinetics. For mixtures slightly asymmetric in composition the fluid phase
separates into bulk and lamellar phases with the lamellae forming distinctive
spiral structures to minimise their elastic energy.Comment: 19 pages, 5 figure
Incomplete equilibrium in long-range interacting systems
We use a Hamiltonian dynamics to discuss the statistical mechanics of
long-lasting quasi-stationary states particularly relevant for long-range
interacting systems. Despite the presence of an anomalous single-particle
velocity distribution, we find that the Central Limit Theorem implies the
Boltzmann expression in Gibbs' -space. We identify the nonequilibrium
sub-manifold of -space characterizing the anomalous behavior and show
that by restricting the Boltzmann-Gibbs approach to this sub-manifold we obtain
the statistical mechanics of the quasi-stationary states.Comment: Title changed, throughout revision of the tex
Average Structures of a Single Knotted Ring Polymer
Two types of average structures of a single knotted ring polymer are studied
by Brownian dynamics simulations. For a ring polymer with N segments, its
structure is represented by a 3N -dimensional conformation vector consisting of
the Cartesian coordinates of the segment positions relative to the center of
mass of the ring polymer. The average structure is given by the average
conformation vector, which is self-consistently defined as the average of the
conformation vectors obtained from a simulation each of which is rotated to
minimize its distance from the average conformation vector. From each
conformation vector sampled in a simulation, 2N conformation vectors are
generated by changing the numbering of the segments. Among the 2N conformation
vectors, the one closest to the average conformation vector is used for one
type of the average structure. The other type of the averages structure uses
all the conformation vectors generated from those sampled in a simulation. In
thecase of the former average structure, the knotted part of the average
structure is delocalized for small N and becomes localized as N is increased.
In the case of the latter average structure, the average structure changes from
a double loop structure for small N to a single loop structure for large N,
which indicates the localization-delocalization transition of the knotted part.Comment: 15 pages, 19 figures, uses jpsj2.cl
Dynamical scaling of the DNA unzipping transition
We report studies of the equilibrium and the dynamics of a general set of
lattice models which capture the essence of the force-induced or mechanical DNA
unzipping transition. Besides yielding the whole equilibrium phase diagram in
the force vs temperature plane, which reveals the presence of an interesting
re-entrant unzipping transition for low T, these models enable us to
characterize the dynamics of the process starting from a non-equilibrium
initial condition. The thermal melting of the DNA strands displays a model
dependent time evolution. On the contrary, our results suggest that the
dynamical mechanism for the unzipping by force is very robust and the scaling
behaviour does not depend on the details of the description we adopt.Comment: 6 pages, 4 figures, A shorter version of this paper appeared in Phys.
Rev. Lett. 88, 028102 (2002
Steady-state hydrodynamic instabilities of active liquid crystals: Hybrid lattice Boltzmann simulations
We report hybrid lattice Boltzmann (HLB) simulations of the hydrodynamics of
an active nematic liquid crystal sandwiched between confining walls with
various anchoring conditions. We confirm the existence of a transition between
a passive phase and an active phase, in which there is spontaneous flow in the
steady state. This transition is attained for sufficiently ``extensile'' rods,
in the case of flow-aligning liquid crystals, and for sufficiently
``contractile'' ones for flow-tumbling materials. In a quasi-1D geometry, deep
in the active phase of flow-aligning materials, our simulations give evidence
of hysteresis and history-dependent steady states, as well as of spontaneous
banded flow. Flow-tumbling materials, in contrast, re-arrange themselves so
that only the two boundary layers flow in steady state. Two-dimensional
simulations, with periodic boundary conditions, show additional instabilities,
with the spontaneous flow appearing as patterns made up of ``convection
rolls''. These results demonstrate a remarkable richness (including dependence
on anchoring conditions) in the steady-state phase behaviour of active
materials, even in the absence of external forcing; they have no counterpart
for passive nematics. Our HLB methodology, which combines lattice Boltzmann for
momentum transport with a finite difference scheme for the order parameter
dynamics, offers a robust and efficient method for probing the complex
hydrodynamic behaviour of active nematics.Comment: 18 eps figures, accepted for publication in Phys. Rev.
Scaling in DNA unzipping models: denaturated loops and end-segments as branches of a block copolymer network
For a model of DNA denaturation, exponents describing the distributions of
denaturated loops and unzipped end-segments are determined by exact enumeration
and by Monte Carlo simulations in two and three dimensions. The loop
distributions are consistent with first order thermal denaturation in both
cases. Results for end-segments show a coexistence of two distinct power laws
in the relative distributions, which is not foreseen by a recent approach in
which DNA is treated as a homogeneous network of linear polymer segments. This
unexpected feature, and the discrepancies with such an approach, are explained
in terms of a refined scaling picture in which a precise distinction is made
between network branches representing single stranded and effective double
stranded segments.Comment: 8 pages, 8 figure
Topological Constraints at the Theta Point: Closed Loops at Two Loops
We map the problem of self-avoiding random walks in a Theta solvent with a
chemical potential for writhe to the three-dimensional symmetric
U(N)-Chern-Simons theory as N goes to 0. We find a new scaling regime of
topologically constrained polymers, with critical exponents that depend on the
chemical potential for writhe, which gives way to a fluctuation-induced
first-order transition.Comment: 5 pages, RevTeX, typo
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