944 research outputs found
How dsDNA breathing enhances its flexibility and instability on short length scales
We study the unexpected high flexibility of short dsDNA which recently has
been reported by a number of experiments. Via the Langevin dynamics simulation
of our Breathing DNA model, first we observe the formation of bubbles within
the duplex and also forks at the ends, with the size distributions independent
of the contour length. We find that these local denaturations at a
physiological temperature, despite their rare and transient presence, can lower
the persistence length drastically for a short DNA segment in agreement with
experiment
INCORPORATION OF QUANTUM STATISTICAL FEATURES IN MOLECULAR DYNAMICS
We formulate a method for incorporating quantum fluctuations into molecular-
dynamics simulations of many-body systems, such as those employed for energetic
nuclear collision processes. Based on Fermi's Golden Rule, we allow spontaneous
transitions to occur between the wave packets which are not energy eigenstates.
The ensuing diffusive evolution in the space of the wave packet parameters
exhibits appealing physical properties, including relaxation towards quantum-
statistical equilibrium.Comment: 8 latex pages + 1 uuencoded ps figur
Surface Polymer Network Model and Effective Membrane Curvature Elasticity
A microscopic model of a surface polymer network - membrane system is
introduced, with contact polymer surface interactions that can be either
repulsive or attractive and sliplinks of functionality four randomly
distributed over the supporting membrane surface anchoring the polymers to it.
For the supporting surface perturbed from a planar configuration and a small
relative number of surface sliplinks, we investigate an expansion of the free
energy in terms of the local curvatures of the surface and the surface density
of sliplinks, obtained through the application of the Balian - Bloch -
Duplantier multiple surface scattering method. As a result, the dependence of
the curvature elastic modulus, the Gaussian modulus as well as of the
spontaneous curvature of the "dressed" membrane, ~{\sl i.e.} polymer network
plus membrane matrix, is obtained on the mean polymer bulk end to end
separation and the surface density of sliplinks.Comment: 15 pages with one included compressed uuencoded figure
Effect of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei
Using an isospin-dependent transport model, we study the effects of nuclear
symmetry energy on two-nucleon correlation functions in heavy ion collisions
induced by neutron-rich nuclei. We find that the density dependence of the
nuclear symmetry energy affects significantly the nucleon emission times in
these collisions, leading to larger values of two-nucleon correlation functions
for a symmetry energy that has a stronger density dependence. Two-nucleon
correlation functions are thus useful tools for extracting information about
the nuclear symmetry energy from heavy ion collisions.Comment: Revised version, to appear in Phys. Rev. Let
Intermittency and Exotic Channels
It is pointed out that accurate measurements of short-range two-particle
correlations in like-charge and in channels should be
very helpful in determining the origin of the \lq\lq intermittency\rq\rq\
phenomenon observed recently for the like-charge pion pairs.Comment: 5 p., plain tex, preprint T94/078(Saclay), LPTHE 94/58(Orsay
Molecular Model of the Contractile Ring
We present a model for the actin contractile ring of adherent animal cells.
The model suggests that the actin concentration within the ring and
consequently the power that the ring exerts both increase during contraction.
We demonstrate the crucial role of actin polymerization and depolymerization
throughout cytokinesis, and the dominance of viscous dissipation in the
dynamics. The physical origin of two phases in cytokinesis dynamics ("biphasic
cytokinesis") follows from a limitation on the actin density. The model is
consistent with a wide range of measurements of the midzone of dividing animal
cells.Comment: PACS numbers: 87.16.Ka, 87.16.Ac
http://www.ncbi.nlm.nih.gov/pubmed/16197254
http://www.weizmann.ac.il/complex/tlusty/papers/PhysRevLett2005.pd
The Interplay of Nonlinearity and Architecture in Equilibrium Cytoskeletal Mechanics
The interplay between cytoskeletal architecture and the nonlinearity of the
interactions due to bucklable filaments plays a key role in modulating the
cell's mechanical stability and affecting its structural rearrangements. We
study a model of cytoskeletal structure treating it as an amorphous network of
hard centers rigidly cross-linked by nonlinear elastic strings, neglecting the
effects of motorization. Using simulations along with a self-consistent phonon
method, we show that this minimal model exhibits diverse thermodynamically
stable mechanical phases that depend on excluded volume, crosslink
concentration, filament length and stiffness. Within the framework set by the
free energy functional formulation and making use of the random first order
transition theory of structural glasses, we further estimate the characteristic
densities for a kinetic glass transition to occur in this model system. Network
connectivity strongly modulates the transition boundaries between various
equilibrium phases, as well as the kinetic glass transition density.Comment: 17 pages, 18 figure
Phase Transitions in Warm, Asymmetric Nuclear Matter
A relativistic mean-field model of nuclear matter with arbitrary proton
fraction is studied at finite temperature. An analysis is performed of the
liquid-gas phase transition in a system with two conserved charges (baryon
number and isospin) using the stability conditions on the free energy, the
conservation laws, and Gibbs' criteria for phase equilibrium. For a binary
system with two phases, the coexistence surface (binodal) is two-dimensional.
The Maxwell construction through the phase-separation region is discussed, and
it is shown that the stable configuration can be determined uniquely at every
density. Moreover, because of the greater dimensionality of the binodal
surface, the liquid-gas phase transition is continuous (second order by
Ehrenfest's definition), rather than discontinuous (first order), as in
familiar one-component systems. Using a mean-field equation of state calibrated
to the properties of nuclear matter and finite nuclei, various phase-separation
scenarios are considered. The model is then applied to the liquid-gas phase
transition that may occur in the warm, dilute matter produced in energetic
heavy-ion collisions. In asymmetric matter, instabilities that produce a
liquid-gas phase separation arise from fluctuations in the proton concentration
(chemical instability), rather than from fluctuations in the baryon density
(mechanical instability).Comment: Postscript file, 50 pages including 23 figure
An Improved Quantum Molecular Dynamics Model and its Applications to Fusion Reaction near Barrier
An improved Quantum Molecular Dynamics model is proposed. By using this
model, the properties of ground state of nuclei from Li to Pb can
be described very well with one set of parameters. The fusion reactions for
Ca+Zr, Ca+Zr and Ca+Zr at energy near
barrier are studied by this model. The experimental data of the fusion cross
sections for Ca+Zr at the energy near barrier can be
reproduced remarkably well without introducing any new parameters. The
mechanism for the enhancement of fusion probability for fusion reactions with
neutron-rich projectile or target is analyzed.Comment: 20 pages, 12 figures, 3 table
Bose-Einstein Correlations of Pion Wavepackets
A wavepacket model for a system of free pions, which takes into account the
full permutation symmetry of the wavefunction and which is suitable for any
phase space parametrization is developed. The properties of the resulting mixed
ensembles and the two-particle correlation function are discussed. A physical
interpretation of the chaoticity lambda as localizat of the pions in the source
is presented.
Two techniques to generate test-particles, which satisfy the probability
densities of the wavepacket state, are studied:
1. A Monte Carlo procedure in momentum space based on the standard Metropolis
technique.
2. A molecular dynamic procedure using Bohm's quantum theory of motion.
In order to reduce the numerical complexity, the separation of the
wavefunction into momentum space clusters is discussed. In this context th
influence of an unauthorized factorization of the state, i. e. the omissio of
interference terms, is investigated. It is shown that the correlation radius
remains almost uneffected, but the chaoticity parameter decreases
substantially. A similar effect is observed in systems with high multiplic
where the omission of higher order corrections in the analysis of two-part
correlations causes a reduction of the chaoticity and the radius.
The approximative treatment of the Coulomb interaction between pions and
source is investigated. The results suggest that Coulomb effects on the co
radii are not symmetric for pion pairs of different charges. For negative the
radius, integrated over the whole momentum spectrum, increases substan while
for positive pions the radius remains almost unchanged.Comment: 15 pages, 8 figures, 0.8 Mb, uses ljour2-macro, Submitted to Z. Phys.
A (1997
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