602 research outputs found
Ring polymers in melts and solutions: scaling and crossover
We propose a simple mean-field theory for the structure of ring polymer
melts. By combining the notion of topological volume fraction and a classical
van der Waals theory of fluids, we take into account many body effects of
topological origin in dense systems. We predict that although the compact
statistics with the Flory exponent is realized for very long chains,
most practical cases fall into the crossover regime with the apparent exponent
during which the system evolves toward a topological dense-packed
limit.Comment: 4 pages, 3 figure
Transient-linking activity enhances subnuclear dynamics by affecting chromatin remodeling
Spatiotemporal coordination of chromatin and subnuclear compartments is
crucial for cells. A plethora of enzymes act inside nucleus, and some of those
transiently link two chromatin segments. Here, we theoretically study how such
transient-linking activities affect fluctuating dynamics of an inclusion in the
chromatic medium. Numerical simulations and a coarse-grained model analysis
categorize inclusion dynamics into three distinct modes. The transient-linking
activity speeds up the inclusion dynamics by affecting a slow mode associated
with chromatin remodeling, viz., size and shape of the chromatin meshes
Semiflexible polymer conformation, distribution and migration in microcapillary flows
The flow behavior of a semiflexible polymer in microchannels is studied using
Multiparticle Collision Dynamics (MPC), a particle-based hydrodynamic
simulation technique. Conformations, distributions, and radial cross-streamline
migration are investigated for various bending rigidities, with persistence
lengths Lp in the range 0.5 < Lp/Lr < 30. The flow behavior is governed by the
competition between a hydrodynamic lift force and steric wall-repulsion, which
lead to migration away from the wall, and a locally varying flow-induced
orientation, which drives polymer away from the channel center and towards the
wall. The different dependencies of these effects on the polymer bending
rigidity and the flow velocity results in a complex dynamical behavior.
However, a generic effect is the appearance of a maximum in the monomer and the
center-of-mass distributions, which occurs in the channel center for small flow
velocities, but moves off-center at higher velocities.Comment: in press at J. Phys. Condens. Matte
Photon generation by laser-Compton scattering at the KEK-ATF
We performed a photon generation experiment by laser-Compton scattering at
the KEK-ATF, aiming to develop a Compton based polarized positron source for
linear colliders. In the experiment, laser pulses with a 357 MHz repetition
rate were accumulated and their power was enhanced by up to 250 times in the
Fabry-Perot optical resonant cavity. We succeeded in synchronizing the laser
pulses and colliding them with the 1.3 GeV electron beam in the ATF ring while
maintaining the laser pulse accumulation in the cavity. As a result, we
observed 26.0 +/- 0.1 photons per electron-laser pulse crossing, which
corresponds to a yield of 10^8 photons in a second.Comment: 3 pages, 5 figures, Preprint submitted to TIPP09 Proceedings in NIM
Dragging a polymer chain into a nanotube and subsequent release
We present a scaling theory and Monte Carlo (MC) simulation results for a
flexible polymer chain slowly dragged by one end into a nanotube. We also
describe the situation when the completely confined chain is released and
gradually leaves the tube. MC simulations were performed for a self-avoiding
lattice model with a biased chain growth algorithm, the pruned-enriched
Rosenbluth method. The nanotube is a long channel opened at one end and its
diameter is much smaller than the size of the polymer coil in solution. We
analyze the following characteristics as functions of the chain end position
inside the tube: the free energy of confinement, the average end-to-end
distance, the average number of imprisoned monomers, and the average stretching
of the confined part of the chain for various values of and for the number
of monomers in the chain, . We show that when the chain end is dragged by a
certain critical distance into the tube, the polymer undergoes a
first-order phase transition whereby the remaining free tail is abruptly sucked
into the tube. This is accompanied by jumps in the average size, the number of
imprisoned segments, and in the average stretching parameter. The critical
distance scales as . The transition takes place when
approximately 3/4 of the chain units are dragged into the tube. The theory
presented is based on constructing the Landau free energy as a function of an
order parameter that provides a complete description of equilibrium and
metastable states. We argue that if the trapped chain is released with all
monomers allowed to fluctuate, the reverse process in which the chain leaves
the confinement occurs smoothly without any jumps. Finally, we apply the theory
to estimate the lifetime of confined DNA in metastable states in nanotubes.Comment: 13pages, 14figure
Dynamics of Excited Electrons in Copper: Role of Auger Electrons
Within a theoretical model based on the Boltzmann equation, we analyze in
detail the structure of the unusual peak recently observed in the relaxation
time in Cu. In particular, we discuss the role of Auger electrons in the
electron dynamics and its dependence on the d-hole lifetime, the optical
transition matrix elements and the laser pulse duration. We find that the Auger
contribution to the distribution is very sensitive to both the d-hole lifetime
tau_h and the laser pulse duration tau_l and can be expressed as a monotonic
function of tau_l/tau_h. We have found that for a given tau_h, the Auger
contribution is significantly smaller for a short pulse duration than for a
longer one. We show that the relaxation time at the peak depends linearly on
the d-hole lifetime, but interestingly not on the amount of Auger electrons
generated. We provide a simple expression for the relaxation time of excited
electrons which shows that its shape can be understood by a phase space
argument and its amplitude is governed by the d-hole lifetime. We also find
that the height of the peak depends on both the ratio of the optical transition
matrix elements R=|M_{d \to sp}|^2/|M_{sp \to sp}|^2 and the laser pulse
duration. Assuming a reasonable value for the ratio, namely R = 2, and a d-hole
lifetime of tau_h=35 fs, we obtain for the calculated height of the peak Delta
tau_{th}=14 fs, in fair agreement with Delta tau_{exp} \approx 17 fs measured
for polycrystalline Cu.Comment: 6 pages, 6 figure
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