7,916 research outputs found
Nuclear reactions in hot stellar matter and nuclear surface deformation
Cross-sections for capture reactions of charged particles in hot stellar
matter turn out be increased by the quadrupole surface oscillations, if the
corresponding phonon energies are of the order of the star temperature. The
increase is studied in a model that combines barrier distribution induced by
surface oscillations and tunneling. The capture of charged particles by nuclei
with well-deformed ground-state is enhanced in stellar matter. It is found that
the influence of quadrupole surface deformation on the nuclear reactions in
stars grows, when mass and proton numbers in colliding nuclei increase.Comment: 12 pages, 10 figure
ac-driven Brownian motors: a Fokker-Planck treatment
We consider a primary model of ac-driven Brownian motors, i.e., a classical
particle placed in a spatial-time periodic potential and coupled to a heat
bath. The effects of fluctuations and dissipations are studied by a
time-dependent Fokker-Planck equation. The approach allows us to map the
original stochastic problem onto a system of ordinary linear algebraic
equations. The solution of the system provides complete information about
ratchet transport, avoiding such disadvantages of direct stochastic
calculations as long transients and large statistical fluctuations. The
Fokker-Planck approach to dynamical ratchets is instructive and opens the space
for further generalizations
Dispersion of particles in an infinite-horizon Lorentz gas
We consider a two-dimensional Lorentz gas with infinite horizon. This
paradigmatic model consists of pointlike particles undergoing elastic
collisions with fixed scatterers arranged on a periodic lattice. It was
rigorously shown that when , the distribution of particles is
Gaussian. However, the convergence to this limit is ultraslow, hence it is
practically unattainable. Here we obtain an analytical solution for the Lorentz
gas' kinetics on physically relevant timescales, and find that the density in
its far tails decays as a universal power law of exponent . We also show
that the arrangement of scatterers is imprinted in the shape of the
distribution.Comment: Article with supplemental material: 10 pages, 4 figure
Biased diffusion in a piecewise linear random potential
We study the biased diffusion of particles moving in one direction under the
action of a constant force in the presence of a piecewise linear random
potential. Using the overdamped equation of motion, we represent the first and
second moments of the particle position as inverse Laplace transforms. By
applying to these transforms the ordinary and the modified Tauberian theorem,
we determine the short- and long-time behavior of the mean-square displacement
of particles. Our results show that while at short times the biased diffusion
is always ballistic, at long times it can be either normal or anomalous. We
formulate the conditions for normal and anomalous behavior and derive the laws
of biased diffusion in both these cases.Comment: 11 pages, 3 figure
Magnetic relaxation in finite two-dimensional nanoparticle ensembles
We study the slow phase of thermally activated magnetic relaxation in finite
two-dimensional ensembles of dipolar interacting ferromagnetic nanoparticles
whose easy axes of magnetization are perpendicular to the distribution plane.
We develop a method to numerically simulate the magnetic relaxation for the
case that the smallest heights of the potential barriers between the
equilibrium directions of the nanoparticle magnetic moments are much larger
than the thermal energy. Within this framework, we analyze in detail the role
that the correlations of the nanoparticle magnetic moments and the finite size
of the nanoparticle ensemble play in magnetic relaxation.Comment: 21 pages, 4 figure
Energy diffusion in hard-point systems
We investigate the diffusive properties of energy fluctuations in a
one-dimensional diatomic chain of hard-point particles interacting through a
square--well potential. The evolution of initially localized infinitesimal and
finite perturbations is numerically investigated for different density values.
All cases belong to the same universality class which can be also interpreted
as a Levy walk of the energy with scaling exponent 3/5. The zero-pressure limit
is nevertheless exceptional in that normal diffusion is found in tangent space
and yet anomalous diffusion with a different rate for perturbations of finite
amplitude. The different behaviour of the two classes of perturbations is
traced back to the "stable chaos" type of dynamics exhibited by this model.
Finally, the effect of an additional internal degree of freedom is
investigated, finding that it does not modify the overall scenarioComment: 16 pages, 15 figure
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