7,054 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
Space-time velocity correlation function for random walks
Space-time correlation functions constitute a useful instrument from the
research toolkit of continuous-media and many-body physics. We adopt here this
concept for single-particle random walks and demonstrate that the corresponding
space-time velocity auto-correlation functions reveal correlations which extend
in time much longer than estimated with the commonly employed temporal
correlation functions. A generic feature of considered random-walk processes is
an effect of velocity echo identified by the existence of time-dependent
regions where most of the walkers are moving in the direction opposite to their
initial motion. We discuss the relevance of the space-time velocity correlation
functions for the experimental studies of cold atom dynamics in an optical
potential and charge transport on micro- and nano-scales.Comment: Phys. Rev. Lett., in pres
Levy walks with velocity fluctuations
The standard Levy walk is performed by a particle that moves ballistically
between randomly occurring collisions, when the intercollision time is a random
variable governed by a power-law distribution. During instantaneous collision
events the particle randomly changes the direction of motion but maintains the
same constant speed. We generalize the standard model to incorporate velocity
fluctuations into the process. Two types of models are considered, namely, (i)
with a walker changing the direction and absolute value of its velocity during
collisions only, and (ii) with a walker whose velocity continuously fluctuates.
We present full analytic evaluation of both models and emphasize the importance
of initial conditions. We show that the type of the underlying Levy walk
process can be identified by looking at the ballistic regions of the diffusion
profiles. Our analytical results are corroborated by numerical simulations
ac-driven atomic quantum motor
We invent an ac-driven quantum motor consisting of two different, interacting
ultracold atoms placed into a ring-shaped optical lattice and submerged in a
pulsating magnetic field. While the first atom carries a current, the second
one serves as a quantum starter. For fixed zero-momentum initial conditions the
asymptotic carrier velocity converges to a unique non-zero value. We also
demonstrate that this quantum motor performs work against a constant load.Comment: 4 pages, 4 figure
Nuclear collective motion with a coherent coupling interaction between quadrupole and octupole modes
A collective Hamiltonian for the rotation-vibration motion of nuclei is
considered, in which the axial quadrupole and octupole degrees of freedom are
coupled through the centrifugal interaction. The potential of the system
depends on the two deformation variables and . The system is
considered to oscillate between positive and negative -values, by
rounding an infinite potential core in the -plane with
. By assuming a coherent contribution of the quadrupole and octupole
oscillation modes in the collective motion, the energy spectrum is derived in
an explicit analytic form, providing specific parity shift effects. On this
basis several possible ways in the evolution of quadrupole-octupole
collectivity are outlined. A particular application of the model to the energy
levels and electric transition probabilities in alternating parity spectra of
the nuclei Nd, Sm, Gd and Dy is presented.Comment: 25 pages, 13 figures. Accepted in Phys. Rev.
Large-scale Ferrofluid Simulations on Graphics Processing Units
We present an approach to molecular-dynamics simulations of ferrofluids on
graphics processing units (GPUs). Our numerical scheme is based on a
GPU-oriented modification of the Barnes-Hut (BH) algorithm designed to increase
the parallelism of computations. For an ensemble consisting of one million of
ferromagnetic particles, the performance of the proposed algorithm on a Tesla
M2050 GPU demonstrated a computational-time speed-up of four order of magnitude
compared to the performance of the sequential All-Pairs (AP) algorithm on a
single-core CPU, and two order of magnitude compared to the performance of the
optimized AP algorithm on the GPU. The accuracy of the scheme is corroborated
by comparing the results of numerical simulations with theoretical predictions
Mapping the Arnold web with a GPU-supercomputer
The Arnold diffusion constitutes a dynamical phenomenon which may occur in
the phase space of a non-integrable Hamiltonian system whenever the number of
the system degrees of freedom is . The diffusion is mediated by a
web-like structure of resonance channels, which penetrates the phase space and
allows the system to explore the whole energy shell. The Arnold diffusion is a
slow process; consequently the mapping of the web presents a very
time-consuming task. We demonstrate that the exploration of the Arnold web by
use of a graphic processing unit (GPU)-supercomputer can result in distinct
speedups of two orders of magnitude as compared to standard CPU-based
simulations.Comment: 7 pages, 4 figures, a video supplementary provided at
http://www.physik.uni-augsburg.de/~seiberar/arnold/Energy15_HD_frontNback.av
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