966 research outputs found
Magnetization precession due to a spin polarized current in a thin nanoelement: numerical simulation study
In this paper a detailed numerical study (in frames of the Slonczewski
formalism) of magnetization oscillations driven by a spin-polarized current
through a thin elliptical nanoelement is presented. We show that a
sophisticated micromagnetic model, where a polycrystalline structure of a
nanoelement is taken into account, can explain qualitatively all most important
features of the magnetization oscillation spectra recently observed
experimentally (S.I. Kiselev et al., Nature, vol. 425, p. 380 (2003), namely:
existence of several equidistant spectral bands, sharp onset and abrupt
disappearance of magnetization oscillations with increasing current, absence of
the out-of-plane regime predicted by a macrospin model and the relation between
frequencies of so called small-angle and quasichaotic oscillations. However, a
quantitative agreement with experimental results (especially concerning the
frequency of quasichaotic oscillations) could not be achieved in the region of
reasonable parameter values, indicating that further model refinement is
necessary for a complete understanding of the spin-driven magnetization
precession even in this relatively simple experimental situation.Comment: Submitted to Phys. Rev. B; In this revised version figure positions
on the page have been changed to ensure correct placements of the figure
caption
Destabilizing Taylor-Couette flow with suction
We consider the effect of radial fluid injection and suction on
Taylor-Couette flow. Injection at the outer cylinder and suction at the inner
cylinder generally results in a linearly unstable steady spiralling flow, even
for cylindrical shears that are linearly stable in the absence of a radial
flux. We study nonlinear aspects of the unstable motions with the energy
stability method. Our results, though specialized, may have implications for
drag reduction by suction, accretion in astrophysical disks, and perhaps even
in the flow in the earth's polar vortex.Comment: 34 pages, 9 figure
Subdiffusion-limited reactions
We consider the coagulation dynamics A+A -> A and A+A A and the
annihilation dynamics A+A -> 0 for particles moving subdiffusively in one
dimension. This scenario combines the "anomalous kinetics" and "anomalous
diffusion" problems, each of which leads to interesting dynamics separately and
to even more interesting dynamics in combination. Our analysis is based on the
fractional diffusion equation
Alternating Kinetics of Annihilating Random Walks Near a Free Interface
The kinetics of annihilating random walks in one dimension, with the
half-line x>0 initially filled, is investigated. The survival probability of
the nth particle from the interface exhibits power-law decay,
S_n(t)~t^{-alpha_n}, with alpha_n approximately equal to 0.225 for n=1 and all
odd values of n; for all n even, a faster decay with alpha_n approximately
equal to 0.865 is observed. From consideration of the eventual survival
probability in a finite cluster of particles, the rigorous bound alpha_1<1/4 is
derived, while a heuristic argument gives alpha_1 approximately equal to 3
sqrt{3}/8 = 0.2067.... Numerically, this latter value appears to be a stringent
lower bound for alpha_1. The average position of the first particle moves to
the right approximately as 1.7 t^{1/2}, with a relatively sharp and asymmetric
probability distribution.Comment: 6 pages, RevTeX, 5 eps figures include
Transition Phenomena Induced by Internal Noise and Quasi-absorbing State
We study a simple chemical reaction system and effects of the internal noise.
The chemical reaction system causes the same transition phenomenon discussed by
Togashi and Kaneko [Phys. Rev. Lett. 86 (2001) 2459; J. Phys. Soc. Jpn. 72
(2003) 62]. By using the simpler model than Togashi-Kaneko's one, we discuss
the transition phenomenon by means of a random walk model and an effective
model. The discussion makes it clear that quasi-absorbing states, which are
produced by the change of the strength of the internal noise, play an important
role in the transition phenomenon. Stabilizing the quasi-absorbing states
causes bifurcation of the peaks in the stationary probability distribution
discontinuously.Comment: 6 pages, 5 figure
A Method of Intervals for the Study of Diffusion-Limited Annihilation, A + A --> 0
We introduce a method of intervals for the analysis of diffusion-limited
annihilation, A+A -> 0, on the line. The method leads to manageable diffusion
equations whose interpretation is intuitively clear. As an example, we treat
the following cases: (a) annihilation in the infinite line and in infinite
(discrete) chains; (b) annihilation with input of single particles, adjacent
particle pairs, and particle pairs separated by a given distance; (c)
annihilation, A+A -> 0, along with the birth reaction A -> 3A, on finite rings,
with and without diffusion.Comment: RevTeX, 13 pages, 4 figures, 1 table. References Added, and some
other minor changes, to conform with final for
General Reaction-Diffusion Processes With Separable Equations for Correlation Functions
We consider general multi-species models of reaction diffusion processes and
obtain a set of constraints on the rates which give rise to closed systems of
equations for correlation functions. Our results are valid in any dimension and
on any type of lattice. We also show that under these conditions the evolution
equations for two point functions at different times are also closed. As an
example we introduce a class of two species models which may be useful for the
description of voting processes or the spreading of epidemics.Comment: 17 pages, Latex, No figure
Noise delayed decay of unstable states: theory versus numerical simulations
We study the noise delayed decay of unstable nonequilibrium states in
nonlinear dynamical systems within the framework of the overdamped Brownian
motion model. We give the exact expressions for the decay times of unstable
states for polynomial potential profiles and obtain nonmonotonic behavior of
the decay times as a function of the noise intensity for the unstable
nonequilibrium states. The analytical results are compared with numerical
simulations.Comment: 9 pages, 6 figures, in press in J. Phys.
Quantum decay rates for driven barrier potentials in the strong friction limit
Quantum decay rates for barrier potentials driven by external stochastic and
periodic forces in the strong damping regime are studied. Based on the recently
derived quantum Smoluchowski equation [Phys. Rev. Lett. {\bf 87}, 086802
(2001)] explicit analytical and numerical results are presented for the case of
the resonant activation phenomenon in a bistable potential and the escape from
a metastablwell with oscillating barrier, respectively. The significant impact
of quantum fluctuations is revealed.Comment: Rapid Communication, Phys. Rev. E, in pres
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