6,189 research outputs found
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
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
The mean electromotive force due to turbulence of a conducting fluid in the presence of mean flow
The mean electromotive force caused by turbulence of an electrically
conducting fluid, which plays a central part in mean--field electrodynamics, is
calculated for a rotating fluid. Going beyond most of the investigations on
this topic, an additional mean motion in the rotating frame is taken into
account. One motivation for our investigation originates from a planned
laboratory experiment with a Ponomarenko-like dynamo. In view of this
application the second--order correlation approximation is used. The
investigation is of high interest in astrophysical context, too. Some
contributions to the mean electromotive are revealed which have not been
considered so far, in particular contributions to the --effect and
related effects due to the gradient of the mean velocity. Their relevance for
dynamo processes is discussed. In a forthcoming paper the results reported here
will be specified to the situation in the laboratory and partially compared
with experimental findings.Comment: 16 pages, 2 figures, in PRE pres
Dynamical and thermal effects in nanoparticle systems driven by a rotating magnetic field
We study dynamical and thermal effects that are induced in nanoparticle
systems by a rotating magnetic field. Using the deterministic Landau-Lifshitz
equation and appropriate rotating coordinate systems, we derive the equations
that characterize the steady-state precession of the nanoparticle magnetic
moments and study a stability criterion for this type of motion. On this basis,
we describe (i) the influence of the rotating field on the stability of the
small-angle precession, (ii) the dynamical magnetization of nanoparticle
systems, and (iii) the switching of the magnetic moments under the action of
the rotating field. Using the backward Fokker-Planck equation, which
corresponds to the stochastic Landau-Lifshitz equation, we develop a method for
calculating the mean residence times that the driven magnetic moments dwell in
the up and down states. Within this framework, the features of the induced
magnetization and magnetic relaxation are elucidated.Comment: 18 pages, 5 figure
Mean first-passage times for an ac-driven magnetic moment of a nanoparticle
The two-dimensional backward Fokker-Planck equation is used to calculate the
mean first-passage times (MFPTs) of the magnetic moment of a nanoparticle
driven by a rotating magnetic field. It is shown that a magnetic field that is
rapidly rotating in the plane {\it perpendicular} to the easy axis of the
nanoparticle governs the MFPTs just in the same way as a static magnetic field
that is applied {\it along} the easy axis. Within this framework, the features
of the magnetic relaxation and net magnetization of systems composed of
ferromagnetic nanoparticles arising from the action of the rotating field are
revealed.Comment: 7 pages, 1 figur
Rapidly driven nanoparticles: Mean first-passage times and relaxation of the magnetic moment
We present an analytical method of calculating the mean first-passage times
(MFPTs) for the magnetic moment of a uniaxial nanoparticle which is driven by a
rapidly rotating, circularly polarized magnetic field and interacts with a heat
bath. The method is based on the solution of the equation for the MFPT derived
from the two-dimensional backward Fokker-Planck equation in the rotating frame.
We solve these equations in the high-frequency limit and perform precise,
numerical simulations which verify the analytical findings. The results are
used for the description of the rates of escape from the metastable domains
which in turn determine the magnetic relaxation dynamics. A main finding is
that the presence of a rotating field can cause a drastic decrease of the
relaxation time and a strong magnetization of the nanoparticle system. The
resulting stationary magnetization along the direction of the easy axis is
compared with the mean magnetization following from the stationary solution of
the Fokker-Planck equation.Comment: 24 pages, 4 figure
Magnetic Properties of 2-Dimensional Dipolar Squares: Boundary Geometry Dependence
By means of the molecular dynamics simulation on gradual cooling processes,
we investigate magnetic properties of classical spin systems only with the
magnetic dipole-dipole interaction, which we call dipolar systems. Focusing on
their finite-size effect, particularly their boundary geometry dependence, we
study two finite dipolar squares cut out from a square lattice with
and , where is an angle between the direction of the lattice axis
and that of the square boundary. Distinctly different results are obtained in
the two dipolar squares. In the square, the ``from-edge-to-interior
freezing'' of spins is observed. Its ground state has a multi-domain structure
whose domains consist of the two among infinitely (continuously) degenerated
Luttinger-Tisza (LT) ground-state orders on a bulk square lattice, i.e., the
two antiferromagnetically aligned ferromagnetic chains (af-FMC) orders directed
in parallel to the two lattice axes. In the square, on the other
hand, the freezing starts from the interior of the square, and its ground state
is nearly in a single domain with one of the two af-FMC orders. These geometry
effects are argued to originate from the anisotropic nature of the
dipole-dipole interaction which depends on the relative direction of sites in a
real space of the interacting spins.Comment: 21 pages, 13 figures, submitted to Journal of Physical Society Japa
- …