126 research outputs found
Supersonic Discrete Kink-Solitons and Sinusoidal Patterns with "Magic" wavenumber in Anharmonic Lattices
The sharp pulse method is applied to Fermi-Pasta-Ulam (FPU) and Lennard-Jones
(LJ) anharmonic lattices. Numerical simulations reveal the presence of high
energy strongly localized ``discrete'' kink-solitons (DK), which move with
supersonic velocities that are proportional to kink amplitudes. For small
amplitudes, the DK's of the FPU lattice reduce to the well-known ``continuous''
kink-soliton solutions of the modified Korteweg-de Vries equation. For high
amplitudes, we obtain a consistent description of these DK's in terms of
approximate solutions of the lattice equations that are obtained by restricting
to a bounded support in space exact solutions with sinusoidal pattern
characterized by the ``magic'' wavenumber . Relative displacement
patterns, velocity versus amplitude, dispersion relation and exponential tails
found in numerical simulations are shown to agree very well with analytical
predictions, for both FPU and LJ lattices.Comment: Europhysics Letters (in print
Weakly nonlinear theory of grain boundary motion in patterns with crystalline symmetry
We study the motion of a grain boundary separating two otherwise stationary
domains of hexagonal symmetry. Starting from an order parameter equation
appropriate for hexagonal patterns, a multiple scale analysis leads to an
analytical equation of motion for the boundary that shares many properties with
that of a crystalline solid. We find that defect motion is generically opposed
by a pinning force that arises from non-adiabatic corrections to the standard
amplitude equation. The magnitude of this force depends sharply on the
mis-orientation angle between adjacent domains so that the most easily pinned
grain boundaries are those with an angle between four and eight degrees.
Although pinning effects may be small, they do not vanish asymptotically near
the onset of this subcritical bifurcation, and can be orders of magnitude
larger than those present in smectic phases that bifurcate supercritically
Magnetization reversal of ferromagnetic nanodisc placed above a superconductor
Using numerical simulation we have studied a magnetization distribution and a
process of magnetization reversal in nanoscale magnets placed above a
superconductor plane. In order to consider an influence of superconductor on
magnetization distribution in the nanomagnet we have used London approximation.
We have found that for usual values of London penetration depth the ground
state magnetization is mostly unchanged. But at the same time the fields of
vortex nucleation and annihilation change significantly: the interval where
vortex is stable enlarges on 100-200 Oe for the particle above the
superconductor. Such fields are experimentally observable so there is a
possibility of some practical applications of this effect.Comment: 8 pages, 9 figure
Switching phenomena in magnetic vortex dynamics
A magnetic nanoparticle in a vortex state is a promising candidate for the
information storage. One bit of information corresponds to the upward or
downward magnetization of the vortex core (vortex polarity). Generic properties
of the vortex polarity switching are insensitive of the way how the vortex
dynamics was excited: by an AC magnetic field, or by an electrical current. We
study theoretically the switching process and describe in detail its mechanism,
which involves the creation and annihilation of an intermediate
vortex-antivortex pair.Comment: REVTeX, 8 pages, 2 figures; to appear in a special issue of Low
Temperature Physics in memory of A.M.Kosevic
Energy localization, Fano resonances, and nonlinear meta-optics
This paper reflects on some memories of the research topics developed at Department No. 29 of the Institute for Low Temperature Physics and Engineering in Kharkov more than 30 years ago. It also provides some recent advances on my major research activities related to those topics, including energy localization and solitons in nonlinear lattices, Fano resonances in photonics and phononics, and nonlinear effects in meta-optics and nanophotonics. Curiously enough, each of those topics can be associated with some memories and discussions that happened in Kharkov a long time ago
Vortex Polarity Switching in Magnets with Surface Anisotropy
Vortex core reversal in magnetic particle is essentially influenced by a
surface anisotropy. Under the action of a perpendicular static magnetic field
the vortex core undergoes a shape deformationof pillow- or barrel-shaped type,
depending on the type of the surface anisotropy. This deformation plays a key
point in the switching mechanism: We predict that the vortex polarity switching
is accompanied (i) by a linear singularity in case of Heisenberg magnet with
bulk anisotropy only and (ii) by a point singularities in case of surface
anisotropy or exchange anisotropy. We study in details the switching process
using spin-lattice simulations and propose a simple analytical description
using a wired core model, which provides an adequate description of the Bloch
point statics, its dynamics and the Bloch point mediated switching process. Our
analytical predictions are confirmed by spin-lattice simulations for Heisenberg
magnet and micromagnetic simulations for nanomagnet with account of a dipolar
interaction.Comment: 17 pages, 15 figure
Solitary Waves of Planar Ferromagnets and the Breakdown of the Spin-Polarized Quantum Hall Effect
A branch of uniformly-propagating solitary waves of planar ferromagnets is
identified. The energy dispersion and structures of the solitary waves are
determined for an isotropic ferromagnet as functions of a conserved momentum.
With increasing momentum, their structure undergoes a transition from a form
ressembling a droplet of spin-waves to a Skyrmion/anti-Skyrmion pair. An
instability to the formation of these solitary waves is shown to provide a
mechanism for the electric field-induced breakdown of the spin-polarized
quantum Hall effect.Comment: 5 pages, 3 eps-figures, revtex with epsf.tex and multicol.st
A supersonic crowdion in mica: Ultradiscrete kinks with energy between K recoil and transmission sputtering
In this chapter we analyze in detail the behaviour and properties of the
kinks found in an one dimensional model for the close packed rows of potassium
ions in mica muscovite. The model includes realistic potentials obtained from
the physics of the problem, ion bombardment experiments and molecular dynamics
fitted to experiments. These kinks are supersonic and have an unique velocity
and energy. They are ultradiscrete involving the translation of an interstitial
ion, which is the reason they are called 'crowdions'. Their energy is below the
most probable source of energy, the decay of the K isotope and above the
energy needed to eject an atom from the mineral, a phenomenon that has been
observed experimentallyComment: 28 pages, 15 figure
Haldane gap in the quasi one-dimensional nonlinear -model
This work studies the appearance of a Haldane gap in quasi one-dimensional
antiferromagnets in the long wavelength limit, via the nonlinear
-model. The mapping from the three-dimensional, integer spin Heisenberg
model to the nonlinear -model is explained, taking into account two
antiferromagnetic couplings: one along the chain axis () and one along the
perpendicular planes () of a cubic lattice. An implicit equation for
the Haldane gap is derived, as a function of temperature and coupling ratio
. Solutions to these equations show the existence of a critical
coupling ratio beyond which a gap exists only above a transition temperature
. The cut-off dependence of these results is discussed.Comment: 14 pages (RevTeX 3.0), 3 PostScript figures appended (printing
instructions included
Nonlinear parametric instability in double-well lattices
A possibility of a nonlinear resonant instability of uniform oscillations in
dynamical lattices with harmonic intersite coupling and onsite nonlinearity is
predicted. Numerical simulations of a lattice with a double-well onsite
anharmonic potential confirm the existence of the nonlinear instability with an
anomalous value of the corresponding power index, 1.57, which is intermediate
between the values 1 and 2 characterizing the linear and nonlinear (quadratic)
instabilities. The anomalous power index may be a result of competition between
the resonant quadratic instability and nonresonant linear instabilities. The
observed instability triggers transition of the lattice into a chaotic
dynamical state.Comment: A latex text file and three pdf files with figures. Physical Review
E, in pres
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