83 research outputs found
Magnetoelastic nature of solid oxygen epsilon-phase structure
For a long time a crystal structure of high-pressure epsilon-phase of solid
oxygen was a mistery. Basing on the results of recent experiments that have
solved this riddle we show that the magnetic and crystal structure of
epsilon-phase can be explained by strong exchange interactions of
antiferromagnetic nature. The singlet state implemented on quaters of O2
molecules has the minimal exchange energy if compared to other possible singlet
states (dimers, trimers). Magnetoelastic forces that arise from the spatial
dependence of the exchange integral give rise to transformation of 4(O2)
rhombuses into the almost regular quadrates. Antiferromagnetic character of the
exchange interactions stabilizes distortion of crystal lattice in epsilon-phase
and impedes such a distortion in long-range alpha- and delta-phases.Comment: 11 pages, 4 figures, Changes: corrected typos, reference to the
recent paper is adde
Long-range effects on superdiffusive solitons in anharmonic chains
Studies on thermal diffusion of lattice solitons in Fermi-Pasta-Ulam
(FPU)-like lattices were recently generalized to the case of dispersive
long-range interactions (LRI) of the Kac-Baker form. The position variance of
the soliton shows a stronger than linear time-dependence (superdiffusion) as
found earlier for lattice solitons on FPU chains with nearest neighbour
interactions (NNI). In contrast to the NNI case where the position variance at
moderate soliton velocities has a considerable linear time-dependence (normal
diffusion), the solitons with LRI are dominated by a superdiffusive mechanism
where the position variance mainly depends quadratic and cubic on time. Since
the superdiffusion seems to be generic for nontopological solitons, we want to
illuminate the role of the soliton shape on the superdiffusive mechanism.
Therefore, we concentrate on a FPU-like lattice with a certain class of
power-law long-range interactions where the solitons have algebraic tails
instead of exponential tails in the case of FPU-type interactions (with or
without Kac-Baker LRI). A collective variable (CV) approach in the continuum
approximation of the system leads to stochastic integro-differential equations
which can be reduced to Langevin-type equations for the CV position and width.
We are able to derive an analytical result for the soliton diffusion which
agrees well with the simulations of the discrete system. Despite of
structurally similar Langevin systems for the two soliton types, the algebraic
solitons reach the superdiffusive long-time limit with a characteristic
time-dependence much faster than exponential solitons. The soliton
shape determines the diffusion constant in the long-time limit that is
approximately a factor of smaller for algebraic solitons.Comment: 7 figure
Magnetic structures of -O resulting from competition of interplane exchange interactions
Solid oxygen is a unique molecular crystal whose phase diagram is mostly
imposed by magnetic ordering, i.e., each crystal phase has a specific magnetic
structure. However, recent experiments showed that high-pressure -phase
is implemented in different magnetic structures. In the present paper we study
the role of interplane exchange interactions in formation of the magnetic
structures with different stacking sequences of the close-packed planes. We
show that temperature-induced variation of intermolecular distances can give
rise to compensation of the exchange coupling between the nearest close-packed
planes and result in the phase transition between different magnetic structures
within -O. Variation of the magnetic ordering is, in turn,
accompanied by the step-wise variation of interplane distance governed by space
and angular dependence of interplane exchange constants.Comment: 16 pages, 6 figure
Effects of finite curvature on soliton dynamics in a chain of nonlinear oscillators
We consider a curved chain of nonlinear oscillators and show that the
interplay of curvature and nonlinearity leads to a number of qualitative
effects. In particular, the energy of nonlinear localized excitations centered
on the bending decreases when curvature increases, i.e. bending manifests
itself as a trap for excitations. Moreover, the potential of this trap is
double-well, thus leading to a symmetry breaking phenomenon: a symmetric
stationary state may become unstable and transform into an energetically
favorable asymmetric stationary state. The essentials of symmetry breaking are
examined analytically for a simplified model. We also demonstrate a threshold
character of the scattering process, i.e. transmission, trapping, or reflection
of the moving nonlinear excitation passing through the bending.Comment: 13 pages (LaTeX) with 10 figures (EPS
Kinks in the discrete sine-Gordon model with Kac-Baker long-range interactions
We study effects of Kac-Baker long-range dispersive interaction (LRI) between
particles on kink properties in the discrete sine-Gordon model. We show that
the kink width increases indefinitely as the range of LRI grows only in the
case of strong interparticle coupling. On the contrary, the kink becomes
intrinsically localized if the coupling is under some critical value.
Correspondingly, the Peierls-Nabarro barrier vanishes as the range of LRI
increases for supercritical values of the coupling but remains finite for
subcritical values. We demonstrate that LRI essentially transforms the internal
dynamics of the kinks, specifically creating their internal localized and
quasilocalized modes. We also show that moving kinks radiate plane waves due to
break of the Lorentz invariance by LRI.Comment: 11 pages (LaTeX) and 14 figures (Postscript); submitted to Phys. Rev.
Solitons in anharmonic chains with ultra-long-range interatomic interactions
We study the influence of long-range interatomic interactions on the
properties of supersonic pulse solitons in anharmonic chains. We show that in
the case of ultra-long-range (e.g., screened Coulomb) interactions three
different types of pulse solitons coexist in a certain velocity interval: one
type is unstable but the two others are stable. The high-energy stable soliton
is broad and can be described in the quasicontinuum approximation. But the
low-energy stable soliton consists of two components, short-range and
long-range ones, and can be considered as a bound state of these components.Comment: 4 pages (LaTeX), 5 figures (Postscript); submitted to Phys. Rev.
Stationary and moving breathers in a simplified model of curved alpha--helix proteins
The existence, stability and movability of breathers in a model for
alpha-helix proteins is studied. This model basically consists a chain of
dipole moments parallel to it. The existence of localized linear modes brings
about that the system has a characteristic frequency, which depends on the
curvature of the chain. Hard breathers are stable, while soft ones experiment
subharmonic instabilities that preserve, however the localization. Moving
breathers can travel across the bending point for small curvature and are
reflected when it is increased. No trapping of breathers takes place.Comment: 19 pages, 11 figure
On the theory of pseudogap anisotropy in the cuprate superconductors
We show by means of the theory of the order parameter phase fluctuations that
the temperature of "closing" (or "opening") of the gap (and pseudogap) in the
electron spectra of superconductors with anisotropic order parameter takes
place within a finite temperature range. Every Fourier-component of the order
parameter has its own critical temperature
Localization of nonlinear excitations in curved waveguides
Motivated by the example of a curved waveguide embedded in a photonic
crystal, we examine the effects of geometry in a ``quantum channel'' of
parabolic form. We study the linear case and derive exact as well as
approximate expressions for the eigenvalues and eigenfunctions of the linear
problem. We then proceed to the nonlinear setting and its stationary states in
a number of limiting cases that allow for analytical treatment. The results of
our analysis are used as initial conditions in direct numerical simulations of
the nonlinear problem and localized excitations are found to persist, as well
as to have interesting relaxational dynamics. Analogies of the present problem
in contexts related to atomic physics and particularly to Bose-Einstein
condensation are discussed.Comment: 14 pages, 4 figure
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Mesoscale Dzyaloshinskii-Moriya interaction: Geometrical tailoring of the magnetochirality
Crystals with broken inversion symmetry can host fundamentally appealing and technologically relevant periodical or localized chiral magnetic textures. The type of the texture as well as its magnetochiral properties are determined by the intrinsic Dzyaloshinskii-Moriya interaction (DMI), which is a material property and can hardly be changed. Here we put forth a method to create new artificial chiral nanoscale objects with tunable magnetochiral properties from standard magnetic materials by using geometrical manipulations. We introduce a mesoscale Dzyaloshinskii-Moriya interaction that combines the intrinsic spin-orbit and extrinsic curvature-driven DMI terms and depends both on the material and geometrical parameters. The vector of the mesoscale DMI determines magnetochiral properties of any curved magnetic system with broken inversion symmetry. The strength and orientation of this vector can be changed by properly choosing the geometry. For a specific example of nanosized magnetic helix, the same material system with different geometrical parameters can acquire one of three zero-temperature magnetic phases, namely, phase with a quasitangential magnetization state, phase with a periodical state and one intermediate phase with a periodical domain wall state. Our approach paves the way towards the realization of a new class of nanoscale spintronic and spinorbitronic devices with the geometrically tunable magnetochirality
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