1,385 research outputs found
Excursion sets of stable random fields
Studying the geometry generated by Gaussian and Gaussian- related random
fields via their excursion sets is now a well developed and well understood
subject. The purely non-Gaussian scenario has, however, not been studied at
all. In this paper we look at three classes of stable random fields, and obtain
asymptotic formulae for the mean values of various geometric characteristics of
their excursion sets over high levels.
While the formulae are asymptotic, they contain enough information to show
that not only do stable random fields exhibit geometric behaviour very
different from that of Gaussian fields, but they also differ significantly
among themselves.Comment: 35 pages, 1 figur
High level excursion set geometry for non-Gaussian infinitely divisible random fields
We consider smooth, infinitely divisible random fields ,
, with regularly varying Levy measure, and are
interested in the geometric characteristics of the excursion sets over high levels u. For a large class of such random fields, we
compute the asymptotic joint distribution of the numbers of
critical points, of various types, of X in , conditional on being
nonempty. This allows us, for example, to obtain the asymptotic conditional
distribution of the Euler characteristic of the excursion set. In a significant
departure from the Gaussian situation, the high level excursion sets for these
random fields can have quite a complicated geometry. Whereas in the Gaussian
case nonempty excursion sets are, with high probability, roughly ellipsoidal,
in the more general infinitely divisible setting almost any shape is possible.Comment: Published in at http://dx.doi.org/10.1214/11-AOP738 the Annals of
Probability (http://www.imstat.org/aop/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Nonlinear evolution of the plasma beatwave: Compressing the laser beatnotes via electromagnetic cascading
The near-resonant beatwave excitation of an electron plasma wave (EPW) can be
employed for generating the trains of few-femtosecond electromagnetic (EM)
pulses in rarefied plasmas. The EPW produces a co-moving index grating that
induces a laser phase modulation at the difference frequency. The bandwidth of
the phase-modulated laser is proportional to the product of the plasma length,
laser wavelength, and amplitude of the electron density perturbation. The laser
spectrum is composed of a cascade of red and blue sidebands shifted by integer
multiples of the beat frequency. When the beat frequency is lower than the
electron plasma frequency, the red-shifted spectral components are advanced in
time with respect to the blue-shifted ones near the center of each laser
beatnote. The group velocity dispersion of plasma compresses so chirped
beatnotes to a few-laser-cycle duration thus creating a train of sharp EM
spikes with the beat periodicity. Depending on the plasma and laser parameters,
chirping and compression can be implemented either concurrently in the same, or
sequentially in different plasmas. Evolution of the laser beatwave end electron
density perturbations is described in time and one spatial dimension in a
weakly relativistic approximation. Using the compression effect, we demonstrate
that the relativistic bi-stability regime of the EPW excitation [G. Shvets,
Phys. Rev. Lett. 93, 195004 (2004)] can be achieved with the initially
sub-threshold beatwave pulse.Comment: 13 pages, 11 figures, submitted to Physical Review
Quantum Criticality in Dimerized Spin Ladders
We analyze a possibility of quantum criticality (gaplessness) in dimerized
antiferromagnetic two- and three-leg spin-1/2 ladders. Contrary to earlier
studies of these models, we examine different dimerization patterns in the
ladder. We find that ladders with the columnar dimerization order have lower
zero-temperature energies and they are always gapped. For the staggered
dimerization order, we find the quantum critical lines, in agreement with
earlier analyses. The bond mean-field theory we apply, demonstrates its
quantitative accuracy and agrees with available numerical results. We conclude
that unless some mechanism for locking dimerization into the energetically less
favorable staggered configuration is provided, the dimerized ladders do not
order into the phase where the quantum criticality occurs.Comment: 7 pages, 9 figure
Peculiarities of evolutions of elastic-plastic shock compression waves in different materials
In the paper, we discuss such unexpected features in the wave evolution in solids as strongly nonlinear uniaxial elastic compression in a picosecond time range, a departure from self-similar development of the wave process which is accompanied with apparent sub-sonic wave propagation, changes of shape of elastic precursor wave as a result of variations in the material structure and the temperature, unexpected peculiarities of reflection of elastic-plastic waves from free surface
Nonlinear Dynamics of Dipoles in Microtubules: Pseudo-Spin Model
We perform a theoretical study of the dynamics of the electric field
excitations in a microtubule by taking into consideration the realistic
cylindrical geometry, dipole-dipole interactions of the tubulin-based protein
heterodimers, the radial electric field produced by the solvent, and a possible
degeneracy of energy states of individual heterodimers. The consideration is
done in the frames of the classical pseudo-spin model. We derive the system of
nonlinear dynamical ordinary differential equations of motion for interacting
dipoles, and the continuum version of these equations. We obtain the solutions
of these equations in the form of snoidal waves, solitons, kinks, and localized
spikes. Our results will help to a better understanding of the functional
properties of microtubules including the motor protein dynamics and the
information transfer processes. Our considerations are based on classical
dynamics. Some speculations on the role of possible quantum effects are also
made.Comment: 14 pages, 15 figures. The high resolution figure files are available
by reques
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