93 research outputs found
Wave turbulence in Bose-Einstein condensates
The kinetics of nonequilibrium Bose-Einstein condensates are considered
within the framework of the Gross-Pitaevskii equation. A systematic derivation
is given for weak small-scale perturbations of a steady confined condensate
state. This approach combines a wavepacket WKB description with the weak
turbulence theory. The WKB theory derived in this paper describes the effect of
the condensate on the short-wave excitations which appears to be different from
a simple renormalization of the confining potential suggested in previous
literature.Comment: 33 pages 2 figure
Joint statistics of amplitudes and phases in Wave Turbulence
Random Phase Approximation (RPA) provides a very convenient tool to study the
ensembles of weakly interacting waves, commonly called Wave Turbulence. In its
traditional formulation, RPA assumes that phases of interacting waves are
random quantities but it usually ignores randomness of their amplitudes.
Recently, RPA was generalised in a way that takes into account the amplitude
randomness and it was applied to study of the higher momenta and probability
densities of wave amplitudes. However, to have a meaningful description of wave
turbulence the RPA properties assumed for the initial fields must be proven to
survive over the nonlinear evolution time, and such a proof is the main goal of
the present paper. We derive an evolution equation for the full probability
density function which contains the complete information about the joint
statistics of all wave amplitudes and phases. We show that, for any initial
statistics of the amplitudes, the phase factors remain statistically
independent uniformly distributed variables. If in addition the initial
amplitudes are also independent variables (but with arbitrary distributions)
they will remain independent when considered in small sets which are much less
than the total number of modes. However, if the size of a set is of order of
the total number of modes then the joint probability density for this set is
not factorisable into the product of one-mode probabilities. In the other
words, the modes in such a set are involved in a ``collective'' (correlated)
motion. We also study new type of correlators describing the phase statistics.Comment: 27 pages, uses feynmf packag
Generation of spin-wave dark solitons with phase engineering
We generate experimentally spin-wave envelope dark solitons from rectangular
high-frequency dark input pulses with externally introduced phase shifts in
yttrium-iron garnet magnetic fims. We observe the generation of both odd and
even numbers of magnetic dark solitons when the external phase shift varies.
The experimental results are in a good qualitative agreement with the theory of
the dark-soliton generation in magnetic films developed earlier [Phys. Rev.
Lett. 82, 2583 (1999)].Comment: 6 pages, including 7 figures, submitted to Phys. Rev.
Spatial Optical Solitons due to Multistep Cascading
We introduce a novel class of parametric optical solitons supported
simultaneously by two second-order nonlinear cascading processes,
second-harmonic generation and sum-frequency mixing. We obtain, analytically
and numerically, the solutions for three-wave spatial solitons and show that
the presence of an additional cascading mechanism can change dramatically the
properties and stability of two-wave quadratic solitary waves.Comment: 6 pages, 4 figure
Hard QCD. Plenary talk at the Quark Matter -97 Conference, December 1997, Tsukuba, Japan
Status of hard/perturbative QCD phenomena is briefly reviewed.
Landau-Pomeranchuk-Migdal effect is discussed as a means for establishing links
between particle and nuclear high-energy physics.Comment: 13 pages, 4 Postscript figures, uses espcrc1.st
Hamiltonian formalism and the Garrett-Munk spectrum of internal waves in the ocean
Wave turbulence formalism for long internal waves in a stratified fluid is
developed, based on a natural Hamiltonian description. A kinetic equation
appropriate for the description of spectral energy transfer is derived, and its
self-similar stationary solution corresponding to a direct cascade of energy
toward the short scales is found. This solution is very close to the high
wavenumber limit of the Garrett-Munk spectrum of long internal waves in the
ocean. In fact, a small modification of the Garrett-Munk formalism includes a
spectrum consistent with the one predicted by wave turbulence.Comment: 4 pages latex fil
Soliton transverse instabilities in anisotropic nonlocal self-focusing media
We study, both theoretically and experimentally, the transverse modulational
instability of spatial stripe solitons in anisotropic nonlocal photorefractive
media. We demonstrate that the instability scenarios depend strongly on the
stripe orientation, but the anisotropy-induced features are largely suppressed
for spatial solitons created by self-trapping of partially incoherent light.Comment: 3 pages including 4 figures (2 theoretical and 2 experimental).
submitted to Optics Letter
Early Triassic (Olenekian) ammonoids from Khentey Province, Mongolia,and their paleobiogeographic significance
Energy spectra of the ocean's internal wave field: theory and observations
The high-frequency limit of the Garrett and Munk spectrum of internal waves
in the ocean and the observed deviations from it are shown to form a pattern
consistent with the predictions of wave turbulence theory. In particular, the
high frequency limit of the Garrett and Munk spectrum constitutes an {\it
exact} steady state solution of the corresponding kinetic equation.Comment: 4 pages, one color figur
A Hamiltonian Formulation for Long Internal Waves
A novel canonical Hamiltonian formalism is developed for long internal waves
in a rotating environment. This includes the effects of background vorticity
and shear on the waves. By restricting consideration to flows in hydrostatic
balance, superimposed on a horizontally uniform background of vertical shear
and vorticity, a particularly simple Hamiltonian structure arises, which can be
thought of as describing a nonlinearly coupled infinite collection of shallow
water systems. The kinetic equation describing the time evolution of the
spectral energy of internal waves is subsequently derived, and a stationary
Kolmogorov solution is found in the high frequency limit. This is surprisingly
close to the Garrett--Munk spectrum of oceanic internal waves
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