98 research outputs found
Svortices and the fundamental modes of the "snake instability": Possibility of observation in the gaseous Bose-Einstein Condensate
The connection between quantized vortices and dark solitons in a long and
thin, waveguide-like trap geometry is explored in the framework of the
non-linear Schr\"odinger equation. Variation of the transverse confinement
leads from the quasi-1D regime where solitons are stable to 2D (or 3D)
confinement where soliton stripes are subject to a transverse modulational
instability known as the ``snake instability''. We present numerical evidence
of a regime of intermediate confinement where solitons decay into single,
deformed vortices with solitonic properties, also called svortices, rather than
vortex pairs as associated with the ``snake'' metaphor. Further relaxing the
transverse confinement leads to production of 2 and then 3 vortices, which
correlates perfectly with a Bogoliubov-de Gennes stability analysis. The decay
of a stationary dark soliton (or, planar node) into a single svortex is
predicted to be experimentally observable in a 3D harmonically confined dilute
gas Bose-Einstein condensate.Comment: 4 pages, 4 figure
Route to nonlocality and observation of accessible solitons
We develop a general theory of spatial solitons in a liquid crystalline
medium exhibiting a nonlinearity with an arbitrary degree of effective
nonlocality. The model accounts the observability of "accessible solitons" and
establishes an important link with parametric solitons.Comment: 4 pages, 2 figure
Two dimensional modulational instability in photorefractive media
We study theoretically and experimentally the modulational instability of
broad optical beams in photorefractive nonlinear media. We demonstrate the
impact of the anisotropy of the nonlinearity on the growth rate of periodic
perturbations. Our findings are confirmed by experimental measurements in a
strontium barium niobate photorefractive crystal.Comment: 8 figure
Watching dark solitons decay into vortex rings in a Bose-Einstein condensate
We have created spatial dark solitons in two-component Bose-Einstein
condensates in which the soliton exists in one of the condensate components and
the soliton nodal plane is filled with the second component. The filled
solitons are stable for hundreds of milliseconds. The filling can be
selectively removed, making the soliton more susceptible to dynamical
instabilities. For a condensate in a spherically symmetric potential, these
instabilities cause the dark soliton to decay into stable vortex rings. We have
imaged the resulting vortex rings.Comment: 4 pages, 4 figure
Stationary solutions of the one-dimensional nonlinear Schroedinger equation: II. Case of attractive nonlinearity
All stationary solutions to the one-dimensional nonlinear Schroedinger
equation under box or periodic boundary conditions are presented in analytic
form for the case of attractive nonlinearity. A companion paper has treated the
repulsive case. Our solutions take the form of bounded, quantized, stationary
trains of bright solitons. Among them are two uniquely nonlinear classes of
nodeless solutions, whose properties and physical meaning are discussed in
detail. The full set of symmetry-breaking stationary states are described by
the character tables from the theory of point groups. We make
experimental predictions for the Bose-Einstein condensate and show that, though
these are the analog of some of the simplest problems in linear quantum
mechanics, nonlinearity introduces new and surprising phenomena.Comment: 11 pages, 9 figures -- revised versio
Renormalization of the nonequilibrium dynamics of fermions in a flat FRW universe
We derive the renormalized equations of motion and the renormalized
energy-momentum tensor for fermions coupled to a spatially homogeneous scalar
field (inflaton) in a flat FRW geometry. The fermion back reaction to the
metric and to the inflaton field is formulated in one-loop approximation.
Having determined the infinite counter terms in an scheme we
formulate the finite terms in a form suitable for numerical computation. We
comment on the trace anomaly which is inferred from the standard analysis. We
also address the problem of initial singularities and determine the Bogoliubov
transformation by which they are removed.Comment: 26 pages, LaTe
Modulational instability of solitary waves in non-degenerate three-wave mixing: The role of phase symmetries
We show how the analytical approach of Zakharov and Rubenchik [Sov. Phys.
JETP {\bf 38}, 494 (1974)] to modulational instability (MI) of solitary waves
in the nonlinear Schr\"oedinger equation (NLS) can be generalised for models
with two phase symmetries. MI of three-wave parametric spatial solitons due to
group velocity dispersion (GVD) is investigated as a typical example of such
models. We reveal a new branch of neck instability, which dominates the usual
snake type MI found for normal GVD. The resultant nonlinear evolution is
thereby qualitatively different from cases with only a single phase symmetry.Comment: 4 pages with figure
Non-Gaussianity from Symmetry
We point out that a light scalar field fluctuating around a symmetry-enhaced
point can generate large non-Gaussianity in density fluctuations. We name such
a particle as an "ungaussiton", a scalar field dominantly produced by the
quantum fluctuations,generating sizable non-Gaussianity in the density
fluctuations. We derive a consistency relation between the bispectrum and the
trispectrum, tau_NL = 10^3 f_NL^(4/3), which can be extended to arbitrary high
order correlation functions. If such a relation is confirmed by future
observations, it will strongly support this mechanism.Comment: 26 pages, 1 figure;v2 discussion and references added. To appear in
JCA
Electron correlation effects and magnetic ordering at the Gd(0001) surface
Effects of electron correlation on the electronic structure and magnetic
properties of the Gd(0001) surface are investigated using of the full-potential
linearized augmented plane wave implementation of correlated band theory
("LDA+U"). The use of LDA+U instead of LDA (local density approximation) total
energy calculations produces the correct ferromagnetic ground state for both
bulk Gd and the Gd surface. Surface strain relaxation leads to an 90 %
enhancement of the interlayer surface-to-bulk effective exchange coupling.
Application of a Landau-Ginzburg type theory yields a 30 % enhancement of the
Curie temperature at the surface, in very good agreement with the experiment.Comment: revised version: minor typos correcte
Creation, doubling, and splitting, of vortices in intracavity second harmonic generation
We demonstrate generation and frequency doubling of unit charge vortices in a
linear astigmatic resonator. Topological instability of the double charge
harmonic vortices leads to well separated vortex cores that are shown to
rotate, and become anisotropic, as the resonator is tuned across resonance
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