223 research outputs found
Breathers in oscillator chains with Hertzian interactions
We prove nonexistence of breathers (spatially localized and time-periodic
oscillations) for a class of Fermi-Pasta-Ulam lattices representing an
uncompressed chain of beads interacting via Hertz's contact forces. We then
consider the setting in which an additional on-site potential is present,
motivated by the Newton's cradle under the effect of gravity. Using both direct
numerical computations and a simplified asymptotic model of the oscillator
chain, the so-called discrete p-Schr\"odinger (DpS) equation, we show the
existence of discrete breathers and study their spectral properties and
mobility. Due to the fully nonlinear character of Hertzian interactions,
breathers are found to be much more localized than in classical nonlinear
lattices and their motion occurs with less dispersion. In addition, we study
numerically the excitation of a traveling breather after an impact at one end
of a semi-infinite chain. This case is well described by the DpS equation when
local oscillations are faster than binary collisions, a situation occuring e.g.
in chains of stiff cantilevers decorated by spherical beads. When a hard
anharmonic part is added to the local potential, a new type of traveling
breather emerges, showing spontaneous direction-reversing in a spatially
homogeneous system. Finally, the interaction of a moving breather with a point
defect is also considered in the cradle system. Almost total breather
reflections are observed at sufficiently high defect sizes, suggesting
potential applications of such systems as shock wave reflectors
The Krein Matrix: General Theory and Concrete Applications in Atomic Bose-Einstein Condensates
When finding the nonzero eigenvalues for Hamiltonian eigenvalue problems it
is especially important to locate not only the unstable eigenvalues (i.e.,
those with positive real part), but also those which are purely imaginary but
have negative Krein signature. These latter eigenvalues have the property that
they can become unstable upon collision with other purely imaginary
eigenvalues, i.e., they are a necessary building block in the mechanism leading
to the so-called Hamiltonian-Hopf bifurcation. In this paper we review a
general theory for constructing a meromorphic matrix-valued function, the
so-called Krein matrix, which has the property of not only locating the
unstable eigenvalues, but also those with negative Krein signature. These
eigenvalues are realized as zeros of the determinant. The resulting finite
dimensional problem obtained by setting the determinant of the Krein matrix to
zero presents a valuable simplification. In this paper the usefulness of the
technique is illustrated through prototypical examples of spectral analysis of
states that have arisen in recent experimental and theoretical studies of
atomic Bose-Einstein condensates. In particular, we consider one-dimensional
settings (the cigar trap) possessing real-valued multi-dark-soliton solutions,
and two-dimensional settings (the pancake trap) admitting complex multi-vortex
stationary waveforms.Comment: 26 pages, 16 figures (revised version on April 18 2013
Dark solitons in external potentials
We consider the persistence and stability of dark solitons in the
Gross-Pitaevskii (GP) equation with a small decaying potential. We show that
families of black solitons with zero speed originate from extremal points of an
appropriately defined effective potential and persist for sufficiently small
strength of the potential. We prove that families at the maximum points are
generally unstable with exactly one real positive eigenvalue, while families at
the minimum points are generally unstable with exactly two complex-conjugated
eigenvalues with positive real part. This mechanism of destabilization of the
black soliton is confirmed in numerical approximations of eigenvalues of the
linearized GP equation and full numerical simulations of the nonlinear GP
equation with cubic nonlinearity. We illustrate the monotonic instability
associated with the real eigenvalues and the oscillatory instability associated
with the complex eigenvalues and compare the numerical results of evolution of
a dark soliton with the predictions of Newton's particle law for its position.Comment: 39 pages, 10 figure
- …