265 research outputs found
PT-symmetric lattices with spatially extended gain/loss are generically unstable
We illustrate, through a series of prototypical examples, that linear
parity-time (PT) symmetric lattices with extended gain/loss profiles are
generically unstable, for any non-zero value of the gain/loss coefficient. Our
examples include a parabolic real potential with a linear imaginary part and
the cases of no real and constant or linear imaginary potentials. On the other
hand, this instability can be avoided and the spectrum can be real for
localized or compact PT-symmetric potentials. The linear lattices are analyzed
through discrete Fourier transform techniques complemented by numerical
computations.Comment: 6 pages, 4 figure
Static and rotating domain-wall crosses in Bose-Einstein condensates
For a Bose-Einstein condensate (BEC) in a two-dimensional (2D) trap, we
introduce cross patterns, which are generated by intersection of two domain
walls (DWs) separating immiscible species, with opposite signs of the wave
functions in each pair of sectors filled by the same species. The cross pattern
remains stable up to the zero value of the immiscibility parameter ,
while simpler rectilinear (quasi-1D) DWs exist only for values of
essentially exceeding those in BEC mixtures (two spin states of the same
isotope) currently available to the experiment. Both symmetric and asymmetric
cross configurations are investigated, with equal or different numbers
of atoms in the two species. In rotating traps, ``propellers''
(stable revolving crosses) are found too. A full stability region for of the
crosses and propellers in the system's parameter space is identified, unstable
crosses evolving into arrays of vortex-antivortex pairs. Stable rotating
rectilinear DWs are found too, at larger vlues of . All the patterns
produced by the intersection of three or more DWs are unstable, rearranging
themselves into ones with two DWs. Optical propellers are also predicted in a
twisted nonlinear photonic-crystal fiber carrying two different wavelengths or
circular polarizations, which can be used for applications to switching and
routing.Comment: 9 pages, 10 figures, Phys. Rev. A (in press
Soliton Dynamics in Linearly Coupled Discrete Nonlinear Schr\"odinger Equations
We study soliton dynamics in a system of two linearly coupled discrete
nonlinear Schr\"odinger equations, which describe the dynamics of a
two-component Bose gas, coupled by an electromagnetic field, and confined in a
strong optical lattice. When the nonlinear coupling strengths are equal, we use
a unitary transformation to remove the linear coupling terms, and show that the
existing soliton solutions oscillate from one species to the other. When the
nonlinear coupling strengths are different, the soliton dynamics is numerically
investigated and the findings are compared to the results of an effective
two-mode model. The case of two linearly coupled Ablowitz-Ladik equations is
also investigated.Comment: to be published in Mathematics and Computers in Simulation,
proceedings of the fifth IMACS International Conference on Nonlinear
Evolution Equations and Wave Phenomena: Computation and Theory (Athens,
Georgia - April 2007
Multi-channel pulse dynamics in a stabilized Ginzburg-Landau system
We study the stability and interactions of chirped solitary pulses in a
system of nonlinearly coupled cubic Ginzburg-Landau (CGL) equations with a
group-velocity mismatch between them, where each CGL equation is stabilized by
linearly coupling it to an additional linear dissipative equation. In the
context of nonlinear fiber optics, the model describes transmission and
collisions of pulses at different wavelengths in a dual-core fiber, in which
the active core is furnished with bandwidth-limited gain, while the other,
passive (lossy) one is necessary for stabilization of the solitary pulses.
Complete and incomplete collisions of pulses in two channels in the cases of
anomalous and normal dispersion in the active core are analyzed by means of
perturbation theory and direct numerical simulations. It is demonstrated that
the model may readily support fully stable pulses whose collisions are
quasi-elastic, provided that the group-velocity difference between the two
channels exceeds a critical value. In the case of quasi-elastic collisions, the
temporal shift of pulses, predicted by the analytical approach, is in
semi-quantitative agrement with direct numerical results in the case of
anomalous dispersion (in the opposite case, the perturbation theory does not
apply). We also consider a simultaneous collision between pulses in three
channels, concluding that this collision remains quasi-elastic, and the pulses
remain completely stable. Thus, the model may be a starting point for the
design of a stabilized wavelength-division-multiplexed (WDM) transmission
system.Comment: a text file in the revtex4 format, and 16 pdf files with figures.
Physical Review E, in pres
Ag-coverage-dependent symmetry of the electronic states of the Pt(111)-Ag-Bi interface: The ARPES view of a structural transition
We studied by angle-resolved photoelectron spectroscopy the strain-related
structural transition from a pseudomorphic monolayer (ML) to a striped
incommensurate phase in an Ag thin film grown on Pt(111). We exploited the
surfactant properties of Bi to grow ordered Pt(111)-xMLAg-Bi trilayers with 0 <
x < 5 ML, and monitored the dispersion of the Bi-derived interface states to
probe the structure of the underlying Ag film. We find that their symmetry
changes from threefold to sixfold and back to threefold in the Ag coverage
range studied. Together with previous scanning tunneling microscopy and
photoelectron diffraction data, these results provide a consistent microscopic
description of the coverage-dependent structural transition.Comment: 10 pages, 9 figure
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