11,617 research outputs found
Cross-diffusion systems for image processing: II. The nonlinear case
In this paper the use of nonlinear cross-diffu\-sion systems to model image
restoration is investigated, theoretically and numerically. In the first case,
well-posedness, scale-space properties and long time behaviour are analyzed.
From a numerical point of view, a computational study of the performance of the
models is carried out, suggesting their diversity and potentialities to treat
image filtering problems. The present paper is a continuation of a previous
work of the same authors, devoted to linear cross-diffusion models.
\keywords{Cross-diffusion \and Complex diffusion \and Image restoration
Theory of Andreev reflection in a two-orbital model of iron-pnictide superconductors
A recently developed theory for the problem of Andreev reflection between a
normal metal (N) and a multiband superconductor (MBS) assumes that the incident
wave from the normal metal is coherently transmitted through several bands
inside the superconductor. Such splitting of the probability amplitude into
several channels is the analogue of a quantum waveguide. Thus, the appropriate
matching conditions for the wave function at the N/MBS interface are derived
from an extension of quantum waveguide theory. Interference effects between the
transmitted waves inside the superconductor manifest themselves in the
conductance. We provide results for a FeAs superconductor, in the framework of
a recently proposed effective two-band model and two recently proposed gap
symmetries: in the sign-reversed s-wave () scenario
resonant transmission through surface Andreev bound states (ABS) at nonzero
energy is found as well as destructive interference effects that produce zeros
in the conductance; in the extended s-wave ()
scenario no ABS at finite energy are found.Comment: 4 pages, 5 figure
Robust self-trapping of vortex beams in a saturable optical medium
We report the first observation of robust self-trapping of vortex beams
propagating in a uniform condensed medium featuring local saturable
self-focusing nonlinearity. Optical vortices with topological charge m=1, that
remain self-trapped over ~ 5 Rayleigh lengths, are excited in carbon disulfide
using a helical light beam at 532 nm and intensities from 8 to 10 GW/cm^2. At
larger intensities, the vortex beams lose their stability, spontaneously
breaking into two fragments. Numerical simulations based on the nonlinear
Schr\"odinger equation including the three-photon absorption and nonpolynomial
saturation of the refractive nonlinearity demonstrate close agreement with the
experimental findings.Comment: 27 pages, 7 figures,to be published in Phys. Rev. A (2016
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