701 research outputs found
Spatial solitons in a medium composed of self-focusing and self-defocusing layers
We introduce a model combining Kerr nonlinearity with a periodically changing
sign ("nonlinearity management") and a Bragg grating (BG). The main result,
obtained by means of systematic simulations, is presented in the form of a
soliton's stability diagram on the parameter plane of the model; the diagram
turns out to be a universal one, as it practically does not depend on the
soliton's power. Moreover, simulations of the nonlinear Schroedinger (NLS)
model subjected to the same "nonlinearity management" demonstrate that the same
diagram determines the stability of the NLS solitons, unless they are very
narrow. The stability region of very narrow NLS solitons is much smaller, and
soliton splitting is readily observed in that case. The universal diagram shows
that a minimum non-zero average value of the Kerr coefficient is necessary for
the existence of stable solitons. Interactions between identical solitons with
an initial phase difference between them are simulated too in the BG model,
resulting in generation of stable moving solitons. A strong spontaneous
symmetry breaking is observed in the case when in-phase solitons pass through
each other due to attraction between them.Comment: a latex text file and 9 eps files with figures. Physics Letters A, in
pres
Source identities and kernel functions for deformed (quantum) Ruijsenaars models
We consider the relativistic generalization of the quantum
Calogero-Sutherland models due to Ruijsenaars, comprising the rational,
hyperbolic, trigonometric and elliptic cases. For each of these cases, we find
an exact common eigenfunction for a generalization of Ruijsenaars analytic
difference operators that gives, as special cases, many different kernel
functions; in particular, we find kernel functions for Chalykh-
Feigin-Veselov-Sergeev-type deformations of such difference operators which
generalize known kernel functions for the Ruijsenaars models. We also discuss
possible applications of our results.Comment: 24 page
Stable solitons in coupled Ginzburg-Landau equations describing Bose-Einstein condensates and nonlinear optical waveguides and cavities
We introduce a model of a two-core system, based on an equation of the
Ginzburg-Landau (GL) type, coupled to another GL equation, which may be linear
or nonlinear. One core is active, featuring intrinsic linear gain, while the
other one is lossy. The difference from previously studied models involving a
pair of linearly coupled active and passive cores is that the stabilization of
the system is provided not by a linear diffusion-like term, but rather by a
cubic or quintic dissipative term in the active core. Physical realizations of
the models include systems from nonlinear optics (semiconductor waveguides or
optical cavities), and a double-cigar-shaped Bose-Einstein condensate with a
negative scattering length, in which the active ``cigar'' is an atom laser. The
replacement of the diffusion term by the nonlinear loss is principally
important, as diffusion does not occur in these physical media, while nonlinear
loss is possible. A stability region for solitary pulses is found in the
system's parameter space by means of direct simulations. One border of the
region is also found in an analytical form by means of a perturbation theory.
Moving pulses are studied too. It is concluded that collisions between them are
completely elastic, provided that the relative velocity is not too small. The
pulses withstand multiple tunneling through potential barriers. Robust
quantum-rachet regimes of motion of the pulse in a time-periodic asymmetric
potential are found as well.Comment: 14 pages, 7 figure
The 4-beaches survey in Uganda: Nkombe Beach
This paper analyses the location, potentialities and set-backs of Nkombe Beach, the landing site chosen in Uganda for the 4-beaches survey
The quantum vacuum as the origin of the speed of light
We show that the vacuum permeability and permittivity may originate from the
magnetization and the polarization of continuously appearing and disappearing
fermion pairs. We then show that if we simply model the propagation of the
photon in vacuum as a series of transient captures within these ephemeral
pairs, we can derive a finite photon velocity. Requiring that this velocity is
equal to the speed of light constrains our model of vacuum. Within this
approach, the propagation of a photon is a statistical process at scales much
larger than the Planck scale. Therefore we expect its time of flight to
fluctuate. We propose an experimental test of this prediction.Comment: 6 pages, Accepted for publication in European Physical Journal D.
arXiv admin note: substantial text overlap with arXiv:1111.1847,
arXiv:1106.399
Micromedex: a brief review for a UK School of Pharmacy
Micromedex is one of the most popular tools used by pharmacists to provide medicines information. This paper outlines a brief review of the product to determine whether it would be suitable for use by undergraduates in a particular UK School of Pharmacy, that of UCL London
- …