842 research outputs found
Fusion, collapse, and stationary bound states of incoherently coupled waves in bulk cubic media
We study the interaction between two localized waves that propagate in a bulk (two transverse dimensions) Kerr medium, while being incoherently coupled through cross-phase modulation. The different types of stationary solitary wave solutions are found and their stability is discussed. The results of numerical simulations suggest that the solitary waves are unstable. We derive sufficient conditions for when the wave function is bound to collapse or spread out, and we develop a theory to describe the regions of different dynamical behavior. For localized waves with the same center we confirm these sufficient conditions numerically and show that only when the equations and the initial conditions are symmetric are they also close to being necessary conditions. Using Gaussian initial conditions we predict and confirm numerically the power-dependent characteristic initial separations that divide the phase space into collapsing and diffracting solutions, and further divide each of these regions into subregions of coupled (fusion) and uncoupled dynamics. Finally we illustrate how, close to the threshold of collapse, the waves can cross several times before eventually collapsing or diffracting
Destruction of superconductivity in disordered materials : a dimensional crossover
The disorder-induced Superconductor-to-Insulator Transition in amorphous
NbSi two-dimensional thin films is studied for different niobium
compositions through a variation of the sample thickness . We show that
the critical thickness , separating a superconducting regime from an
insulating one, increases strongly with diminishing , thus attaining values
of over 100 {\AA}. The corresponding phase diagram in the plane is
inferred and related to the three-dimensional situation. The two-dimensional
Superconductor-to-Insulator Transition well connects with the three-dimensional
Superconductor-to-Metal Transition
Collapsing dynamics of attractive Bose-Einstein condensates
The self-similar collapse of 3D and quasi-2D atom condensates with negative
scattering length is examined. 3D condensates are shown to blow up following
the scenario of {\it weak collapse}: The inner core of the condensate diverges
with an almost zero particle number, while its tail distribution spreads out to
large distances with a constant density profile. For this case, the 3-body
recombination arrests the collapse, but it weakly dissipates the atoms. The
confining trap then reforms the condensate at later times. In contrast, 2D
condensates undergo a {\it strong collapse}: The atoms stay mainly located at
center and recombination sequentially absorbs a significant amount of
particles.Comment: 4 pages, submitted for publicatio
Observation of thermally activated glassiness and memory dip in a-NbSi insulating thin films
We present electrical conductance measurements on amorphous NbSi insulating
thin films. These films display out-of equilibrium electronic features that are
markedly different from what has been reported so far in disordered insulators.
Like in the most studied systems (indium oxide and granular Al films), a slow
relaxation of the conductance is observed after a quench to liquid helium
temperature which gives rise to the growth of a memory dip in MOSFET devices.
But unlike in these systems, this memory dip and the related conductance
relaxations are still visible up to room temperature, with clear signatures of
a temperature dependent dynamics
Influence of Four-Wave Mixing and Walk-Off on the Self-Focusing of Coupled Waves
Four-wave mixing and walk-off between two optical beams are! investigated For focusing Kerr media. It is shown that four-wave mixing reinforces the self-focusing of mutually trapped waves by lowering their power threshold for collapse, only when their phase mismatch is small. On the contrary, walk-off inhibits the collapse by detrapping the beams, whose partial centroids experience nonlinear oscillations
Multidimensional solitons in a low-dimensional periodic potential
Using the variational approximation(VA) and direct simulations, we find
stable 2D and 3D solitons in the self-attractive Gross-Pitaevskii equation
(GPE) with a potential which is uniform in one direction () and periodic in
the others (but the quasi-1D potentials cannot stabilize 3D solitons). The
family of solitons includes single- and multi-peaked ones. The results apply to
Bose-Einstein condensates (BECs) in optical lattices (OLs), and to spatial or
spatiotemporal solitons in layered optical media. This is the first prediction
of {\em mobile} 2D and 3D solitons in BECs, as they keep mobility along .
Head-on collisions of in-phase solitons lead to their fusion into a collapsing
pulse. Solitons colliding in adjacent OL-induced channels may form a bound
state (BS), which then relaxes to a stable asymmetric form. An initially
unstable soliton splits into a three-soliton BS. Localized states in the
self-repulsive GPE with the low-dimensional OL are found too.Comment: 4 pages, 5 figure
The fundamental solution of the unidirectional pulse propagation equation
The fundamental solution of a variant of the three-dimensional wave equation
known as "unidirectional pulse propagation equation" (UPPE) and its paraxial
approximation is obtained. It is shown that the fundamental solution can be
presented as a projection of a fundamental solution of the wave equation to
some functional subspace. We discuss the degree of equivalence of the UPPE and
the wave equation in this respect. In particular, we show that the UPPE, in
contrast to the common belief, describes wave propagation in both longitudinal
and temporal directions, and, thereby, its fundamental solution possesses a
non-causal character.Comment: accepted to J. Math. Phy
Effect of annealing on the superconducting properties of a-Nb(x)Si(1-x) thin films
a-Nb(x)Si(1-x) thin films with thicknesses down to 25 {\AA} have been
structurally characterized by TEM (Transmission Electron Microscopy)
measurements. As-deposited or annealed films are shown to be continuous and
homogeneous in composition and thickness, up to an annealing temperature of
500{\deg}C. We have carried out low temperature transport measurements on these
films close to the superconductor-to-insulator transition (SIT), and shown a
qualitative difference between the effect of annealing or composition, and a
reduction of the film thickness on the superconducting properties of a-NbSi.
These results question the pertinence of the sheet resistance R_square as the
relevant parameter to describe the SIT.Comment: 9 pages, 12 figure
Modeling terahertz emissions from energetic electrons and ions in foil targets irradiated by ultraintense femtosecond laser pulses
Terahertz (THz) emissions from fast electron and ion currents driven in
relativistic, femtosecond laser-foil interactions are examined theoretically.
We first consider the radiation from the energetic electrons exiting the
backside of the target. Our kinetic model takes account of the coherent
transition radiation due to these electrons crossing the plasma-vacuum
interface as well as of the synchrotron radiation due to their deflection and
deceleration in the sheath field they set up in vacuum. After showing that both
mechanisms tend to largely compensate each other when all the electrons are
pulled back into the target, we investigate the scaling of the net radiation
with the sheath field strength. We then demonstrate the sensitivity of this
radiation to a percent-level fraction of escaping electrons. We also study the
influence of the target thickness and laser focusing. The same sheath field
that confines most of the fast electrons around the target rapidly sets into
motion the surface ions. We describe the THz emission from these accelerated
ions and their accompanying hot electrons by means of a plasma expansion model
that allows for finite foil size and multidimensional effects. Again, we
explore the dependencies of this radiation mechanism on the laser-target
parameters. Under conditions typical of current ultrashort laser-solid
experiments, we find that the THz radiation from the expanding plasma is much
less energetic -- by one to three orders of magnitude -- than that due to the
early-time motion of the fast electrons
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