220 research outputs found
Discrete breathers in a two-dimensional Fermi-Pasta-Ulam lattice
Using asymptotic methods, we investigate whether discrete
breathers are supported by a two-dimensional Fermi-Pasta-Ulam lattice. A scalar (one-component) two-dimensional
Fermi-Pasta-Ulam lattice is shown to model the charge stored
within an electrical transmission lattice. A third-order multiple-scale analysis in the semi-discrete limit fails, since at this order, the lattice equations reduce to the (2+1)-dimensional cubic nonlinear Schrödinger (NLS) equation which does not support stable soliton solutions for the breather envelope. We therefore extend
the analysis to higher order and find a generalised
-dimensional NLS equation which incorporates higher order dispersive and nonlinear terms as perturbations. We find an ellipticity criterion for the wave numbers of the carrier wave. Numerical simulations suggest that both stationary and moving breathers are supported by the system. Calculations of the energy show the expected threshold behaviour whereby the energy of breathers does {\em not} go to zero with the amplitude; we find
that the energy threshold is maximised by stationary breathers, and becomes arbitrarily small as the boundary of the domain of ellipticity is approached
Asymptotic analysis of combined breather-kink modes in a Fermi-Pasta-Ulam chain
We find approximations to travelling breather solutions of the
one-dimensional Fermi-Pasta-Ulam (FPU) lattice. Both bright
breather and dark breather solutions are found. We find that the
existence of localised (bright) solutions depends upon the
coefficients of cubic and quartic terms of the potential energy,
generalising an earlier inequality derived by James [CR Acad Sci
Paris 332, 581, (2001)]. We use the method of multiple scales to
reduce the equations of motion for the lattice to a nonlinear
Schr{\"o}dinger equation at leading order and hence construct an
asymptotic form for the breather. We show that in the absence of
a cubic potential energy term, the lattice supports combined
breathing-kink waveforms. The amplitude of breathing-kinks can be
arbitrarily small, as opposed to traditional monotone kinks, which
have a nonzero minimum amplitude in such systems. We also present
numerical simulations of the lattice, verifying the shape and
velocity of the travelling waveforms, and confirming the
long-lived nature of all such modes
Asymptotic analysis of combined breather-kink modes in a Fermi-Pasta-Ulam chain
We find approximations to travelling breather solutions of the
one-dimensional Fermi-Pasta-Ulam (FPU) lattice. Both bright
breather and dark breather solutions are found. We find that the
existence of localised (bright) solutions depends upon the
coefficients of cubic and quartic terms of the potential energy,
generalising an earlier inequality derived by James [CR Acad Sci
Paris 332, 581, (2001)]. We use the method of multiple scales to
reduce the equations of motion for the lattice to a nonlinear
Schr{\"o}dinger equation at leading order and hence construct an
asymptotic form for the breather. We show that in the absence of
a cubic potential energy term, the lattice supports combined
breathing-kink waveforms. The amplitude of breathing-kinks can be
arbitrarily small, as opposed to traditional monotone kinks, which
have a nonzero minimum amplitude in such systems. We also present
numerical simulations of the lattice, verifying the shape and
velocity of the travelling waveforms, and confirming the
long-lived nature of all such modes
Discrete breathers in a two-dimensional Fermi-Pasta-Ulam lattice
Using asymptotic methods, we investigate whether discretebreathers are supported by a two-dimensional Fermi-Pasta-Ulam lattice. A scalar (one-component) two-dimensionalFermi-Pasta-Ulam lattice is shown to model the charge storedwithin an electrical transmission lattice. A third-order multiple-scale analysis in the semi-discrete limit fails, since at this order, the lattice equations reduce to the (2+1)-dimensional cubic nonlinear Schrödinger (NLS) equation which does not support stable soliton solutions for the breather envelope. We therefore extendthe analysis to higher order and find a generalised-dimensional NLS equation which incorporates higher order dispersive and nonlinear terms as perturbations. We find an ellipticity criterion for the wave numbers of the carrier wave. Numerical simulations suggest that both stationary and moving breathers are supported by the system. Calculations of the energy show the expected threshold behaviour whereby the energy of breathers does {\em not} go to zero with the amplitude; we findthat the energy threshold is maximised by stationary breathers, and becomes arbitrarily small as the boundary of the domain of ellipticity is approached
Discrete breathers in a two-dimensional hexagonal Fermi-Pasta-Ulam lattice
We consider a two-dimensional Fermi-Pasta-Ulam (FPU) latticewith hexagonal symmetry. Using asymptotic methods based onsmall amplitude ansatz, at third order we obtain a eduction to a cubic nonlinear Schr{\"o}dinger equation (NLS) for the breather envelope. However, this does not support stable soliton solutions, so we pursue a higher-order analysis yielding a generalised NLS, which includes known stabilising terms. We present numerical results which suggest that long-lived stationary and moving breathersare supported by the lattice. We find breather solutions which move in an arbitrary direction, an ellipticity criterion for the wavenumbers of the carrier wave, symptotic estimates for the breather energy,and a minimum threshold energy below which breathers cannot be found. This energy threshold is maximised for stationary breathers, and becomes vanishingly small near the boundary of the elliptic domain where breathers attain a maximum speed. Several of the results obtained are similar to those obtained for the square FPU lattice (Butt \& Wattis, {\em J Phys A}, {\bf 39}, 4955, (2006)), though we find that the square and hexagonal lattices exhibit different properties in regard to the generation of harmonics, and the isotropy of the generalised NLS equation
Asymptotic analysis of combined breather-kink modes in a Fermi-Pasta-Ulam chain
We find approximations to travelling breather solutions of theone-dimensional Fermi-Pasta-Ulam (FPU) lattice. Both brightbreather and dark breather solutions are found. We find that theexistence of localised (bright) solutions depends upon thecoefficients of cubic and quartic terms of the potential energy,generalising an earlier inequality derived by James [CR Acad SciParis 332, 581, (2001)]. We use the method of multiple scales toreduce the equations of motion for the lattice to a nonlinearSchr{\"o}dinger equation at leading order and hence construct anasymptotic form for the breather. We show that in the absence ofa cubic potential energy term, the lattice supports combinedbreathing-kink waveforms. The amplitude of breathing-kinks can bearbitrarily small, as opposed to traditional monotone kinks, whichhave a nonzero minimum amplitude in such systems. We also presentnumerical simulations of the lattice, verifying the shape andvelocity of the travelling waveforms, and confirming thelong-lived nature of all such modes
Precision X-ray spectroscopy of kaonic atoms as a probe of low-energy kaon-nucleus interaction
In the exotic atoms where one atomic electron is replaced by a ,
the strong interaction between the and the nucleus introduces an energy
shift and broadening of the low-lying kaonic atomic levels which are determined
by only the electromagnetic interaction. By performing X-ray spectroscopy for
Z=1,2 kaonic atoms, the SIDDHARTA experiment determined with high precision the
shift and width for the state of and the state of kaonic
helium-3 and kaonic helium-4. These results provided unique information of the
kaon-nucleus interaction in the low energy limit.Comment: 4 pages, 1 figure, proceedings for oral presentation at the ICNFP2015
conference, Kolymbari, Cret
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
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