141 research outputs found
Elasticity effects on the stability of growing films
It is shown how the combination of atomic deposition and nonlinear diffusion
may lead, below a critical temperature, to the growth of nonuniform layers on a
substrate. The dynamics of such a system is of the Cahn-Hilliard type,
supplemented by reaction terms representing adsorption-desorption processes.
The instability of uniform layers leads to the formation of nanostructures
which correspond to regular spatial variations of substrate coverage. Since
coverage inhomogeneities generate internal stresses, the coupling between
coverage evolution and film elasticity fields is also considered, for film
thickness below the critical thickness for misfit dislocation nucleation. It is
shown that this coupling is destabilizing and favors nanostructure formation.
It also favors square planforms which could compete, and even dominate over the
haxagonal or stripe nanostructures induced by coverage dynamics alon
Convective and Absolute Instabilities in the Subcritical Ginzburg-Landau Equation
We study the nature of the instability of the homogeneous steady states of
the subcritical Ginzburg-Landau equation in the presence of group velocity. The
shift of the absolute instability threshold of the trivial steady state,
induced by the destabilizing cubic nonlinearities, is confirmed by the
numerical analysis of the evolution of its perturbations. It is also shown that
the dynamics of these perturbations is such that finite size effects may
suppress the transition from convective to absolute instability. Finally, we
analyze the instability of the subcritical middle branch of steady states, and
show, analytically and numerically, that this branch may be convectively
unstable for sufficiently high values of the group velocity.Comment: 13 pages, 10 figures (fig1.ps, fig2.eps, fig3.ps, fog4a.ps, fig
4b.ps, fig5.ps, fig6.eps, fig7a.ps, fig7b.ps, fig8.p
Patterns arising from the interaction between scalar and vectorial instabilities in two-photon resonant Kerr cavities
We study pattern formation associated with the polarization degree of freedom
of the electric field amplitude in a mean field model describing a nonlinear
Kerr medium close to a two-photon resonance, placed inside a ring cavity with
flat mirrors and driven by a coherent -polarized plane-wave field. In
the self-focusing case, for negative detunings the pattern arises naturally
from a codimension two bifurcation. For a critical value of the field intensity
there are two wave numbers that become unstable simultaneously, corresponding
to two Turing-like instabilities. Considered alone, one of the instabilities
would originate a linearly polarized hexagonal pattern whereas the other
instability is of pure vectorial origin and would give rise to an elliptically
polarized stripe pattern. We show that the competition between the two
wavenumbers can originate different structures, being the detuning a natural
selection parameter.Comment: 21 pages, 6 figures. http://www.imedea.uib.es/PhysDep
Theory for the spatiotemporal dynamics of domain walls close to a nonequilibrium Ising-Bloch transition
© 2015 American Physical Society. We derive a generic model for the interaction of domain walls close to a nonequilibrium-Bloch transition. The universal scenario predicted by the model includes stationary Ising and Bloch localized structures (dissipative solitons), as well as drifting and oscillating Bloch structures. Our theory also explains the behavior of Bloch walls during a collision. The results are confirmed by numerical simulations of the Ginzburg-Landau equation forced at twice its natural frequency and are in agreement with previous observations in several physical systems.Peer Reviewe
Wave-unlocking transition in resonantly coupled complex Ginzburg-Landau equations
We study the effect of spatial frequency forcing on standing-wave solutions of coupled complex Ginzburg-Landau equations. The model considered describes several situations of nonlinear counterpropagating waves and also of the dynamics of polarized light waves. We show that forcing introduces spatial modulations on standing waves which remain frequency locked with a forcing-independent frequency. For forcing above a threshold the modulated standing waves unlock, bifurcating into a temporally periodic state. Below the threshold the system presents a kind of excitability.Financial support from DGICYT Projects PB94-1167 and PB94-1172 is acknowledged.Peer Reviewe
Magnetization and spin-spin energy diffusion in the XY model: a diagrammatic approach
It is shown that the diagrammatic cluster expansion technique for equilibrium
averages of spin operators may be straightforwardly extended to the calculation
of time-dependent correlation functions of spin operators. We use this
technique to calculate exactly the first two non-vanishing moments of the
spin-spin and energy-energy correlation functions of the XY model with
arbitrary couplings, in the long-wavelength, infinite temperature limit
appropriate for spin diffusion. These moments are then used to estimate the
magnetization and spin-spin energy diffusion coefficients of the model using a
phenomenological theory of Redfield. Qualitative agreement is obtained with
recent experiments measuring diffusion of dipolar energy in calcium fluoride.Comment: 28 pages, 15 embedded .eps figures, Elsevier preprint forma
Numerical Solution of the Walgraef-Aifantis Model for Simulation of Dislocation Dynamics in Materials Subjected to Cyclic Loading
Abstract. Strain localization and dislocation pattern formation are typical features of plastic deformation in metals and alloys. Glide and climb dislocation motion along with accompanying production/annihilation processes of dislocations lead to the occurrence of instabilities of initially uniform dislocation distributions. These instabilities result into the development of various types of dislocation micro-structures, such as dislocation cells, slip and kink bands, persistent slip bands, labyrinth structures, etc., depending on the externally applied loading and the intrinsic lattice constraints. The Walgraef-Aifantis (WA) (Walgraef and Aifanits, J. Appl. Phys., 58, 668, 1985) model is an example of a reaction-diffusion model of coupled nonlinear equations which describe 0 formation of forest (immobile) and gliding (mobile) dislocation densities in the presence of cyclic loading. This paper discuss two versions of the WA model, the first one comprising linear diffusion of the density of mobile dislocations and the second one, with nonlinear diffusion of said variable. Subsequently, the paper focus on a finite difference, second order in time Cranck-Nicholson semi-implicit scheme, with internal iterations at each time step and a spatial splitting using the Stabilizing, Correction (Christov and Pontes, Mathematical and Computer 0, 35, 87, 2002) for solving the model evolution equations in two dimensions. The discussion on the WA model and on the numerical scheme was already presented on a conference paper by the authors (Pontes et al., AIP Conference Proceedings, Vol. 1301 pp. 511-519, 2010. The first results of four simulations, one with linear diffusion of the mobile dislocations and three with nonlinear diffusion are presented. Several phenomena were observed in the numerical simulations, like the increase of the fundamental wavelength of the structure, the increase of the walls height and the decrease of its thickness
Polarization coupling and pattern selection in a type-II optical parametric oscillator
We study the role of a direct intracavity polarization coupling in the
dynamics of transverse pattern formation in type-II optical parametric
oscillators. Transverse intensity patterns are predicted from a stability
analysis, numerically observed, and described in terms of amplitude equations.
Standing wave intensity patterns for the two polarization components of the
field arise from the nonlinear competition between two concentric rings of
unstable modes in the far field. Close to threshold a wavelength is selected
leading to standing waves with the same wavelength for the two polarization
components. Far from threshold the competition stabilizes patterns in which two
different wavelengths coexist.Comment: 14 figure
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