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Nonlinear parametric instability in double-well lattices

A possibility of a nonlinear resonant instability of uniform oscillations in dynamical lattices with harmonic intersite coupling and onsite nonlinearity is predicted. Numerical simulations of a lattice with a double-well onsite anharmonic potential confirm the existence of the nonlinear instability with an anomalous value of the corresponding power index, 1.57, which is intermediate between the values 1 and 2 characterizing the linear and nonlinear (quadratic) instabilities. The anomalous power index may be a result of competition between the resonant quadratic instability and nonresonant linear instabilities. The observed instability triggers transition of the lattice into a chaotic dynamical state.

A possibility of a nonlinear resonant instability of uniform oscillations in dynamical lattices with harmonic intersite coupling and onsite nonlinearity is predicted. Numerical simulations of a lattice with a double-well onsite anharmonic potential confirm the existence of the nonlinear instability with an anomalous value of the corresponding power index, ≈ 1.57, which is intermediate between the values 1 and 2 characterizing the linear and nonlinear (quadratic) instabilities. The anomalous power index may be a result of a competition between the resonant quadratic instability and nonresonant linear instabilities. The observed instability triggers transition of the lattice into a chaotic dynamical state.
Dynamical lattices with onsite nonlinearity and harmonic intersite coupling constitute a vast class of models which have numerous physical application and are an object of great interest in their own right, see, e.g., Ref. 1 . Among these models, the ones with a double-well onsite potential [given, e.g., by the expression (12) below] have special importance, as they directly apply to the description of structural transitions in dielectrics, semiconductors, superconductors, and optical lattices (see recent works 2 and references therein), and find other applications 3 .
The simplest dynamical state in lattices represents spatially homogeneous oscillations. This state in conservative lattice models is sometimes stable, and sometimes it is subject to linear modulational instabilities initiating a transition to nontrivial dynamics 7 . An objective of the present work is to demonstrate analytically, and verify by direct simulations, that homogeneous oscillatory states in lattices with the double-well anharmonicity may be subject to a specific nonlinear instability, which triggers transition of the lattice into a chaotic dynamical state.
The nonlinear parametric instability, which is a subject of this work, is inherently related to the phonon anharmonism in the lattice: the instability is caused by a resonance involving the uniform oscillations of the lattice and a second harmonic of the phonon mode. A possibility of a nonlinear instability of an intrinsic localized mode 4 in the lattice due to the phonon anharmonism was first considered in Ref. 5 . An opposite, and more common, type of the resonance, namely, between a strictly linear phonon mode and a higher harmonic of an intrinsic localized mode is well known to give rise to a slow decay of the localized mode into phonons (see recent works 6 and references therein).
The general form of the lattice equation of motion is where u n are real dynamical variables on the lattice, the overdot stands for d/dt, f (u n ) is a polynomial function accounting for the onsite nonlinearity (in fact, nonpolynomial functions can be considered too), and the right-hand side of the equation accounts for the intersite harmonic coupling. The linearized version of Eq. (1) gives rise to phonon modes with an arbitrary infinitesimal amplitude A and the dispersion relation ω = 2 |sin(k/2)| inside the phonon band, ω ≤ 2 (by definition, the frequencies are positive). A homogeneous oscillatory state U 0 (t) is a timeperiodic solution to the equation with a fundamental frequency Ω. The linear stability of the homogeneous state is determined by a linearized equation for small perturbations δu n , which is produced by the substitution of u n = U 0 (t) + δu n into Eq. (1): In the mean-field approximation, one may describe phonon modes of the type (2) on top of the homogeneous oscillations, replacing the coefficient in front of δu n on the left-hand side of Eq. (4) by its time-average value . The corresponding dispersion relation for the phonon modes acquires a gap ω 0 , so that which gives rise to the phonon band (if ω 2 0 < 0, the homogeneous oscillations are immediately unstable).
Beyond this simple approximation, the Fourier decomposition of the coefficient f ′ (U 0 (t)) U 0 (t) + f (U 0 (t)) in Eq. (4) gives rise to parametrically driven terms ∼ cos(mΩt) · δu n with all integer values of m. The linear parametric drive resonates with a perturbation frequency ω, i.e., it may give rise to a resonant linear instability, under the condition mΩ − ω = ω, or If any resonant frequency (m/2)Ω gets into the phonon band (6), the homogeneous oscillatory state is expected to be modulationally unstable, otherwise the resonant linear instability does not take place.
In the latter case, it makes sense to seek for nonlinear parametric instabilities, the simplest of which may be generated by a cubic term, or any higher-order one, in the onsite nonlinearity. Indeed, the cubic term generates a nonlinear correction ∼ U 0 (t) · (δu n ) 2 to Eq. (4), which may be regarded as a parametric drive that can give rise to a nonlinear parametric resonance under the condition mΩ − 2ω = ω, or where m is an arbitrary integer different from a multiple of 3, cf. Eq. (7) (if m is a multiple of 3, the linear parametric resonance takes place at the same frequency, so that the nonlinear resonance is insignificant). Of course, this description has a very approximate nature for two reasons. First, the phonon band (6) was defined in the framework of the mean-field approximation, hence one cannot be sure in the accuracy of the predictions based on the comparison of the resonant frequencies with this band. Second, the full lattice model (1) may give rise to other instabilities, which are not related to the parametric resonance. Therefore, the above consideration should only be considered as a qualitative clue, and an actual possibility of dynamical regimes dominated by the nonlinear resonance must be checked by direct simulations.
Continuing the consideration, we note that, if none of the linear-resonance frequencies (7) gets into the renormalized band (6), but a nonlinear-resonance frequency (8) can be found inside the band, an evolution equation for the amplitude of the corresponding resonantperturbation mode, δu n = A(t) cos (ω res t)·v n , with some spatial profile v n (it may be, for instance, the abovementioned localized intrinsic mode), has a general form where C is a constant which depends on a particular form of Eq. (1) and the homogeneous solution U 0 (t); cf. a similar equation governing the nonlinear instability of the so-called embedded solitons 9 . A solution to Eq. (9) is where A 0 is the initial value of the perturbation amplitude. A drastic difference of the perturbation growth law (10) from the exponential growth in the case of the linear instability is that the nonlinear instability is initially growing much slower than an exponential, and a characteristic time scale of the growth, ∼ 1/ (CA 0 ), depends on the initial perturbation A 0 , while in the case of the exponential growth it is a fixed constant. However, the nonlinear instability is self-accelerating, and, as a manifestation of that, Eq. (10) formally predicts a singularity at t = 1/ (CA 0 ). In reality, of course, the singularity may not occur, as the above approximation, taking into regard the first nonlinear correction to Eq. (4), becomes irrelevant if A(t) is too large. A natural conjecture, that will be corroborated by direct simulations below, is that the nonlinear instability leads to a chaotic dynamical state. It is relevant to mention that, although nonlinear instabilities are less common than the usual linear instability, they occur and play an important role in many physical problems, as diverse as optical solitons in media with competing quadratic and cubic nonlinearities 9 , Bose gases, plasma turbulence, contact lines in flows, etc. 10 . In this work, we will check the possibility of the nonlinear instability of the homogeneous oscillations in the lattice model (1) with which corresponds to the double-well onsite anharmonic potential, In this case, Eq. (3) can be solved in terms of elliptic functions, but an explicit result is very cumbersome. As a typical example, we take homogeneous oscillations produced by Eq. (3) with ν = 0.01 and initial conditions U 0 (0) = 1 and . U 0 (0) = 0. The variable U 0 then performs strongly anharmonic oscillations between the values (U 0 ) min = 1 and (U 0 ) max = 12.247 at the fundamental frequency Ω = 1. 694, and the gap in the renormalized phonon spectrum (5) is calculated to be ω 0 = 3. 518, so that the renormalized band (6) is, in the mean-field approximation, 3.518 < ω < 4.047. (13) Then, it is straightforward to check that all the linearresonance frequencies (7) do not get into this band (the band as whole fits between the linear resonant frequencies 3.392 and 4.240, which correspond to m = 4 and m = 5).
On the other hand, the nonlinear-resonance frequency (8) corresponding to m = 7 is ω To directly test the instability, small perturbations of the form δu n (0) = A 0 cos(2πp 0 n/N ), where N is the net number of sites in the lattice and p 0 is an integer, were added to the homogeneous oscillatory state. The lattice equations of motion were solved for N = 1000 and periodic boundary conditions by means of the eighth-order explicit Runge-Kutta scheme with a stepsize control such that the time step was dynamically changed within the range 0.05 -0.3. It was checked that the relative (per site) error at each step did not exceed 10 −10 . The simulations were performed for the perturbations (14) with p 0 taking values in the interval 1 ≤ p 0 ≤ 30. In all the cases considered, results were quite similar. Here, we demonstrate a typical example with p 0 = 20. Longtime evolution initiated by the small perturbation (14) with A 0 = 0.05 is displayed is Fig. 1 in the form of a set of plots showing the temporal development of several components in the Fourier transform of the lattice field, which are defined as follows: It is obvious that the small perturbation triggers a transition of the lattice into a chaotic state. Fully developed chaos, i.e., a state in which all the lattice modes are involved into the chaotic motion, is attained at t ≈ 22, when the phonon mode with p = p 0 + 1 gets chaotically excited too, see Fig. 1. To further illustrate the transition to chaos, in Fig. 2 we additionally show in detail, on the logarithmic scale, the growth of the amplitude |U p0+1 (t)|. More detailed studies of the established chaotic state may be of interest in their own right, but this problem is beyond the scope of the present work. As concerns the nonlinear character of the instability, a crucial issue is the growth of the perturbation at the initial stage. It is necessary to check whether it is indeed essentially different from the familiar exponential law, being, instead, close to the Eq. (10). To this end, in Fig. 3 we display the best fit of the time evolution of the numerically computed Fourier amplitude |U p0 (t)| to a function where the parameters are found to be ∆ = 0.041, γ = 0.560 and α = 1.750. Comparison of these results with Eq. (10) shows a difference in the (most essential) power parameter α. Note that the expression ( 16) with the empirically found value α = 1.750 formally corresponds to a solution to the nonlinear evolution equation dA/dt = CA β , with an anomalous value of the power index, β ≡ 1 + α −1 ≈ 1.5714, that should be compared to Eq. ( 9), valid in case of the ordinary nonlinear instability 9 . This anomalous value is sort of intermediate between β = 1 and β = 2, which are expected for the linear and nonlinear instabilities, respectively. This result may suggest that, in fact, in the present model we have a competition between the resonant nonlinear instability, qualitatively considered above, and linear instabilities against nonresonant perturbations, which were not taken into regard in the above consideration. While an accurate analysis of the full linear stability problem of the homogeneous oscillations is a technically complex problem, that we do not aim to consider here, Fig. 3 clearly shows that the resonant nonlinear instability dominates in the growth of the perturbations.
In conclusion, we have proposed a possibility of a nonlinear resonant instability of homogeneous oscillations in harmonically coupled nonlinear lattices, which is expected to play a dominant role, provided that no resonant frequency accounting for the linear parametric resonant instability gets into the renormalized phonon band, while a frequency that gives rise to a quadratic parametric resonance is found in the band. Numerical simulations of the lattice with a double-well onsite anharmonic potential confirm the existence of nonlinear instability with an anomalous value of the power index ≈ 1.57, which is intermediate between the values 1 and 2, characteristic of the linear and nonlinear instabilities. The onset of the nonlinear instability triggers transition of the lattice into a chaotic dynamical state.
A valuable discussion with P.G. Kevrekidis is acknowledged. B.A.M. appreciates hospitality of the Department of Theoretical Physics at the Palacký University (Olomouc, the Czech Republic). E.M. acknowledges a partial support by grants No. 202/01/1450 from the agency GACR and by VEGA No. 2/7174/20.  Fig. 1. The time dependence for selected Fourier amplitudes |U p (t)|, defined as per Eq. (15). The results are shown for p = 0, p = p 0 , p = 2p 0 , and p = p 0 + 1, where p 0 = 20. The lattice size is N = 1000 (with periodic boundary conditions), and the initial amplitude of the perturbation is A 0 = 0.05. Fig. 2. Details of the evolution of the Fourier amplitude |U p0+1 (t)|, shown on the logarithmic scale. Fig. 3. Fitting the time dependence of the amplitude |U p0 (t)| to the function (16) with ∆ = 0.0411, γ = 0.560 and α = 1.750. Diamonds stand for numerical data, and stars (which almost completely overlap with the diamonds) show the closest values provided by the fitting function.