# Nonlinear response of the sine-Gordon breather to an ac driver

Nonlinear response of the sine-Gordon breather to an ac driver

PHYSD25982 K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12and substitute into the Matsuda identity we obtain anapproximate solution for the SG breather in the presenceof the a.c. driver. The solution in the presence ofthe driver is only approximate because the identity isonly exact for the undriven breather. However, as weshow in Section 3, the approximate CV solution forthe breather is very close to the exact simulation resultfor the a.c.-driven SG for driver amplitudes below theamplitude for the breakup of the breather into K andA. In some cases even at breakup of the breather intoK and A the CV solution is very close to the exactsimulation result. Consequently we are able to obtainquite-detailed knowledge of the nonlinear response ofthe SG solution to an a.c. driver by analyzing the solutionof a one-dimensional ordinary differential equationfor r(t) which yields the exact breather solutionwhen the a.c. driver is not present.The frequencies that appear in r(t) due to the nonlinearparticle potential, U(r), agree very well qualitativelyand quantitatively with the simulation of thefull a.c.-driven SG equation. Also the amplitude of thedriver required for the breakup of the breather into Kand A agree qualitatively and fairly well quantitatively.For the range of driver amplitudes and frequenciesused in this paper we find the intensity of the phononsradiated to be small even in the case of breakup of thebreather into K–A. The main reason that phonon radiationis small is that r(t) has to be driven nonlinearlyaway from its (nonlinear) motion in the undriven casebefore there can be radiation of phonons. Below thethreshold for breakup the radiation intensity, I = ε 2m ,where m>1 and ε is the dimensionless driver amplitudewhich we take in this paper to be in the range0.04 ≤ ε ≤ 0.2. Near breakup only a linear deviationfrom the undriven r(t) is needed to cause the radiationof the phonons observed in the simulations.We compare the frequency spectrum of the CV approximationwith the exact simulation of the drivenSG by using a wavelet description which enables usto see the frequencies that appear in both the CV approximationand the simulation of the full a.c.-drivencontinuum SG equation at different times in their evolution.For example we find that below the thresholdfor breakup both the CV approximation and the exactsimulation show a nonlinear frequency “beating” betweenthe driver and the breather which shows up inthe phonon band. We also find that in all cases whenthe exact simulation undergoes breakup into aKandA the CV approximation also breaks up into aKandA.The nonlinear response of the undamped a.c.-drivenSG breather in this paper is time dependent and containsa spectrum of frequencies with variable amplitudeswhich depend sensitively on the amplitude andfrequency of the driver. The nonlinear response contrastswith the damped a.c.-driven SG [3] where themain response is a constant shift (modulation) of theundriven breather frequency which depends on thedriver amplitudes, driver frequency and on the fixedvalue of the energy of the kink. The fixed energy is determinedby balancing the energy loss due to dampingwith the energy gain from the driver. In Section 2, wederive the equations of motion for r(t).We comparethe results for the CV approximation with the simulationof the a.c.-driven SG in Section 3. In Section 4,we summarize and conclude the paper.2. CV equations of motionWe first briefly review the results for the rigoroussingle CV theory of the breather and the K–A system.The analytic solution for the breather is[ ]φ b (x, t) = 4 tan −1 Γb cos(ωt − θ), (2.1)ω b cosh(Γ x)where the center of mass of the breather is located atzero, θ determines the initial phase, ω b is the breatherfrequency and Γ b < 1 obeys the dispersion law ωb 2 +Γb 2 = 1. From Matsuda’s identity for the breather [2],we define r(t) byr(t) ≡ 2 [ ]sinh −1 Γbcos(ωt − θ) . (2.2)Γ b ω bWhen we substitute Eq. (2.2) in the analytic solution,(2.1), and use the following identity tan −1 y −tan −1 z = tan −1 [y − z/(1 + yz)], we obtainφ(x,t)= 4 tan −1 e Γ(x−r(t)/2)−4 tan −1 e Γ(x+r(t)/2) , (2.3)UNCORRECTED PROOF

PHYSD2598where here and in the remainder of the paper we dropthe subscript b on Γ , φ and ω. Eq. (2.3) is also theexact solution for K–A when we replace Γ by γ ≡(1 − 4ṙ2 1 ) −1/2 and r(t) is given by[ ] 2r(t) = 2γ −1 sinh −1 γ ṙtsinh .ṙ 2For details, see [1]. The close relationship between theexact breather and the K–A solution follows from thefact that the breather is a bound state and the K–Ais an unbound state of the “same” potential U(Γ,r),where Γ>1 for K–A and Γ 1 is substitutedin Eq. (2.1) we get the exact K–A solution. Nextwe add the a.c. driver whose potential is V ext (r) ≡4εf (t) ∫ ∞−∞ dx{ tan−1 Γ(x−2 1 r)− tan−1 Γ(x+2 1 r)},where f(t)= ε sin Ωt.Note that dV ext /dr =−16εΓf (t) and dV ext /dΓ =0. Thus the a.c. driver drives r(t) directly. However,even if Γ was treated as a dynamical variable the a.c.driver would not drive Γ directly, i.e., Γ(t) wouldexperience the driver only through the driver’s effecton r(t).K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12 33. Comparison of CV approximation withsimulationIn this section, we analyze some typical cases of thea.c.-driven SG for the full SG partial differential equation(S) and for the CV description determined by thesolution r(t) of the ordinary differential equation (2.4)substituted in Eq. (2.3). Our purpose is to first examinethe time-dependent nonlinear response of the SGbreather to the a.c. driver and second to show how themuch simpler CV description which is completely determinedby the solution r(t) of Eq. (2.4), provides uswith information that appreciably increases our understandingof the nonlinear response of the SG breatherto an a.c. driver. In order to compare the CV calculationwith the full simulation equations (S) of theSG breather they were both iterated with a standardfifth-order Runge–Kutta method [4] and their outputcompared. Typically in the absence of a driver with atime step of 0.01 the energy is conserved to an accuracyof 0.001% over a whole simulation which lastedon the order of 300 time units. With a driver the energyis not expected to be conserved exactly, however,in all cases the energy fluctuated around a constant averageenergy which did not increase or decrease significantlyover the length of the simulations. Differentinitial phase conditions were examined and found toaffect mainly the time needed until breakup (drivingin resonance was extremely sensitive). To avoid questionsrelated to phase the driver was turned on veryslowly (over 10–20 time units) in most cases exceptwhere noted.We find that a very useful technique to present frequencyand time-dependent information in our simulationsis to use a wavelet transform [5]. The underlyingidea of the wavelet transform used in this paper issimilar to that of the “windowed” Fourier transformdefined byTf(t, ξ) =∫ ∞−∞f(x)g(x− t)e −i2πξx dx (3.1)which is the Fourier transform with a window functiong inserted, which limits the calculation of frequency toan interval of time around time t. (The window functiong can be chosen to be a Gaussian, which results inUNCORRECTED PROOF

PHYSD2598K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12 5Fig. 1. Simulation and CV calculation of the breather shape captured at maximum amplitude shortly before 300 time units. The breatherfrequency is 0.9, the driver frequency is 0.7 and the driver amplitude is 0.02 in dimensionless units.S and CV breakdown when the a.c. driver has a smallamplitude.The amplitude of the driver required for breakupinto K and A as a function of the driver frequency ispresented in Fig. 2. Except at very low driving frequencyΩ = 0.1 and resonance Ω = 0.9 the thresholdamplitude for breakup for S and CV are close toeach other. When the driving frequency is resonantwith the breather frequency the agreement with S andCV breaks down because at resonance a large numberof phonons are radiated. It is somewhat surprisingthat the agreement between S and CV at breakup awayfrom the resonance is so good. Different initial phaseswere tried and found only to affect the length of timeuntil breakup. Fig. 2 is for in-phase initial conditions.In Fig. 3a and b we show the breakup of the breatherinto K–A for S and for CV for Ω = 0.4, ω = 0.7 anddriving amplitude of 0.1. In the time before breakupS and CV agree very well and the times of breakupdiffer by only seven time steps. After breakup K andA recede from each other at the same speed and Kand A each oscillate at the driving frequency. However,it is clear that there is a difference in the speedof the K (and A) in S which is 0.17 and the speed ofthe K (and A) in CV which is 0.70. The reason forthe difference speeds in S and CV is that at breakupS radiates phonons while the CV solution cannot radiatephonons. The total energy is approximately thesame in both cases and the total rest energy is the samein both cases. Thus the energy radiated by phononsin S shows up as higher kinetic of K and A in theCV calculation. An examination of the amplitude atsmaller scales shows phonons present in the simulationcase. Most breakups were into K–A pairs but oc-UNCORRECTED PROOF

PHYSD25986 K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12Fig. 2. Driving amplitude needed to cause breakup within 300 time units with various driver frequencies for a breather with a frequencyof 0.9, driver on from the beginning. Simulation values are denoted by ×, CV calculation by +.casionally the simulation broke up into two smallerbreathers.In Fig. 4a and b we show the wavelet transform of abreather with ω = 0.9, Ω = 0.6 and amplitude = 0.1which is well below breakup. The driver is turned ongradually over 10 time units starting at t = 50. We seethat there are frequencies above the band edge around1.15 in both S and CV. These frequencies arise fromthe nonlinear breather oscillation driven by the a.c.driver. In the simulation these frequencies lead to radiationof phonons. A beating between ω and Ω canbe seen in both case with a time duration of about 10time units. (The small variations in intensity of approximatelyfive time units are an artifact of the calculationand are not physical.) There is a faint intermediatefrequency between and just below the frequency0.8 in both S and CV. Also there is a faint frequency atthe band edge in both S and CV. The CV calculationshows the breather frequency shifts downward slightlywhile the simulation does not because the simulationis radiating phonons which causes the frequency torise thus canceling the drops in frequency.In Fig. 5a and b we show the wavelet transformwith ω = 0.9, Ω = 0.8 and amplitude = 0.1 which isjust above breakup for S and just below breakup forCV with the driver turned on gradually over 10 timeunits starting at t = 50. The frequency shifts down tothe driver frequency in both S and CV. At t = 150 thebreakup of S into K and A is complete. In the timebetween t = 50 and 150 the agreement between Sand CV for frequencies at 0.9 and below is very goodwith bands around 0.8, between 0.6 and 0.7 and be-UNCORRECTED PROOF

PHYSD2598K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12 7Fig. 3. Breather breakup into kink–antikink pair (simulation (a), CV calculation (b)). Driving frequency is 0.4, breather frequency is 0.7and driving amplitude is 0.1. Speed of the kink (or antikink) after breakup is approximately 0.17 for the simulation and 0.70 for the CVcalculation.UNCORRECTED PROOF

PHYSD25988 K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12Fig. 4. Wavelet transform of the central location of the breather with initial frequency 0.9, driver frequency 0.6 and amplitude 0.1 (wellbelow breakup). The wavelet transform of the simulation is shown in (a), the CV calculation in (b). The small variations in intensity ofapproximately five time units are an artifact of the calculation and are not physical.tween 0.4 and 0.5. Since the CV solution cannot radiateor have shape mode oscillations we can reasonablyconclude that the frequencies in the phonon bandin the CV solution arise from the interaction betweenthe driver frequency and the frequencies which arisefrom the difference between the undriven and drivenr(t). The frequency spectra of the simulation and theCV calculation are nearly identical (as was the casefor all wavelet transforms for cases below breakup).As a final example of the use of the CV, r(t),toincrease our understanding of the a.c.-driven SG weconsider the cases with the same driving frequencyΩ = 0.7, breather frequency ω = 0.9 and threedifferent but close driving amplitudes of 0.119765,0.119768 and 0.119770 shown in Fig. 6. In all threecases the SG breather breaks up into K and A. In mostcases we examined the breakup occurs smoothly asin Fig. 6a. In Fig. 6b (which differs by only a rela-UNCORRECTED PROOF

PHYSD2598K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12 9Fig. 5. Wavelet transform of the central location of the breather with initial frequency 0.9, driver frequency 0.8 and amplitude 0.1 (right atbreakup for simulation (a), just below breakup for CV calculation (b)). The small variations in intensity of approximately five time unitsare an artifact of the calculation and are not physical.tive percentage of the driver amplitude from Fig. 6aof 0.0025%), we see irregular behavior for an appreciabletime before breakup. In Fig. 6c (which differsonly by a relative percentage change of the driver amplitudefrom Fig. 6b of 0.0017%), we see the developmentof a multifrequency quasiperiodic modulation ofthe breather frequency which might be signaling theonset of chaotic behavior. The three very different resultsfor three nearly equal driver amplitudes demonstratevery effectively the efficacy of using the CV,r(t), to understand the a.c.-driven SG in the breakupregion.The cases we discussed in this section are typicaland are a representative selection from the many caseswe simulated. In the region away from breakup theagreement is very good. The only significant qualitativedifference is that for S phonons can be radiatedwhich only has an appreciable effect for cases nearbreakup. For cases of breakup where phonon radiationis low there is good agreement between S and CV.UNCORRECTED PROOF

PHYSD259810 K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12Fig. 6. Plot of r(t) showing breather breakup into K–A pair for the driving frequency 0.7, breather frequency 0.9 and three differentamplitudes. (a) Driver amplitude = 0.119765; (b) driver amplitude = 0.119768; (c) driver amplitude = 0.119770. In these cases the driverwas turned on gradually over 20 time units starting at t = 10.UNCORRECTED PROOF

PHYSD25984. Discussion and conclusionsPreviously the authors of [7] investigated thebreakup of the SG breather into K and A by atime-independent constant force and obtained goodagreement between their theory and their numerical results.In a comprehensive review paper on “Solitons innearly integrable systems” [8] the authors carried outextensive investigations of several problems includingkink–antikink interactions and breather dynamics usinggeneral perturbation induced evolution equationsfor the parameters where the time dependence of theparameters arises solely from the perturbation. Theyobtained results for a range of problems from breakupof the breather into kink and antikink and for caseswith external damping and driving terms, includingthe results of [7]. The most fundamental difference ofour approach from theirs is that the CV, r(t), has timedependence even without a perturbation that gives therigorous analytic solution, using the Matsuda identity,for the breather solution to the SG partial differentialequation. Any external perturbation only produces achange in the time dependence of the solution of ther(t) equation of motion which in turn causes a changein the breather time dependence. Consequently thetime-dependent change in the spectra of r(t) due tothe perturbation directly, using the Matsuda identityleads to the change of the time-dependent spectra ofthe SG breather.The good agreement between S and CV belowbreakup of the breather into K and A for the parameterrange we use in this paper indicates the dominateeffect on the breather by the a.c. driver is to changethe distance, r(t), between the center of masses of thebound K and A that constitutes the breather from thevalue of r(t) in the undriven breather. Consequentlythe change in slope, Γ(t), is small or zero in thecases we consider. Note the spatially independent a.c.driver is very different than cases where the breatherexperiences a spatially dependent driver. In thosecases the driver often causes the slope to oscillate.The reason the slope Γ is constant or nearly constantfor the a.c. driver (away from breakup) as opposed tothe spatially dependent driver is that if you introduceΓ(t) as a CV and work out the rigorous equationsK. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12 11of motion for Γ(t) you find that the a.c. driver doesnot appear in the Γ equation of motion. The reasonis that the breather shape mode is orthogonal to thea.c. driver (see [5]). Thus the only way that Γ(t)candevelop a time dependence is through the interactionwith r(t) which is directly driven by the a.c. driver.For the parameter range we use in this paper the onlyplace where the Γ slope mode could start to oscillateis at the breakup of the breather into K and A wherethe slope is altered at breakup. Parenthetically thetime dependence of the slope in Fig. 1 is not directlydue to the a.c. driver but is a consequence of the initialcondition at t = 0 of the a.c.-driven SG equation.In [3] the authors expressed their results for thedamped a.c.-driven SG in terms of a constant frequencymodulation. We can also express our undampedresults as a frequency modulation by solvingEq. (2.2) for θ(t) in terms of r(t) and then taking thetime derivative of θ(t) which is the time-dependentfrequency modulation (which depends only on r(t)and ṙ(t)). In the present paper, we are only consideringthe undamped case so the frequency modulationis an oscillatory function of time with a rich spectra offrequencies which appear in our wavelet transforms.From the wavelet transform we see that below breakupthe frequencies that appear in the phonon band aredue to the nonlinearity in the difference between thedriven and undriven r(t) caused by the a.c. driver.The results on the a.c.-driven SG suggest a set ofproblems worth pursuing. When damping is added tothe SG in the underdamped limit there will be a rangeof phenomena such as those that appear in the treatmentsof the underdamped pendulum which would applyto the r(t) equation of motion. Then with the useof the Matsuda identity we could find the new breatherbehavior of the SG that results from the correspondingr(t) behavior. A second area of study would be theeffect of moderately stronger drivers even though theagreement would not be as good as the weaker driversquantitatively but the qualitative agreement would stillbe good. The case of damped driven SG has beentreated in [9–11] and it would be interesting to comparethose results with the present approach using CV.A fourth region worth studying further is driving atfrequencies in resonance with the breather. Finally theUNCORRECTED PROOF

PHYSD259812 K. Forinash, C.R. Willis / Physica D 2598 (2000) 1–12behavior of r(t) might be an interesting tool for investigatingthe chaotic nature of the breakup.References[1] C.D. Ferguson, C.R. Willis, Physica D 119 (1998) 283.[2] T. Matsuda, Lett. Nuovo Cimento 24 (1979) 207.[3] P.S. Lomdahl, M.R. Samuelsen, Phys. Rev. A 34 (1986)664.[4] W.H. Press, B.P. Flannery, S.A. Teukolsky, W.T. Vetterling,Numerical Recipes, Cambridge University Press, Cambridge,1989.[5] W.C. Lang, K. Forinash, Am. J. Phys. 66 (1998) 794.[6] R. Boesch, C.R. Willis, Phys. Rev. B 42 (1990) 2290.[7] P.S. Lomdahl, O.H. Olsen, M.R. Samuelsen, Phys. Rev. A29 (1984) 350.[8] Y. Kivshar, B. Malomed, Rev. Mod. Phys. 61 (1989) 763.[9] J.D. Kaup, A. Newell, Phys. Rev. B 18 (1978) 5162.[10] R. Grauer, Y. Kivshar, Phys. Rev. E 48 (1993) 4791.[11] B. Birnir, R. Grauer, Commun. Math. Phys. 162 (1994) 539.UNCORRECTED PROOF

More magazines by this user
Similar magazines