Sine-Gordon breathers generation in driven long Josephson junctions Claudio Guarcello∗1,2, Kirill Fedorov4,5,6, Davide Valenti1, Bernardo Spagnolo†1,2,3 and Alexey Ustinov4,5,7 1Group of Interdisciplinary Physics Università di Palermo and CNISM, Unità di 5 Palermo Viale delle Scienze, Edificio 18, 90128 Palermo, Italy 1 2Radiophysics Department, Lobachevsky State University, 23 Gagarin Avenue, 603950 0 Nizhniy Novgorod, Russia 2 3Istituto Nazionale di Fisica Nucleare, Sezione di Catania, Via S. Sofia 64, I-90123 n a Catania, Italy J 4Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Str. 1, 9 D-76131, Karlsruhe, Germany 1 5National University of Science and Technology MISiS, Leninskiy prosp. 4, Moscow, ] 119049, Russia n o 6Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, D-85748 c Garching, Germany - r 7Russian Quantum Center, Novaya str. 100, BC “Ural”, Skolkovo, Moscow region, p u 143025, Russia s . t a m January 20, 2015 - d n o c Abstract [ We consider a long Josephson junction excited by a suitable external ac-signal, in order 2 to generate control and detect breathers. Studying the nonlinear supratransmission phe- v nomenon in a nonlinear sine-Gordon chain sinusoidally driven, Geniet and Leon [12, 13] 7 3 exploredthebifurcationoftheenergytransmittedintothechainandcalculatedathreshold 0 A(ω) for the external driving signal amplitude, at which the energy flows into the system 4 by breathers modes. I numerically study the continuous sine-Gordon model, describing the 0 dynamicsofthephasedifferenceinalongJosephsonjunction,inordertodeeplyinvestigate . 1 the “continuous limit” modifications to this threshold. Wherever the energy flows into the 0 systemduetothenonlinearsupratransmission,apeculiarbreatherlocalizationareasappear 5 ina(A,ω)parametersspace. Theemergenceoftheseareasdependsonthedampingparam- 1 eter value, the bias current, and the waveform of driving external signal. The robustness : v of generated breathers is checked by introducing into the model a thermal noise source to i X mimic the environmental fluctuations. Presented results allows one to consider a cryogenic experiment for creation and detection of Josephson breathers. r a Thebehaviorofanonlinearsystemexcitedbyanexternalsinusoidaldrivingandthebifurca- tion of the energy transmission into the system were deeply studied by Geniet and Leon [12, 13]. The Floquet theorem [8] states that, for linear waves in a periodic structure, a forbidden gap ∗e-mail: [email protected] †e-mail: [email protected] 1 of frequency exists. The frequency gap between acoustic and optical branches in the vibration modes of a diatomic chain well embodies this picture. Waves with frequency within this forbid- den range esponentially vanish in the medium (evanescent waves). In nonlinear systems excited by plane waves, the appearance of gap solitons [9] and the localization of energy due to non- linear instability are matter of many systems [25, 10, 23, 15, 11, 26]. If the nonlinear medium, characterized by a forbidden band gap (FBG), is irradiated to one end by a sinusoidal drive, with frequency within the gap, the energy transmission can be supported. This phenomenon is called nonlinear supratransmission (NST) and is nowadays reported in several other contexts, from Bragg media [21, 30], coupled-wave-guide arrays (nonlinear Schrödinger model) [19, 20], optical waves guide arrays [16], Fermi-Pasta-Ulam model [17], Klein-Gordon (KG) electronic network [7], chains of coupled oscillators [24], discrete inductance-capacitance electrical line [18] to generic multicomponent nonintegrable nonlinear systems [1]. The manifestation of NST in both SG and KG equations, points out that this process is not dependent on the integrability, but is a feature of systems possessing a natural forbidden band gap. Geniet and Leon found a very simple rule proving an explicit formula for the bifurcation of the energy in a (ω,A) di- agram, called the bifurcation diagram, where ω and A are the frequency and the amplitude of the external signal, respectively. The energy flows into the medium as soon as the amplitue A of the driving, at frequency ω, exceeds the maximum amplitude of the static breather of the same frequency. This energy then travels through the system by means of plasma waves and nonlinear localized excitations, that is solitons, antisolitons and breathers. Geniet and Leon extended the results, obtained for a SG chain, to its continuous version, used to describe a long Josephson junction (LJJ) whose extremity is subjected to a sinusoidal excitation (Neumann boundary condition). The soliton solutions are strongly stable, maintaining their shape after collisions or reflections, and producing clean evidences in the I-V characteristics of the junction. A breather is another travelling SG solution formed by a soliton-antisoliton bounded couple, oscillating in an internal frame with a proper internal frequency. The magnetic flux associated with a breather is zero, as the voltage difference across the junction. Indeed, considering that the time variations of ϕ produce a voltage difference across the junction according to the a.c. Josephson relation, the rapid oscillations of the breather phase result in a mean voltage close to zero (or beyond the sensitivity of nowadays high frequencies oscilloscopes). Other pratical difficulties exist: i) a breather has to be efficently generated and trapped, for a sufficent long time, in a confined area to allow measurements, ii) it decays esponentially in time, even more so considering the damping affecting the system, and iii) an applied bias current tends to split the breather in a soliton/antisoliton couple. These are the reasons that underlie the lack of experimental evidences and confirmations of breathers in LJJ. Observations of breathers, called rotobreathers, in Josephson junctions (JJ) are reported only using “Josephson ladders”, i.e. a ladder composed by small JJs [3, 4, 31, 27, 29]. Our investigation points towards an experimen- tal setup devoted to the exclusive generation and detection of breathers in LJJ. The influence of the environment should be take into account to well describe a real experimental framework. Bodo et al [6, 7, 5] enhanced the deterministic analisys developed by Geniet and Leon in a discrete SG chain, using a sinusoidal excitation corrupted by an additive white Gaussian noise. Consideringdrivingamplitudesbelowthecriticalvalue, theyfoundthatnoiseinducedbreathers can be generated with a probability depending on the noise intensity. To check the robustness of the generated breathers, we add to the perturbed SG model a Gaussian white source, to mimic the environment effect on the breather’s dynamics. We calcu- late the percentage of surviving breather, including a white noise source of given amplitude. The continuous SG model. − The electrodynamics of a LJJ in the geometrical configuration ofoverlap,isdescribedintermsofanonlinearpartialdifferentialequationfortheorderparameter 2 ϕ, that is the sine-Gordon equation [2, 22]. Here ϕ is the phase difference between the wave functionsdescribingthesuperconductingcondensateinthetwoelectrodes. Ouranalysisincludes a quasiparticle tunneling term and an additional stochastic contribution, i (x,t), representing TN the Gaussian thermal noise effects. However, the surface resistance of the superconductors is neglected. The resulting perturbed SG equation reads ϕ (x,t)+β ϕ (x,t)−ϕ (x,t) = i (x,t)−sin(ϕ(x,t))+i (x,t), (1) tt J t xx b TN whereasimplifiednotationhasbeenused,withthesubscriptofϕindicatingthepartialderivative in that variable. This notation will be used throughout all paper. The SG equation is written in terms of the dimensionless x and t variables, that are the space and time coordinates normalized to the Josephson penetration depth λ and to the inverse of the plasma frequency ω of the J p junction, respectively. In the limit of small amplitude oscillations, the JJ plasma frequency correspondstotheoscillationfrequencyinthebottomoftheawashboardpotentialwell,modified by the presence of a bias current [2]. Damping parameter is β = (ω R C)−1, where R and J p N N C are the effective normal resistance and capacitance of the junction. The terms i (x,t) and b sin(ϕ) of Eq. (1) are the bias current and supercurrent, respectively, both normalized to the JJ critical current i . Eq. (1) is solved imposing the following boundary conditions c ϕ (0,t) = f(t) ϕ (L,t) = 0, (2) x x where f(t) is the external driving pulse used to excite the LJJ and generate the breathers. The unperturbed SG equation is ϕ (x,t)−ϕ (x,t) = sin(ϕ(x,t)). (3) xx tt and admits solutions in the traveling wave form f = ϕ(x−ut) [2], (cid:26) (cid:20) (cid:21)(cid:27) (x−ut) ϕ(x,t) = 4arctan exp ±√ , (4) 1−u2 where u is the wave propagation velocity normalized to the speed of light, and is called Swihart velocity. Eq. (4) represents a single kink, or soliton. The signs + and − indicate a 2π-kink (soliton) and a 2π-antikink (antisoliton), respectively. In this framework, ϕ gives a normal- ized measure of the magnetic flux through the junction, so that Eq. (3) can also represent the motion of a single fluxon (or antifluxon). In fact, starting from simple electrodynamic consid- erations [2], it is possible to obtain a simple relation between the magnetic field H(y) and the spatial derivative of the phase difference dH(y) Φ H ϕ = 2π , integrating over the entire JJ lenght: ϕ(L)−ϕ(0) = 2π (5) x Φ Φ 0 0 whereΦ = hc/2eisthefluxonandd = λ +λ +tisthemagneticpenetration,λ andλ arethe 0 L R L R Londondepthsintheleftandrightsuperconductorsandtistheinterlayerthickness. Ifthephase evolution shows a single 2π-kink, a single fluxon will propagate along the junction. Analytic expressionsoftravelling-wavesolutionsoftheunperturbedSGequationcanbeexplicitlywritten also for N-solitons, for kink-antikink collisions and for breathers. Also the damped and biased SG equation has slightly perturbed kink as solutions. Indeed, after a series of trasformations in zerothorder inbias and dampingparameter, thePDE describing theevolution ofthis perturbed system reduces to the unperturbed SG differential equation [14]. 3 The analytic expression of a stationary breather oscillating with frequency ω is: √   1−ω2 sin(ωt)  ϕ(x,t) = 4arctan (cid:16) √ (cid:17) . (6) ω  cosh x 1−ω2  The SG solutions are invariant under Lorentz transformations, so this stationary breather can be boosted into a moving frame, resulting in the following moving breather travelling with an envelope velocity v < 1, √   1−ω2 sin[γω(t−vx)]  ϕ(x,t) = 4arctan (cid:104) √ (cid:105) , (7) ω  cosh γ 1−ω2(t)(x−vt)  where γ = (cid:0)1−v2(cid:1)−1/2 is the Lorentz factor. The nonlinear supratransmission: from discrete to continuous SG. − Studying a discrete SG chain of N damped oscillators: u¨ −c2(u −2u +u )+sinu = −γ(n)u˙ n = 1,...,N (8) n n+1 n n−1 n n with boundary conditions u (t) = Asin(ωt) u (0) = 0 u˙ (0) = 0, (9) 0 n n where c is the coupling factor and γ the damping coefficent, Geniet and Leon [12, 13] deduced, exciting the JJ in the frequencies forbidden gap (ω < 1), a threshold A (ω): s (cid:20)c (cid:18) 1−ω2(cid:19)(cid:21) A (ω) = 4arctan arccosh 1+ , (10) s ω ω above which NST accours and the system permits the energy trasmission by means of nonlinear modes generation (breathers, kinks, antikinks). In the continuous limit, that is 1/c → 0, using the boundary condition: ∂ u| = Bsin(ωt). (11) x x=0 the resulting threshold is: B = 2(1−ω2). (12) s TheB expressioniscalculatedimposingthatthesystemadaptsitsphaseprofiletothebreather s derivative at the boundary ∂ u| = ∂ u | . Varying the position of the breather’s center to x x=0 x b x=0 maximize ∂ u | , the simple expression for the maximum amplitude, see Eq. 12, results. x b x=0 Numerical results. − The deterministic analysis is carried on setting i (x,t) = 0 in the TN Eq. 1. Two different bifurcation diagrams are shown in Fig. 1. Due to the time normalization performed for the SG model, set ω < 1 means explore the frequencies lower than the JJ plasma one. In the (A,ω) parametric space shown in Fig. 1a, the dark spots indicate (A,ω) values in correspondence of which only breathers are generated. In particular, the breathers tend to localize in branches above the bifurcation threshold, see Eq. 12, whereas are totally absent below the threshold. The grey background of Fig. 1a indicates the absence of breathers modes uniquely generated into the junction. In detail, for low (A,ω) values (below the threshold B ) s 4 (a) (b) Figure 1: a) Breathers-branches localized in a (A,ω) parametric 2D-space. b) Energy injected into the medium in a (A,ω) parametric 2D-space. thereisnoNSTeffect,whileabovethethresholdeverykindofnonlinearexcitationcanpropagate through the medium. Plasma waves, generated by the external signal and resulting from the breathers decay, are also detected with the breathers. Beetwen these breathers-branches, every combination of kinks, antikinks, breathers and plasma waves can be detected, but due to the stablenatureofsolitonsandthedampingofthemedium,afterashorttimeonlysolitonsolutions survive into the system. The branches for high amplitude values, that is A (cid:38) 1, corresponde to multiple generation of stationary and moving breathers. The results for low frequencies (∼ 0.2) and high amplitudes are consistent with Geniet and Leon’s results [13]. Changing the bias current value, the damping parameter and the duration of the external signal, modifications in the breathers arrangement come to light. The analisys of the energy injected into the system is displayed in Fig. 1b. This (A,ω) bifurcationdiagramshowsthesamebranchedstructures,suggestingmoreoverthatthebreathers tends to localize into isoenergetic regions of this diagram, in particular where the energy is slightly lower than the sorrounding areas. Indeed, the energy of a breather [28]: E = 16γ(1−ω2) (13) breather is less then the energy of a kink/antikink couple E = E = 8γ. (14) kink antikink The Fig. 1b shows also a large white area, sign of the lack of NST (no energy flows into the system). To check the robustness of the generated breathers, a nonzero noise amplitude is setted. The stochastic investigation is developed performing N = 103 numerical realizations, setting the (A,ω) values that deterministically corresponde to breathers generation, including into the SG model, see Eq. 12, a white noise contribute with amplitude γ = 10−3. The thermal TN fluctuations can supply the system with an amount of energy large enough to create kinks instead of a breather, or to break a generated breather into a kink-antikink couple. The results in the Fig. 2 shows the percentage of surviving breathers as a function of the amplitude A and the frequency ω. For large A values, little amount of thermal energy can be enough to destroy a breather. Anyway, a suitable (A,ω) values range exists, in which breathers are still generated 5 Figure 2: (Color online) Percentage of surviving breathers as a function of amplitude A and frequency ω of the external drive, setting γ =10−3 and performing N =103 numerical realizations. TN by external excitation with high probability despite of the thermal influence. Conclusions. − We explore the breathers generation and propagation in a long Josephson junction externally irradiated by a suitable excitation, changing the amplitude A and the fre- quency ω of the signal. The analysis is computationally developed in the framework of the damped and biased continuous sine-Gordon equation, inserting a Gaussian white noise source to include the environmental influence. Taking a cue from the Geniet and Leon results about the nonlinearsupratransmissioninadiscretesine-Gordonchain, wediscoveranewpeculiarlocaliza- tion of the breathers in branches in a particular (A,ω) diagram. We distinguish two parameters regions: for low values of driving frequencies and amplitudes, no energy flowing into the system is detected, otherwise every kind of nonlinear excitations travels through the junction. When this occurs, we are able to recognize where only breathers are generated. 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