Switching near a network of rotating nodes

We study the dynamics of a Z ₂ ⊕ Z ₂-equivariant vector field in the neighbourhood of a heteroclinic network with a periodic trajectory and symmetric equilibria. We assume that around each equilibrium the linearization of the vector field has non-real eigenvalues. Trajectories starting near each node of the network turn around in space either following the periodic trajectory or due to the complex eigenvalues near the equilibria. Thus, in a network with rotating nodes, the rotations combine with transverse intersections of two-dimensional invariant manifolds to create switching near the network; close to the network, there are trajectories that visit neighbourhoods of the saddles following all the heteroclinic connections of the network in any given order. Our results are motivated by an example where switching was observed numerically by forced symmetry breaking of an asymptotically stable network with O(2) symmetry.     

The effect of symmetry breaking on the dynamics near a structurally stable heteroclinic cycle between equilibria and a periodic orbit

The effect of small forced symmetry breaking on the dynamics near a structurally stable heteroclinic cycle connecting two equilibria and a periodic orbit is investigated. This type of system is known to exhibit complicated, possibly chaotic dynamics including irregular switching of sign of various phase space variables, but details of the mechanisms underlying the complicated dynamics have not previously been investigated. We identify global bifurcations that induce the onset of chaotic dynamics and switching near a heteroclinic cycle of this type, and by construction and analysis of approximate return maps, locate the global bifurcations in parameter space. We find there is a threshold in the size of certain symmetry-breaking terms, below which there can be no persistent switching. Our results are illustrated by a numerical example.     

Stability of quasi-simple heteroclinic cycles

The stability of heteroclinic cycles may be obtained from the value of the local stability index along each connection of the cycle. We establish a way of calculating the local stability index for quasi-simple cycles: cycles whose connections are one-dimensional and contained in flow-invariant spaces of equal dimension. These heteroclinic cycles exist both in symmetric and non-symmetric contexts. We make one assumption on the dynamics along the connections to ensure that the transition matrices have a convenient form. Our method applies to all simple heteroclinic cycles of type Z and to various heteroclinic cycles arising in population dynamics, namely non-simple heteroclinic cycles, as well as to cycles that are part of a heteroclinic network. We illustrate our results with a non-simple cycle present in a heteroclinic network of the Rock–Scissors–Paper game.     

Closed trajectories in planar relay feedback systems

A planar system of piecewise linear differential equations with a line of discontinuity, ?₂–symmetry and a linear part having negative determinant is investigated. Using the theory of differential inclusions and an appropriate Poincaré map a complete analysis is provided. This analysis applies to uniqueness and non-uniqueness for the initial value problem, stability of stationary points, sliding motion solutions, number and stability of periodic trajectories and existence of pairs of heteroclinic trajectories connecting two saddle points forming heteroclinic cycles. A complete bifurcation diagram is given.     

Self-pulsing in a Laser Model with Discrete Gain and Loss

Abstract

A new model of a multimode homogeneously broadened unidirectional ring cavity laser is presented. Gain and loss are modelled as discrete elements taking the form of a thin laser medium of negligible extent and a partially reflecting mirror forming a cavity with a finite round-trip time. In this way, an exact analytic solution to the stability problem of the c.w. state can be obtained. It is shown that a second threshold exists for any mirror transmissivity and, when using a semitransparent mirror, that it can be reached by pumping seven times above laser threshold. Numerical solutions are presented, including periodic, quasi-periodic and chaotic oscillations. Well known mean-field limit self-pulsations are here predicted for any mirror transmissivity and suitable pump parameter, and are shown to be particular cases of a more general class of instabilities involving excitation of one or more cavity modes by Rabi frequency modulation; the resulting dynamics reflect a complex interaction among the excited modes and the Rabi frequency modulation. The present model predicts, in particular, transition to chaos from a frequency locked limit cycle or from a two-dimensional torus.     

Dynamical properties and synchronization of complex non-linear equations for detuned lasers

We study the dynamics and synchronization properties of a system of complex non-linear equations describing detuned lasers. These equations possess a whole circle of fixed points, while the corresponding real variable equations have only isolated fixed points. We examine the stability of their equilibrium points and determine conditions under which the complex equations have positive, negative or zero Lyapunov exponents and chaotic, quasiperiodic or periodic attractors for a wide range of parameter values. We investigate the synchronization of chaotic solutions of our detuned laser system, using as a drive a similar set of equations and applying the method of global synchronization. We find attractors whose three-dimensional projection is not at all similar to the well-known shape of the (real) Lorenz attractor. Finally, we apply complex periodic driving to the electric field equation and show that the model can exhibit a transition from chaotic to quasiperiodic oscillations. This leads us to the discovery of an exact periodic solution, whose amplitude and frequency depend on the parameters of the system. Since this solution is stable for a wide range of parameter values, it may be used to control the system by entraining it with the applied periodic forcing.     

Nonreversible homoclinic snaking

Homoclinic snaking refers to the sinusoidal ‘snaking’ continuation curve of homoclinic orbits near a heteroclinic cycle connecting an equilibrium E and a periodic orbit P. Along this curve the homoclinic orbit performs more windings about the periodic orbit. Typically, this behaviour appears in reversible Hamiltonian systems. Here we discuss this phenomenon in systems without any particular structure. We give a rigorous analytical verification of homoclinic snaking under certain assumptions on the behaviour of the stable and unstable manifolds of E and P. We show how the snaking behaviour depends on the signs of the Floquet multipliers of P. Further we present a nonsnaking scenario. Finally, we show numerically that these assumptions are fulfilled in a model equation.     

Heteroclinic cycles and wreath product symmetries

We consider the existence and stability of heteroclinic cycles arising by local bifurcation in dynamical systems with wreath product symmetry = Z 2 G, where Z 2 acts by ±1 on R and G is a transitive subgroup of the permutation group S N (thus G has degree N). The group acts absolutely irreducibly on R N . We consider primary (codimension one) bifurcations from an equilibrium to heteroclinic cycles as real eigenvalues pass through zero. We relate the possibility of such cycles to the existence of non-gradient equivariant vector fields of cubic order. Using Hilbert series and the software package MAGMA we show that apart from the cyclic groups G (previously studied by other authors) only five groups G of degree ≤7 are candidates for the existence of heteroclinic cycles. We establish the existence of certain types of heteroclinic cycle in these cases by making use of the concept of a subcycle. We also discusss edge cycles, and a generalization of heteroclinic cycles which we call a heteroclinic web. We apply our method to three examples.     

Dynamics of surface diffeomorphisms relative to homoclinic and heteroclinic orbits

We show how to obtain information about the dynamics of a two-dimensional discrete-time system from its homoclinic and heteroclinic orbits. The results obtained are based on the theory of 'trellises', which comprise finite-length subsets of the stable and unstable manifolds of a collection of saddle periodic orbits. For any collection of homoclinic or heteroclinic orbits, we show how to associate a canonical 'trellis type' which describes the orbits. Given a trellis type, we show how to compute a 'graph representative' which gives a combinatorial invariant of the trellis type. The orbits of the graph give the dynamics forced by the homoclinic/heteroclinic orbits in the sense that every orbit of the graph representative is 'globally shadowed' by some orbit of the system, and periodic, homoclinic/heteroclinic orbits of the graph representative are shadowed by similar orbits.     

A codimension-two resonant bifurcation from a heteroclinic cycle with complex eigenvalues

Robust heteroclinic cycles between equilibria lose stability either through local bifurcations of their equilibria or through global bifurcations. This paper considers a global loss of stability termed a 'resonant' bifurcation. This bifurcation is usually associated with the birth or death of a nearby periodic orbit, and generically occurs in either a supercritical or subcritical manner. For a specific robust heteroclinic cycle between equilibria with complex eigenvalues we examine the codimension-two point that separates the supercritical and subcritical. We investigate the bifurcation structure and show the existence of further bifurcations of periodic orbits.     

The route from order to chaos of soil moisture dynamics at large spatial scales

Abstract

This study analyzes the relationship between soil moisture and lagged soil moisture according to the nonlinear soil moisture balance equation. The phase plane diagram and chaotic analysis show that the phenomena of the nonlinear dynamics equation, such as fixed point, limit cycle, and chaotic type of behavior, will become quite complex. At given parameters and lagged time, the limit evolution of soil moisture whose time delay is equal to 21 days is a strange attractor and the fractal dimension is 1.56. The ultimate soil moisture evolution is through a period doubling route from order to chaos.     

Multiple periodic solutions with prescribed minimal period to second-order Hamiltonian systems

A new variational method based on Ekeland’s principle is introduced for the existence, localization and multiplicity of periodic solutions of a prescribed minimal period to second-order Hamiltonian systems. The oscillatory property at zero or infinity of only one component of the gradient of the potential function is sufficient for the existence of infinitely many solutions. Also, oscillating properties of several components of the gradient of the potential function yield sequences of solutions with some of the components tending in norm to zero and others to infinity.     

Chaos in the Takens–Bogdanov bifurcation with O(2) symmetry

The Takens–Bogdanov bifurcation is a codimension-two bifurcation that provides a key to the presence of complex dynamics in many systems of physical interest. When the system is translation-invariant in one spatial dimension with no left-right preference the imposition of periodic boundary conditions leads to the Takens–Bogdanov bifurcation with O(2) symmetry. This bifurcation, analyzed by G. Dangelmayr and E. Knobloch, Phil. Trans. R. Soc. London A 322, 243 (1987), describes the interaction between steady states and travelling and standing waves in the nonlinear regime and predicts the presence of modulated travelling waves as well. The analysis reveals the presence of several global bifurcations near which the averaging method (used in the original analysis) fails. We show here, using a combination of numerical continuation and the construction of appropriate return maps, that near the global bifurcation that terminates the branch of modulated travelling waves, the normal form for the Takens–Bogdanov bifurcation admits cascades of period-doubling bifurcations as well as chaotic dynamics of Shil'nikov type. Thus chaos is present arbitrarily close to the codimension-two point.     

Investigations on the Magnetization Switching Dynamics in Spin Valve Multilayers Under Periodic Applied Magnetic Field

Magnetization switching dynamics in a spin valve nanopillar, induced by spin transfer torque in the presence of a periodic applied field is investigated by solving the Landau–Lifshitz–Gilbert–Slonczewski equation. Under steady state conditions, the switching of magnetization occurs in the system, above a threshold current density value J _c. A general expression for the critical current density is derived and it is shown that this further reduces when there is magnetic interface anisotropy present in the free layer of the spin valve. We also investigated the chaotic behavior of the free layer magnetization vector in a periodically varying applied magnetic field, in the presence of a constant DC magnetic field and spin current. Further, it is found that in the presence of a nonzero interfacial anisotropy, chaotic behavior is observed even at much smaller values of the spin current and DC applied field.     

Sources of uncertainty in deterministic dynamics: an informal overview

The discovery of chaotic dynamics implies that deterministic systems may not be predictable in any meaningful sense. The best-known source of unpredictability is sensitivity to initial conditions (popularly known as the butterfly effect), in which small errors or disturbances grow exponentially. However, there are many other sources of uncertainty in nonlinear dynamics. We provide an informal overview of some of these, with an emphasis on the underlying geometry in phase space. The main topics are the butterfly effect, uncertainty in initial conditions in non-chaotic systems, such as coin tossing, heteroclinic connections leading to apparently random switching between states, topological complexity of basin boundaries, bifurcations (popularly known as tipping points) and collisions of chaotic attractors. We briefly discuss possible ways to detect, exploit or mitigate these effects. The paper is intended for non-specialists.     

Quantum network architecture of tight-binding models with substitution sequences

Abstract

We study a two-spin quantum Turing architecture, in which discrete local rotations {α_m} of the Turing head spin alternate with quantum controlled NOT operations. Substitution sequences are known to underlie aperiodic structures. We show that parameter inputs {α_m} described by such sequences can lead here to a quantum dynamics, intermediate between the regular and the chaotic variant. Exponential parameter sensitivity characterizing chaotic quantum Turing machines turns out to be an adequate criterion for induced quantum chaos in a quantum network.     

Multi-cluster dynamics in coupled phase oscillator networks

In this paper, we examine robust clustering behaviour with multiple nontrivial clusters for identically and globally coupled phase oscillators. These systems are such that the dynamics is completely determined by the number of oscillators N and a single scalar function g(?) (the coupling function). Previous work has shown that (a) any clustering can stably appear via choice of a suitable coupling function and (b) open sets of coupling functions can generate heteroclinic network attractors between cluster states of saddle type, though there seem to be no examples where saddles with more than two nontrivial clusters are involved. In this work, we clarify the relationship between the coupling function and the dynamics. We focus on cases where the clusters are inequivalent in the sense of not being related by a temporal symmetry, and demonstrate that there are coupling functions that give robust heteroclinic networks between periodic states involving three or more nontrivial clusters. We consider an example for N = 6 oscillators where the clustering is into three inequivalent clusters. We also discuss some aspects of the bifurcation structure for periodic multi-cluster states and show that the transverse stability of inequivalent clusters can, to a large extent, be varied independently of the tangential stability.     

Bounded solutions for non-autonomous parabolic equations

The existence of bounded solutions (including in particular homoclinic and heteroclinic solutions) is studied for non-autonomous perturbed parabolic partial differential equations, without the restriction that the linear variational equation has a unique non-trivial bounded solution. Specifically, an idea applied to ordinary differential equations by Hale (1984) and by Battelli and Laari (1990) is realised in an infinite-dimensional setting. Like other work on related problems, the main technique is Lyapunov?Schmidt reduction; we use that technique here in the context of bounded solutions, rather than the more usual setting of periodic or homoclinic solutions. Moreover, several technical obstacles are circumvented in the infinite-dimensional setting?in particular in the proof of the existence of a solution to the reduced bifurcation equation. Non-uniqueness is shown to occur for the Kuramoto-Sivashinsky equation, demonstrating the need to remove the uniqueness restriction     

Topological equivalence in the theory of trapped ions

Abstract

We consider an ion whose motion is confined to a plane perpendicular to a constant magnetic field. A harmonic trapping field is applied within the plane of the ion's motion, together with some unspecified irradiating probe beam. By determining the dynamics of the ion we show that the particle theory is formally equivalent to that of a coupled massive (1 + 2)-dimensional vector Boson field. The field characteristics of current and Proca mass take on the roles of the interaction and harmonic potential respectively of the ion. The topological mass appropriate to the restricted configuration space, which in the case of the vector Boson field is given by the Chern-Simons term, is provided by the constant magnetic field.     

Controlling chaotic dynamics of periodically forced spheroids in simple shear flow: Results for an example of a potential application

Recently, we studied the technologically important problem of periodically forced spheroids in simple shear flow and demonstrated the existence of chaotic parametric regimes. Our results indicated a strong dependence of the solutions obtained on the aspect ratio of the spheroids, which can be used to separate particles from a suspension. In this paper we demonstrate that controlling the chaotic dynamics of periodically forced particles by a suitably engineered novel control technique, which needs little information about the system and is easy to implement, leads to the possibility of better separation. Utilizing the flexibility of controlling chaotic dynamics in a desired orbit irrespective of initial state, we show that it is theoretically possible to separate particles much more efficiently than otherwise from a suspension of particles having different shapes but similar sizes especially for particles of aspect ratior _e >1.0. The strong dependence of the controlled orbit on the aspect ratio of the particles may have many applications such as in the development of computer-controlled intelligent rheology. The results suggest that control of chaos as discussed in this work may also have many applications. A list of symbols is given at the end of the paper