• V K Chandrasekar

      Articles written in Pramana – Journal of Physics

    • Nonlinear dynamics of spin transfer nano-oscillators

      B Subash V K Chandrasekar M Lakshmanan

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      The evolution equation of a ferromagnetic spin system described by Heisenberg nearest-neighbour interaction is given by Landau–Lifshitz–Gilbert (LLG) equation, which is a fascinating nonlinear dynamical system. For a nanomagnetic trilayer structure (spin valve or pillar) an additional torque term due to spin-polarized current has been suggested by Slonczewski, which gives rise to a rich variety of dynamics in the free layer. Under appropriate conditions the spin-polarized current gives a time-varying resistance to the magnetic structure thereby inducing magnetization oscillations of frequency which lies in the microwave region. Such a device is called a spin transfer nanooscillator (STNO). However, this interesting nanoscale level source of microwaves lacks efficiency due to its low emitting power typically of the order of nWs. To over-come this difficulty, one has to consider the collective dynamics of synchronized arrays/networks of STNOs as suggested by Fert and coworkers so that the power can be enhanced 𝑁2 times that of a single STNO. We show that this goal can be achieved by applying a common microwave magnetic field to an array of STNOs. In order to make the system technically more feasible to practical level integration with CMOS circuits, we establish suitable electrical connections between the oscillators. Although the electrical connection makes the system more complex, the applied microwave magnetic field drives the system to synchronization in large regions of parameter space.

    • Symmetries of nonlinear ordinary differential equations: The modified Emden equation as a case study

      M Senthilvelan V K Chandrasekar R Mohanasubha

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      Lie symmetry analysis is one of the powerful tools to analyse nonlinear ordinary differential equations. We review the effectiveness of this method in terms of various symmetries. We present the method of deriving Lie point symmetries, contact symmetries, hidden symmetries, nonlocal symmetries, 𝜆-symmetries, adjoint symmetries and telescopic vector fields of a secondorder ordinary differential equation. We also illustrate the algorithm involved in each method by considering a nonlinear oscillator equation as an example. The connections between

      symmetries and integrating factors and

      symmetries and integrals are also discussed and illustrated through the same example.

      The interconnections between some of the above symmetries, i.e.,

      Lie point symmetries and 𝜆-symmetries and

      exponential nonlocal symmetries and 𝜆-symmetries are also discussed.

      The order reduction procedure is invoked to derive the general solution of the second-order equation.

    • Isochronous Liénard-type nonlinear oscillators of arbitrary dimensions

      Ajey K Tiwari A Durga Devi R Gladwin Pradeep V K Chandrasekar

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      In this paper, we briefly present an overview of the recent developments made in identifying/generating systems of Liénard-type nonlinear oscillators exhibiting isochronous properties, including linear, quadratic and mixed cases and their higher-order generalizations. There exists several procedures/methods in the literature to identify/generate isochronous systems. The application of local as well as nonlocal transformations and 𝛺-modified Hamiltonian method in identifying and generating systems exhibiting isochronous properties of arbitrary dimensions is also discussed in detail. The identified oscillators include singular and nonsingular Hamiltonian systems and PT-symmetric systems.

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