• Deepak Kumar

      Articles written in Pramana – Journal of Physics

    • Two site spin correlation function in Bethe-Peierls approximation for Ising model

      Deepak Kumar

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      Two site spin correlation function for an Ising model above Curie temperature has been calculated by generalising Bethe-Peierls approximation. The results derived by a graphical method due to Englert are essentially the same as those obtained earlier by Elliott and Marshall, and Oguchi and Ono. The earlier results were obtained by a direct generalisation of the cluster method of Bethe, while our results are derived by retaining that class of diagrams, which is exact on Bethe lattice.

    • Susceptibility and magnetisation of a random ising model

      Deepak Kumar Vipin Srivastava

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      The susceptibility of a bond disordered Ising model is calculated by configurationally averaging an Ornstein-Zernike type of equation for the two spin correlation function. The equation for the correlation function is derived using a diagrammatic method due to Englert. The averaging is performed using bond CPA. The magnetisation is also calculated by averaging in a similar manner a linearised molecular field equation.

    • Dynamical scaling for spin waves in dilute ferromagnets

      Deepak Kumar

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      The ambiguity in the application of the principle of dynamic scaling to spin waves in dilute ferromagnets is resolved by taking account of the fractal nature of the infinite percolation cluster.

    • Influence of oxygen deficiency on the thermoelectric power of Y1Ba2Cu3O7-x

      S R Jha Y S Reddy D K Suri K D Kundra R G Sharma Deepak Kumar

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      In this paper we report measurements of the thermoelectric power on a series of Y1Ba2Cu3O7-x specimens with varying amounts of oxygen deficiency obtained by changing the cooling rate of the sintered specimens. The specimens have been characterized by X-ray diffraction measurements, electric resistivity and oxygen contents. The temperature variation of the thermopower reveals a peak just before the onset of superconducting transition. We examine possible theoretical explanations of this anomaly. In particular we argue that this anomaly is associated with the pairing fluctuations in the normal state close toTc. We present some theoretical results in support of this conclusion.

    • A new formalism for the statistical dynamics of planar spin systems

      Kamlesh Kumari Deepak Kumar

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      The relaxational dynamics of a classical planar Heisenberg spin system is studied using the Fokker-Planck equation. A new approach is introduced in which we attempt to directly calculate the eigenvalues of the Fokker-Planck operator. In this connection a number space representation is introduced, which enables us to visualize the eigenvalue structure of the Fokker-Planck operator. The mean field approximation is derived and a systematic method to improve the mean field approximation is presented.

    • Optical mode-coupling in a ring due to a back-scatterer

      A Venugopalan Deepak Kumar R Ghosh

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      The coupling of light waves travelling clockwise and counterclockwise along an optical ring due to a back-scattering element is studied. An asymmetric mode splitting occurs as a consequence of the discontinuity suffered by the waves at the scattering point. The mode splitting shows up in an interference pattern and lends itself to an experimental verification.

    • A new formalism for the statistical dynamics of vector spin systems

      Kamlesh Kumari Deepak Kumar

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      The relaxational dynamics of a classical vector Heisenberg spin system is studied using the Fokker-Planck equation. To calculate the eigenvalues of the Fokker-Planck operator, a new approach is introduced. In this connection, a number space repesentation is introduced, which enables us to visualize the eigenvalue structure of the Fokker-Planck operator. The mean field approximation is derived and a systematic method to improve the mean field approximation is presented.

    • Measurement problem in quantum mechanics: Characteristics of an apparatus

      Deepak Kumar

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      The problem of measurement in Quantum Mechanics will be briefly reviewed. Since the measurement process involves a macroscopic apparatus, the attention is focussed on the dynamics of a pointer-like variable of the apparatus when it interacts with a quantum system. It is argued that since the measurement process requires an apparent collapse of the wave function in a certain basis, and collapse is an irreversible process, understanding of irreversibility in a quantum macroscopic system is crucial. The chief characteristics of an apparatus that are important in understanding measurement process are (a) its closely spaced energy levels and (b) its interaction with environment. The coupling with the environment drives the density matrix of the apparatus to diagonal form, but to have persistent correlations between system and apparatus states, it seems necessary to have a pointer variable that has a classical limit

    • Photon states in anisotropic media

      Deepak Kumar

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      Quantum aspects of optical polarization are discussed for waves traveling in anisotropic dielectric media with a view to relate the dynamics of polarization with that of photon spin and its manipulation by classical polarizers.

    • Analysis of adiabatic transfer in cavity quantum electrodynamics

      Joyee Ghosh R Ghosh Deepak Kumar

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      A three-level atom in a 𝛬 configuration trapped in an optical cavity forms a basic unit in a number of proposed protocols for quantum information processing. This system allows for efficient storage of cavity photons into long-lived atomic excitations, and their retrieval with high fidelity, in an adiabatic transfer process through the ‘dark state’ by a slow variation of the control laser intensity. We study the full quantum mechanics of this transfer process with a view to examine the non-adiabatic effects arising from inevitable excitations of the system to states involving the upper level of 𝛬, which is radiative. We find that the fidelity of storage is better, the stronger the control field and the slower the rate of its switching off. On the contrary, unlike the adiabatic notion, retrieval is better with faster rates of switching on of an optimal control field. Also, for retrieval, the behaviour with dissipation is non-monotonic. These results lend themselves to experimental tests. Our exact computations, when applied to slow variations of the control intensity for strong atom–photon couplings, are in very good agreement with Berry’s superadiabatic transfer results without dissipation.

    • Performance evaluation of self-breakdown-based single-gap plasma cathode electron gun

      Niraj Kumar Nalini Pareek Udit Narayan Pal Deepak Kumar Verma Jitendra Prajapati Mahesh Kumar Bharat Lal Meena Ram Prakash

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      This paper presents the experimental studies on self-breakdown-based single-gap plasma cathode electron (PCE) gun (5–20 kV/50–160 A) in argon, gas atmosphere and its performance evaluation based on particle-in-cell (PIC) simulation code `OOPIC-Pro’.The PCE-Gun works in conducting phase (low energy, high current) of pseudospark discharge. It produces an intense electron beam, which can propagate more than 200 mm in the drift space region without external magnetic field. The profile of this beam in the drift space region at different breakdown conditions (i.e., gas pressures and applied voltages) has been studied and the experimental results are compared with simulated values. It is demonstrated that ∼30% beam current is lost during the propagation possibly due to space charge neutralization and collisions with neutral particles and walls.

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