• Volume 22, Issue 5

      May 1984,   pages  387-476

    • Phase transitions of a feedback amplifier

      K Neelakantan G Venkataraman

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      The phase transition behaviour of an amplifier with positive feedback is experimentally studied. The results are interpreted using catastrophe theory language. Zero (in Gilmore’s classification), first and second-order transitions are demonstrated by driving the system along appropriate trajectories in control parameter space and the cusp and the spinodal are mapped. The fluctuations of the order parameter are investigated and their relationship to system response time established. Quench experiments analogous to those familiar in condensed matter have also been performed and with similar results.

    • Perturbation expansions and series acceleration procedures: Part-II. Extrapolation techniques

      M V Sangaranarayanan S K Rangarajan

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      Three new procedures for the extrapolation of series coefficients from a given power series expansion are proposed. They are based on (i) a novel resummation identity, (ii) parametrised Euler transformation (pet) and (iii) a modifiedpet. Several examples taken from the Ising model series expansions, ferrimagnetic systems, etc., are illustrated. Apart from these applications, the higher order virial coefficients for hard spheres and hard discs have also been evaluated using the new techniques and these are compared with the estimates obtained by other methods. A satisfactory agreement is revealed between the two.

    • Magnetization and susceptibility of disordered alloy with competitive interactions

      S K Ghatak

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      A model calculation of magnetization and susceptibility of disordered alloy (ApB1 −p) where bothA andB represent the magnetic atoms is presented. It is based on the cluster-variational method where interactions within the clusters of all possible configurations are treated exactly and the rest of the interaction is replaced by an effective variational field. The frustration effect is introduced taking the exchange interactionsJAB orJBB or both to be antiferromagnetic whereas the exchangeJAA is ferromagnetic. The results are qualitatively similar to the observed behaviour of moment and susceptibility in some metallic glasses. The critical concentration for ferromagnetic state is determined in the presence of competitive interactions.

    • Superconducting transition temperature of a paramagnetic material close to magnetic ordering

      Ranjan Chaudhury Sudhanshu S Jha

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      An explicit expression for the superconducting transition temperatureTc in a paramagnetic material is derived, when the transition occurs just before a possible magnetic ordering. As first noted by Uspenskii, such a transition may arise from electronic mechanism itself, without the necessary role played by the usual phonon-exchange mechanism. The result is discussed in terms of some recent experimental observations on the binary alloy Y9Co7.

    • Some puzzles in the results of ultrasonic attenuation in superconductors

      B K Basu

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      Two aspects of ultrasonic attenuation in superconductors are examined: (i) electron drag on dislocations and (ii) anomalous results in energy gap measurements. None of these features is physically understandable at present.

    • Dynamic calibration of shock overpressure transducers

      J Harvey J Nandakumar L V Krishnan

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      Piezoelectric transducers for dynamic overpressure measurements are commonly calibrated with static or quasistatic loads and the calibration is extrapolated to frequencies up to 30% of the resonant frequency of the piezoelectric crystal. Sinusoidal pressure generators are also used for dynamic calibration up to 500 Hz in the range of 3 MPa. This paper describes a method for dynamic calibration using transient overpressures, with rise time of 2 µsec and width 40 µsec, generated by exploding wires in air. The calibration is done in the range of 600 kPa.

    • Gas pressure regulation in a r.f. ion source

      S S Jayantha Kumar V K Chindhade V N Bhoraskar

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      An experimental technique by which devterium gas pressure in a radio frequency ion source is automatically controlled and maintained to its initial set value is described. The accuracy achieved is ± 0.5 µ in about 5 µ gas pressure being used in the r.f. ion source of the neutron generator of this laboratory.

    • Pion absorption by16O using realistic interactions—II: Free pion absorption followed by emission of proton

      U N L Mathur Y R Waghmare

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      Matrix element of the Gallilean invariant nonrelativistic reduction of the pseudoscalar-pseudovector interaction has been calculated for free pion absorption by a single nucleon inside the nucleus of16O. The Hartree-Fock wavefunctions obtained with the unitary-model-operator approach starting with the hard-core nucleon-nucleon interaction have been used for the π-capturing nucleon in the initial state. The initial pion distortion in the presence of nuclear field of the absorbing nucleus prior to its absorption together with the Coulomb interaction with the finite nuclear size has been taken into account. The distortion of the emitted proton in the field of the residual nucleus has also been considered. The differential cross-sections have been obtained and calculated results are compared with the previous experimental and theoretical work.

    • Structure study of simple fluids

      B B Deo A C Naik

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      The structure function of simple monatomic liquids like neon and argon is studied in an approximation scheme where intermediate functionQ(r) is extended beyond hardcore diameter rather than the direct correlation functionC(r). The calculated values show good agreement with experimental values.

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