• Volume 30, Issue 1

      January 1988,   pages  1-69

    • Critical current in silver clad Y-Ba-Cu-O superconducting wires

      R G Sharma Y S Reddy S R Jha S S Dubey

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      Silver clad wires of highTc superconductor Y1Ba2Cu3O7−x have been fabricated through the powder metallurgy technique. The reacted wires show a midpointTc of 84K. A critical current density of 26·4 A cm−2 (77K, 0T) is obtained in these wires. The wires, however, turn complete normal only at a current density of 280 A cm−2. The reasons for low critical current density obtained in these wires are discussed.

    • The two-body multipole problem of electrodynamics

      Somnath Datta

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      A 2-body system composed of two objects having arbitrary distributions of charge and current is discussed. An expression for the velocity dependent potential between these two objects has been obtained in the non-relativistic approximation. This potential consists of two parts viz. a velocity independent scalar potential Φeff and another part which is linearly dependent on the relative velocity between the objects. The second part naturally suggests a vector potential Aeff. The potentials have been expanded into multipole terms. It has been found that Φeff is a sum of two components viz. ΦEE and ΦMM such that each multipole term in ΦEE represents an interaction between the electric multipoles of the two systems, each term in ΦMM represents an interaction between their magnetic multipoles whereas each term in Aeff represents an interaction between an electric multipole of one and a magnetic multipole of the other. The results have been applied to the interaction between an electric dipole and a magnetic dipole. The symmetry among the multipole terms in Aeff suggests vanishing vector potential between two identical objects. A corollary of this appears to be absence of spin orbit interaction between two identical particles in the same spin state.

    • Nonsingular cosmological models: the massive scalar field case

      B S Sathyaprakash K P Sinha

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      The nonminimal coupling of a massive self-interacting scalar field with a gravitational field is studied. Spontaneous symmetry breaking occurs in the open universe even when the sign on the mass term is positive. In contrast to grand unified theories, symmetry breakdown is more important for the early universe and it is restored only in the limit of an infinite expansion. Symmetry breakdown is shown to occur in flat and closed universes when the mass term carries a wrong sign. The model has a naturally defined effective gravitational coupling coefficient which is rendered time-dependent due to the novel symmetry breakdown. It changes sign below a critical value of the cosmic scale factor indicating the onset of a repulsive field. The presence of the mass term severely alters the behaviour of ordinary matter and radiation in the early universe. The total energy density becomes negative in a certain domain. These features make possible a nonsingular cosmological model for an open universe. The model is also free from the horizon and the flatness problems.

    • A simple apparatus for the measurement of thermoelectric power in the temperature range 4·2–300K

      K P Rajeev N Y Vasanthacharya

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      A simple apparatus to measure the absolute thermoelectric power of solids in the temperature range 4·2–300K is described. The cryostat and the associated instrumentation is simple to operate. Representative data of measurements on metallic wire and pressed pellets are given. An accuracy of better than 10% in absolute thermopower can be obtained in this apparatus.

    • Gribov-Lipatov inequality and inclusivee+e processes

      D K Choudhury A K Misra

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      We study the inclusivee+e processes at the PETRA energy range within QCD and a fixed point theory using the phenomenological Gribov-Lipatov inequality suggested in an earlier analysis. Theoretical justification is provided within QCD and its possible implication in hadronization is discussed.

    • Conformational analysis of allyl halides from the calculation of indirect spin-spin coupling

      V Santhanam J Sobhanadri S Subramaniam

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      A theoretical study on the conformation of allyl halides from the calculation of nuclear spin-spin coupling constants by adopting the finite perturbation theory (FPT), is carried out in terms of the self-consistent, semi-empirical INDO (intermediate neglect of differential overlap) approximation of molecular orbital theory. Results of the calculations performed using ‘s’ and ‘p’ valence orbitals alone (‘sp’ basis) at INDO level approximation seem to replicate the experimental trend quite satisfactorily. Despite the overall agreement of the theoretical values with the experimental ones, the uncertainties in the INDO parametrization scheme lead to overestimation of certain coupling constants. The calculations also show that the orientation of the coupled protons with respect to the substituent halogen atom is an important factor to be considered.

    • Intensity distribution in the bands of the B1 → X1Σ+ system of PbO

      N Rajamanickam

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      Relative intensities of a few bands of the B1→X1Σ+ system of PbO have been measured by the technique of heterochromatic photographic photometry. The Franck-Condon factors andr-centroids have been computed by the more reliable numerical integration procedure, using a suitable potential. The effective vibrational temperature of the source is found to be 3836 K.

    • An age-dependent model of cavity radiation and its detection II. Anti-bunched and thermal characteristics

      S K Srinivasan

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      The model of cavity radiation introduced earlier is analysed further by considering special types of age-specific population growth interpretable in terms of evolution through phases. The model is shown to be versatile enough to admit anti-bunched photon statistics provided the process of spontaneous emission is appropriately modelled. A four-phase model is analysed and the resulting radiation is shown to correspond to the one obtained by the superposition of two independent thermal streams each with a Lorentzian spectrum.

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