• Volume 15, Issue 3

      September 1980,   pages  225-326

    • Space-time singularities and microwave background radiation

      P S Joshi

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      A general relativistic space-time universe is considered together with a radiation such as the microwave background radiation. It is shown that if certain reasonable conditions are satisfied, then the presence of such a radiation would imply space-time singularities in the sense that all time-like curves will be incomplete in the past. The considerations provide an upperbound to the age of the universe, which is consistent with present data.

    • Effect of destabilizing fields on hydrodynamic instabilities in nematics

      U D Kini

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      The effect of destabilizing fields on the roll instability (RI) threshold for shear flow and on the homogeneous instability (HI) threshold for plane Poiseuille flow of nematic HBAB (μs>0) is studied on the basis of the continuum theory of nematics for flow cells of infinite lateral width. It turns out that the critical shear rate and wave vector at RI threshold decrease with increasing destabilizing field but do not approach zero at the Freedericksz transition. However calculations show that beyond the Freedericksz threshold HI may be favourable over a range of destabilizing field with shear in the stabilizing role. For plane Poiseuille flow a similar analysis points to the existence of a HI threshold in the presence of destabilizing field beyond the Freedericksz threshold again with shear acting as a stabilizing field. These results are compared with theoretical results obtained previously for MBBA.

    • Classical φ6-field theory in (1+1) dimensions. A model for structural phase transitions

      S N Behera Avinash Khare

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      The classical φ6-field theory in (1+1) dimensions, is considered as a model for the first order structural phase transitions. The equation of motion is solved exactly; and the presence of domain wall (kink) solutions at and below the transition point, in addition to the usual phonon-like oscillatory solutions, is demonstrated. The domain wall solutions are shown to be stable, and their mass and energies are calculated. Above the transition point there exists exotic unstable kink-like solutions which takes the particle from one hill top to the other of the potential. The partition function of the system is calculated exactly using the functional integral method together with the transfer matrix techniques which necessitates the determination of the eigenvalues of a Schrödinger-like equation. Thus the exact free energy is evaluated which in the low temperature limit has a phonon part and a contribution coming from the domain wall excitations. It was shown that this domain wall free energy differs from that calculated by the use of the domain wall phenomenology proposed by Krumhansl and Schrieffer. The exact solutions of the Schrödinger-like equation are also used to evaluate the displacement-displacement, intensity-intensity correlation functions and the probability distribution function. These results are compared with those obtained from the phenomenology as well as the φ4-field theory. A qualitative picture of the central peak observed in structural phase transitions is proposed.

    • Thermoluminescence in single crystals of RbBr:OH and RbBr:Ca2+

      S B S Sastry S Sapru

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      Optical absorption, thermoluminescence glow and emission spectra of RbBr:Ca2+ and RbBr:OH have been studied and analysed. It is observed that both Ca2+ and OH ions enhance theF-centre concentration.FZ1 band in RbBr:Ca2+ appears at 1.55 eV. TL glow peak corresponding toFZ1 centre on analysis gives a trap depth of 0.84 eV. OH ions in the crystal seem to act as TL ‘killers’. Spectral distribution of emission under the glow peaks shows five bands around 1.5, 1.8, 2.1, 2.5 and 2.9 eV. Probable models of TL mechanism are suggested to explain the observed TL emission bands.

    • Theory of radiation from relativistic positrons moving in the 〈110〉 axial channels of f.c.c. (diamond) crystal

      Ratan Lal

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      The emission of radiation from relativistic positrons moving in the 〈110〉 axial channels of an f.c.c. (diamond) crystal has been studied. An expression for the radiation intensity has been obtained for the general case of positron motion. This expression has been simplified for the particular case of well-collimated incident beam. Enhancement of the radiation over (ordinary) bremsstrahlung has been discussed.

    • Feynman diagram approach to atomic collisions

      Babaji Charan Mishra Trilochan Pradhan

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      A quantum field theoretic formulation of atomic collision phenomena involving non-relativistic free and bound systems is developed and a calculational procedure in terms of Feynman diagrams is prescribed. Matrix elements of several atomic collision processes have been calculated. In most cases standard quantum mechanical results are reproduced. But in some cases new terms appear in the scattering matrix whose contribution though negligibly small in the low energy region, become important at higher energies.

    • Momenta and rapidity characteristics of the multiparticle production in 50 GeV/cπ-Emulsion collison

      V Kumar N S Verma S C Varma A P Sharma

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      The momenta and rapidity characteristics of the particles produced in 50 GeV/cπ-collision with emulsion nuclei have been studied with an emulsion stack exposed under a pulsed magnetic field. The longitudinal rapidity plots suggest that leading pion is attenuated strongly when passing through a heavy nucleus. The average net charge 〈Ns+Ns〉 produced in the final state of the reaction rises rapidly from a negative to a positive value atNh⩽1 and attains a constant value⋍0.60±0.08 at allNh⩾7.

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