• Volume 13, Issue 6

      December 1979,   pages  581-672

    • A new theory of compressible ions — structures of the alkali halides

      Ramesh Narayan S Ramaseshan

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      Ions in ionic crystals are considered to exist in compressible space-filling polyhedral cells analogous to the Wigner-Seitz cell in metals. Repulsion arises from the compression energy of the ions written as a surface integral over the ionic cells. Two adjustable parameters are introduced per ion with the provision that the same parameters can be used in any crystal of any structure in which the ion occurs. The 18 parameters for the 5 alkali and 4 halogen ions have been determined from PV data on the 20 alkali halides. The important successes of the theory are: (i) All the twenty alkali halides are correctly predicted to occur in their observed structures (ii) The thermal transition in CsCl is explained (iii) The pressure transitions in the alkali halides are predicted well (iv) The calculated values of the variation of transition pressures with temperature agree well with experiment. These results are much better than those obtained by earlier theories.

    • Parametric excitation of hybrid mode in magnetised piezoelectric semiconductors

      S Ghosh P K Sen S Guha

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      Using the hydrodynamic model of homogeneous plasma, the parametric decay of a laser beam into an acoustic wave and another electromagnetic wave has been studied in heavily dopedn-type piezoelectric semiconductors in the presence of a transverse magnetostatic field. This decay process results in the parametric excitation of the hybrid mode. The threshold electric field necessary for the onset of instability equals to zero. The magnetostatic field couples the acoustic and the electromagnetic waves and in its absence the instability disappears. The growth rate increases with the square of the magnetic field.

    • Mössbauer study of spinel system ZnMn1−x CrxFeO4

      P D Deshpande P V Khandekar P H Umadikar A J Diwan

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      Mössbauer effect of the system of spinels ZnMn1−xCrx FeO4 is studied forx having values 0, 0·2, 0·4, 0·5, 0·6, 0·8 and 1·0. These compounds show quadrupole splitting which increases with increasing Mn3+ ion concentration. It abruptly increases at Mn3+ ion concentration at which crystal structure also changes.

    • Mechanoluminescence of ferroelectric crystals

      B P Chandra K K Shrivastava

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      Mechanoluminescence spectra of triglycine sulfate, guanidine aluminium sulfate, rochelle salt, diglycine manganous chloride dihydrate, lithium ammonium tartate monohydrate and potassium dihydrogen phosphate crystals resemble the discharge spectra of nitrogen gases. Glycine silver nitrate, thiourea, ammonium sulfate and azobenzene crystals, the structure of which is centrosymmetric at room temperature, do not show mechanoluminescence. Following the role of crystal structure, the mechanoluminescence excitation is discussed on the basis of the piezo-electrification of the new surfaces created during fracture of the crystals.

    • Quantum mechanical calculation of Suryan’s line broadening in nuclear magnetic resonance

      K N Shrivastava

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      The first quantum theory of the classical radiation damping in nuclear magnetic resonance is presented. Relaxation times and life times arising from the interaction of nuclear spin with the radio-frequency radiation field are calculated. Second-order line shifts are predicted and the existence ofIz andIz2-type operators due to photons is pointed out. The predicted line shifts as well as relaxation are found to be measurably large. Numerical estimates are given for protons in water.

    • Temperature dependence of positron lifetime in GaAs crystals with defects

      A Bharathi K P Gopinathan C S Sundar B Viswanathan

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      Positron lifetime has been measured as a function of temperature in Sidoped GaAs single crystals subjected to various heat treatments. Defects produced by these heat treatments trap positrons. In all the GaAs samples containing defects positron lifetime was found to decrease with temperature in the range from 375 K to 16 K. The decrease is explained as due to the decrease in the trapping rate. The trapping rate is mainly controlled by the diffusion of the positron to the trap. The diffusion constant is determined mainly by the scattering from charged Si impurities.

    • Reduction and second quantisation of generalised electromagnetic field for zero mass system

      D C Joshi B S Rajput

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      Reduction and second quantisation of generalised electromagnetic fields in the presence of massless spin-1/2 particles carrying both electric and magnetic charges have been carried out in terms of Lomont-Moses realisation of irreducible representations of Poincare group and the expression for field Hamiltonian has been derived.

    • Normal co-ordinates through coriolis coupling constants

      Rama Natesan V Balasubramanian

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      Normal co-ordinates, force fields and isotopic frequencies have been calculated for someXY3 andXY4-type molecules using coriolis coupling constants.

    • 6j, 9j symbols and 3jm factors for the group chainD5C5

      L S R K Prasad K Bharathi

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      A complete set of nontrivial inequivalent 6j and 9j symbols for the double groupD5 and the complete set of 3jm factors (equivalently isoscalar factors) associated with the group chainD5C5 are calculated using the most general phase choices specified by Butler.

    • Coulomb energy of3He and charge-symmetry breaking in nuclear forces

      S S Mehdi V K Gupta

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      We report a calculation of Coulomb energy of3He with two-term separable potentials of a form proposed recently, and a perturbative calculation of the charge-symmetry-breaking potential, again in a two-term separable form. By using potential sets adjusted to various values of proton-neutron effective range difference, some limit is sought to be placed on this quantity, about which nothing is known experimentally.

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