• Volume 34, Issue 1

      January 1990,   pages  1-90

    • String cosmology in Bianchi I space-time

      A Banerjee Abhik Kumar Sanyal Subenoy Chakraborty

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      Some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel. A class of solutions corresponds to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of stateρ=(1+W)λ.

    • Orthogonality constrained general variational calculation of excited state wavefunctions and energies: A new strategy

      Priyatosh Dutta S P Bhattacharyya

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      A viable strategy is developed for the general variational calculation of excited state wavefunctions which are constrained to remainorthogonal to all the lower-lying states of the samesymmetry. A key element of the strategy is to employ the penalised functional procedure for enforcing the relevant orthogonality constraints and the method of steepest descent to locate the constrained minima with respect to all variables, linear as well as nonlinear. The workability of the algorithm is tested by applying the technique for the optimization of nonlinear parameters in trial functions for the 2s state of H atom and singlet 1s2s states of helium atom and some isoelectronic ions.

    • Structure factor of charged colloidal suspensions using Brownian-dynamics simulation: Comparison of Yukawa and Sogami pair potentials

      B V R Tata A K Sood R Kesavamoorthy

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      Static structure factorsS(Q) are obtained for dilute charged colloidal systems using Brownian dynamics simulation method for the widely used screened Coulomb repulsive Yukawa potential and the recently proposed Sogami pair potential. The latter potential has, in addition to the usual repulsive part, an attractive term which is necessary to understand the reentrant phase transition reported in these colloids. It is shown for the first time thatS(Q) obtained using the Sogami potential for parameters favourable for liquid-like order agrees well with that measured experimentally. Thus it appears that the Sogami potential explains features of a homogeneous liquid as well as phase separated states, whereas Yukawa potential does not.

    • Positron annihilation in copper

      V Sundararajan D G Kanhere

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      Electron momentum distribution in copper is studied through the angular correlation of positron annihilation radiation, by employing self-consistent linear combination of Gaussian orbitals band structure method. The two-photon momentum density is presented and analysed through three dimensional plots and angular momentum decomposed contributions. Calculation of two-dimensional angular correlation has been carried out in the independent particle approximation. The results for the analysis using Lock, Crisp and West theorem and the reciprocal form factors are also presented. The present theoretical results are compared with the experimental ones wherever possible, and the agreement in general, is good.

    • Effect of variation of dielectric constant on the magnetic field modulation of exciplex luminescence

      Deb Narayan Nath Mihir Chowdhury

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      The effect of variation of dielectric constant on the relative magnetic field effect in singlet luminescence has been studied using a typical exciplex system at a saturating field. The study indicates strong specificity in the perturbation of the magnetic field effect by alcoholic solvents. In contrast to alcohols where relative singlet magnetic field effect is of the order of 2% only, the magnetic field effect in non-alcoholic medium reaches as high as 9%. Moreover, dielectric constant variation in alcohols yields curves which are distinctly different from those in non-alcoholic media. It turns out that this dependence of magnetic field effect on dielectric constant is similar in all non-alcoholic solvent mixtures. An analytical study based on Hong and Noolandi’s solution of Smoluchowski equation has been made. Derived expressions can interpret experimental curves reasonably well.

    • Effect of annealing on the opto-electronic properties of Cu0.9In1.0Se2.0 films

      R P Sharma Pankaj Garg J C Garg

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      The influence of annealing on the structure and opto-electronic properties of Cu0.9In1.0Se2.0 films prepared by solution growth technique has been studied. The films annealed at 500–520°C in air, vacuum (10−4 torr), In-vapour and Se-vapour show polycrystalline chalcopyrite structure with orientation perpendicular to the (220) plane. Films annealed in Se-vapour at 500°C for 30 min have maximum grain size (560 Å), minimum optical energy gap, maximum absorption coefficient, lowest resistivity, maximum photosensitivity and thus are suitable for photovoltaic applications. Annealing in In-vapour or in vacuum changesp-type CuInSe2 inton-type which possibly arises due to the increase in Se vacancies.

    • Time-resolved spectra of coumarin 30-rhodamine 6G dye mixture

      R D Singh D Mohan A K Sharma N V Unnikrishnan

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      The effect of acceptor concentration on the energy transfer from Coumarin 30 (donor) to Rhodamine 6G (acceptor) has been studied. The nature of energy transfer reaction has been studied through lifetime measurements by recording the time-resolved fluorescence decay curves. The energy transfer parameters calculated were used to confirm the occurrence of energy transfer on the basis of the emission-reabsorption effect.

    • Comment on a solution of Altarelli Parisi equations

      D K Choudhury A Saikia

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      We show that the assumptions of factorizability of the structure functionF(x, t) inx andt and the equality oft evolution of the singlet and gluon distributions are not necessary to obtain the approximate solutions of Altarelli-Parisi equations derived by us recently.

    • High energy elastice+ − O2 scattering

      K N Joshipura S Mohanan

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      Theoretical differential cross-sections are obtained for the elastic scattering of fast (Ei ⩾ 200 eV) positrons by oxygen molecules. Employing the independent atom model, the atomic scattering amplitudes are calculated with the (static + polarization) model potentials, in the partial wave analysis (PWA). Comparisons made with the theory and experiments on the incident electrons, show significant differences at small angles.

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