• Volume 47, Issue 4

      October 1996,   pages  263-337

    • Symmetric scattering in electron and positron impact ionization of metastable 2s-state hydrogen atoms

      J N Das S Dhar

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      Triple differential cross sections for ionization of hydrogen atoms in the metastable 2s-state by the impact of electrons and positrons have been calculated for coplanar symmetric geometry. In this calculation a multiple scattering theory due to Das [10] and Das and Seal [11] has been used. An analysis of the results reveals that unlike scattering from the ground state, scattering from 2s-state is essentially a higher order process except for the binary collision direction. Moreover, here, the cross section results for 2s-state are much larger compared to those for scattering from the ground state. It is also found that the ionization mechanism at large scattering angles for ionization from the 2s-state is different from that for ionization from the ground state.

    • Structure of some liquid transition metals using integral equation theory

      O Akinlade A M Umar L A Hussain

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      We present the results of calculations of the structure factorS(q) of some liquid 3d transition metals using the self consistent hybridized mean spherical approximation (HMSA) integral equation. The local pseudopotential used is composed of the empty core model and a part that takes care of s-d mixing through an inverse scattering approach to model the interionic pair potential. The results presented are in very good agreement with experiment for most of the systems investigated near freezing, as well as for the noble metals Cu, Ag and Au, thus, confirming the reliability of the pseudopotential in the present integral equation scheme.

    • Nonlinear travelling waves in φ6 polarizable model

      Nabonita D Chowdhury Sankar P Sanyal

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      We present a complete theoretical analysis of the periodic and non-periodic travelling waves in a diatomic chain model, in the continuum limit by incorporating nonlinear sixth order polarization potential (φ6) at the anion site. We have formulated a nonlinear lattice dynamical theory in which various energy curves are obtained for different types and magnitudes of the core-shell force constants. For periodic solutions, we have obtained two types of commensurate wave amplitudes which propagate in the opposite direction with respect to each other. For nonperiodic solutions, we have obtained various travelling excitations such as kink, antikink, excitons etc. for different values of the mass ratio and velocity parameter. The dipole moment per unit charge for SrTiO3 has been calculated and it is found that the nonlinear excitations in this model carry large amount of energy as compared to those obtained from harmonic and anharmonic optical phonons in the φ4-polarizable model.

    • A model of knock-out of oxygen by charged particle irradiation of Bi-2212

      S K Bandyopadhyay Pintu Sen P Barat P Mukherjee S K Das B Ghosh

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      A model of knock-out of oxygen by charged particle (α and proton) irradiation of Bi2Sr2CaCu2O8+x (Bi-2212) is proposed on the basis of Monte Carlo TRIM calculations. In Bi-2212, the loosely bound excess oxygen is vulnerable to be displaced by particle irradiation. Binding energy and hence, displacement energy of this loosely bound excess oxygen is less compared to that of stoichiometric lattice bound oxygen and other atoms. The displaced or knocked out oxygen goes to pores or intergranular region and generates large pressure inside the sample. Because of porosity of the material, this displaced oxygen diffuses out and there is a net reduction of oxygen content of the sample. The irradiation induced oxygen knock-out is dominant in the bulk where nonionizing energy loss is maximum.

    • Enhanced third harmonic generation in periodic structures

      V Mahalakshmi Jolly Jose S Dutta Gupta

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      We consider third harmonic generation in a periodic layered medium with alternate nonlinear media. We show enhanced third harmonic generation when the fundamental frequency matches one of the mode frequencies of the distributed feedback structure. The observed feature is explained in terms of large local field enhancement for the fundamental wave.

    • Radiation stabilization of U5+ in CaO matrix and its thermal stability: Electron paramagnetic resonance and thermally stimulated luminescence studies

      V Natarajan T K Seshagiri M D Sastry

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      Electron paramagnetic resonance (EPR) evidence is presented for the radiation stabilization of pentavalent uranium in CaO matrix. From the theoretical predictions ofg value for U5+ in axial symmetries, it was concluded that U5+ at Ca2+ site is associated with a second neighbour charge compensating Ca2+ vacancy. EPR measurements also revealed the presence of Mn2+, Mn4+ and Cu2+ impurities in the samples. The thermal stability of U5+ was investigated using EPR and thermally stimulated luminescence (TSL) techniques. The TSL and EPR studies on gamma irradiated uranium doped calcium oxide samples had shown that the intense glow peak at 540 K is associated with the reduction in the intensity of EPR signal of U5+ ion around this temperature. This peak is associated with the process U5++hole→U6+*→U6++hv. The activation energy for this process was determined to be 1.4eV.

    • Impurity mediated mechanism of photorefractive effect in BaTiO3: A combination of sangster and piezoelectric effects

      M D Sastry M Moghbel Putcha Venkateswarlu

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      An impurity mediated mechanism of photorefractive effect in BaTiO3 is proposed. The photoinduced changes in the relative concentration of Fe3+ in BaTiO3 results in an electro-optic coupling through a combination of the Sangster and piezoelectric effects. This is based on the examination of the extensive results on the EPR of Fe3+ in the BaTiO3 lattice. This model explains the improved photorefractive behavior of BaTiO3 on doping with Co2+.

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