• Volume 42, Issue 5

      May 1994,   pages  375-446

    • Structure and vibrational properties of carbon tubules

      N Chandrabhas A K Sood D Sundararaman S Raju V S Raghunathan GVN Rao V S Sastry T S Radhakrishnan Y Hariharan A Bharathi C S Sundar

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      The structure of multilayered carbon tubules has been investigated by electron microscopy and X-ray diffraction. The structure of tubules is characterized by disorder in the stacking of cylindrical graphene sheets. Raman scattering measurements have been carried out in tubules and compared with graphite. The observed features in the Raman spectra in tubules can be understood in terms of the influence of disorder. The additional Raman modes predicted for single layer carbon tubules have not been observed.

    • The elastic moduli, the volume contribution and the Cauchy ratio ford andf shell metals

      N Singh B S Yadav

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      The elastic constants of nine transition metals and four rare-earths and actinides are calculated using the ion-ion interaction defined by us recently. The volume contribution to elastic moduli is calculated by exploiting the density dependence of the screening function. The calculated volume contribution to bulk modulus is found to vary between 17.1% and 62.4% for a number of metals, which is quite significant and play an important role for describing quantitatively the violation of the Cauchy ratio for these metals.

    • Temperature dependence of low angle structure factors of liquid alkali metals using electron-ion plasma model

      A Bari P Pal R N Joarder

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      The analytic solution of EIP model in the MSA via the OCP criterion yields temperature dependent results in RPA quite satisfactorily at high temperatures. The same set of parameters produces low and high angle structural properties. The model can work almost near to the critical point if the ion-core radius is allowed to vary systematically in the regionρ<2ρc,ρc being the critical density. The model can also accommodate the general scaling behaviour observed for the structure factors of liquid alkali metals.

    • Electronic structure of non-magnetic impurities in dilute alloys of Al

      P Singh S Prakash

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      The electronic structure of substitutional non-magnetic impurities Cu, Ag, Cd, Mg, Zn, Ga, In, Ge, Si and Sn in Al is studied using density functional theory. A simple physical model is proposed to calculate the effective charges on impurities in trivalent metal Al. A linear relation is found between the effective charges on impurities and impurity vacancy capture radii. The spherical solid model (SSM) is used to account for discrete nature of the host. The impurity-induced change in charge density, scattering phase shifts, host-impurity potential, residual resistivity and impurity self-energy are calculated. Higher order scattering phase shifts are found significant and the host-impurity potential is found proportional to effective charge on impurity in its vicinity. The self-consistently calculated potential is used to calculate the electric field gradients (EFGs) at the first and second nearest neighbours (1NNs, 2NNs) of impurity. The calculated values are in agreement with the experimental results.

    • Transport phenomena of SmSe1−x Asx

      S Ariponnammal S Natarajan

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      Results are presented on the measurements of unit cell parameter and electrical resistivity under pressure on SmSe1−xAsx forx=0.1, 0.2, 0.3 and 0.4. The electrical resistivity values are found to be decreasing with increase of pressure and also with increase of arsenic concentration. The semiconductor to metallic transition is induced by chemical alloying of SmSe with SmAs similar to that observed under pressure. The electrical resistivity values are also calculated which are in good agreement with the experimental values. In this calculation, the carrier mobility is of negative sign and so the sample SmSe1−xAsx is found to ben-type semiconductor.

    • Modification of spectral characteristics and optogalvanic response in neon hollow cathode under laser excitation

      P R Sasi Kumar V P N Nampoori C P G Vallabhan

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      The changes in emission characteristics of a neon hollow cathode discharge by resonant laser excitation of 1s5→2p2 and 1s5→2p4 transition have been studied by simultaneously monitoring the optogalvanic effect and the laser induced fluorescence. It has been observed that resonant excitation causes substantial variation in the relative intensities of lines in the emission spectrum of neon discharge.

    • Plasma-maser interaction of langmuir wave with kinetic Alfvén wave turbulence

      Bipuljyoti Saikia B K Saikia S Bujarbarua M Nambu

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      A theoretical study is made on the generation mechanism of Langmuir mode wave in the presence of kinetic Alfvén wave turbulence in a magnetized plasma on the basis of plasma-maser interaction. It is shown that a test high frequency Langmuir mode wave is unstable in the presence of low frequency kinetic Alfvén wave turbulence. The growth of the Langmuir wave occurs due to direct and polarization coupling terms. Because of the universal existence of the kinetic Alfvén waves in large scale plasmas, the results have potential importance in space and astrophysical radiation processes.

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