• V S Sastry

      Articles written in Pramana – Journal of Physics

    • 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.

    • Annealing effect on transport properties of Nd0.67Sr0.33MnO3 thin films

      M Pattabiraman P Murugaraj G Rangarajan V Prasad S V Subramanyam V S Sastry Sang-Mo Koo K V Rao

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      Annealing dependence of the lattice parameter, resistivity, magnetoresistance and thermopower have been studied on Nd0.7-Sr0.33MnO3 thin films deposited on LaAlO3 and alumina substrates by pulsed laser ablation. Upon annealing at 800°C and 1000°C the lattice constant of the LaAlO3 film tends toward that of the bulk target due to reduction in oxygen vacancies. This results in a metal-insulator transition at temperatures which increase with progressive annealing along with a decrease in the observed low temperature MR. Using a magnon scattering model we estimate the eg bandwidth of the film annealed at 1000°C and show that the magnon contribution to the resistivity is suppressed in a highly oxygen deficient film and gains prominence only upon subsequent annealing. We also show that upon annealing, the polaron concentration and the spin cluster size increases in the paramagnetic phase, using an adiabatic polaron hopping model which takes into account an exchange dependent activation energy above the resistivity peak.

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