• Ajay K Sood

      Articles written in Pramana – Journal of Physics

    • Phonons and fractons in sol-gel alumina: Raman study

      Anushree Roy Ajay K Sood

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      We report Raman scattering from the boehmite,γ-, δ- andα-phases of the alumina gel. Samples are characterized by transmission and scanning electron microscopy, X-ray diffraction and density measurements. The main Raman line in the boehmite phase is red-shifted as well as asymmetrically broadened with respect to that in the crystalline boehmite, signifying the nanocrystalline nature of the gel. Raman signatures are absent in theγ- andδ-phases due to the disorder in cation vacancies. We also show that low frequency Raman scattering from the boehmite phase resembles that from a fractal network, characterized in terms of fraction dimension$$\tilde d$$. Taking Hausdorff dimension D of the boehmite gel to be 2.5 (or 3.0), the value of$$\tilde d$$ is 1.33±0.02 (or 1.44±0.02), which is close to the theoretically predicted value of 4/3.

    • Diffusing wave spectroscopy of dense colloids: Liquid, crystal and glassy states

      Subrata Sanyal Ajay K Sood

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      Using intensity autocorrelation of multiply scattered light, we show that the increase in interparticle interaction in dense, binary colloidal fluid mixtures of particle diameters 0.115µm and 0.089µm results in freezing into a crystalline phase at volume fractionφ of 0.1 and into a glassy state atφ=0.2. The functional form of the field autocorrelation functiong(1)(t) for the binary fluid phase is fitted to exp[−γ(6k02Defft)1/2] wherek0 is the magnitude of the incident light wavevector andγ is a parameter inversely proportional to the photon transport mean free pathl*. TheDeff is thel* weighted average of the individual diffusion coefficients of the pure species. Thel* used in calculatingDeff was computed using the Mie theory. In the solid (crystal or glass) phase, theg(1)(t) is fitted (only with a moderate success) to exp[−γ(6k02W(t))1/2] where the mean-squared displacementW(t) is evaluated for a harmonically bound overdamped Brownian oscillator. It is found that the fitted parameterγ for both the binary and monodisperse suspensions decreases significantly with the increase of interparticle interactions. This has been justified by showing that the calculated values ofl* in a monodisperse suspension using Mie theory increase very significantly with the interactions incorporated inl* via the static structure factor.

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