• A K Sood

      Articles written in Pramana – Journal of Physics

    • Structures of some surfactant—polyelectrolyte complexes

      Rema Krishnaswamy V A Raghunathan A K Sood

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      Structures of complexes formed in aqueous solutions by some anionic polyelectrolytes (double and single stranded (ds and ss) DNA, poly(vinyl sulfonate) (PVS), and poly(styrene sulfonate) (PSS)) with a cationic surfactant system consisting of cetyltrimethylammonium bromide (CTAB) and sodium 3-hydroxy-2-naphthoate (SHN) have been determined using small angle X-ray diffraction. All complexes are found to have a two-dimensional (2-D) hexagonal structure at low SHN concentrations. Analysis of the diffraction data shows that the ds DNA—CTAB complex has an intercalated structure, with each DNA strand surrounded by three cylindrical micelles. On increasing SHN concentration, DNA—CTAB—SHN complexes exhibit a hexagonal-to-lamellar transition, whereas PVS complexes show a hexagonal → centered rectangular → lamellar transition. PSS complexes show yet another sequence of structures. These results indicate the significant influence of the chemical nature of the polyelectrolyte on the structure of the complexes.

    • Electronic and vibrational Raman spectroscopy of Nd0.5Sr0.5MnO3 through the phase transitions

      Md Motin Seikh A K Sood Chandrabhas Narayana

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      Raman scattering experiments have been carried out on single crystals of Nd0.5Sr0.5MnO3 as a function of temperature in the range of 320–50 K, covering the paramagnetic insulator-ferromagnetic metal transition at 250 K and the charge-ordering antiferromagnetic transition at 150 K. The diffusive electronic Raman scattering response is seen in the paramagnetic phase which continue to exist even in the ferromagnetic phase, eventually disappearing below 150 K. We understand the existence of diffusive response in the ferromagnetic phase to the coexistence of the different electronic phases. The frequency and linewidth of the phonons across the transitions show significant changes, which cannot be accounted for only by anharmonic interactions.

    • Foreword

      A K Sood K N Ganesh C S Sundar A K Raychaudhuri

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    • Flow-driven voltage generation in carbon nanotubes

      A K Sood S Ghosh Anindya Das

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      The flow of various liquids and gases over single-walled carbon nanotube bundles induces an electrical signal (voltage/current) in the sample along the direction of the flow. The electrical response generated by the flow of liquids is found to be logarithmic in the flow speed over a wide range. In contrast, voltage generated by the flow of gas is quadratically dependent on the gas flow velocity. It was found that the underlying physics for the generation of electrical signals by liquids and gases are different. For the liquid, the Coulombic interaction between the ions in the liquid and the charge carriers in the nanotube plays a key role while electrical signal generation due to gas flow is due to an interplay of Bernoulli’s principle and Seebeck effect. Unlike the liquid case which is specific to the nanotubes, the gas flow effect can be seen for a variety of solids ranging from single and multi-walled carbon nanotubes, graphite and doped semiconductors.

    • Foreword

      A K Sood K N Ganesh C S Sundar A K Raychaudhuri

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    • Optically-driven red blood cell rotor in linearly polarized laser tweezers

      Manas Khan Samarendra K Mohanty A K Sood

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      We have constructed a dual trap optical tweezers set-up around an inverted microscope where both the traps can be independently controlled and manipulated in all the three dimensions. Here we report our observations on rotation of red blood cells (RBCs) in a linearly polarized optical trap. Red blood cells deform and become twisted in hypertonic phosphate buffer saline and when trapped, experience an unbalanced radiation pressure force. The torque generated from the unbalanced force causes the trapped RBC to rotate. Addition of Ca++ ions in the solution, keeping the osmolarity same, makes the cell membranes stiffer and the cells deform less. Thus the speed of rotation of the red blood cells can be controlled, as less deformation and in turn less asymmetry in shape produces less torque under the radiation pressure resulting in slower rotation at the same laser power

    • Temperature-dependent infrared reflectivity studies of multiferroic TbMnO3: Evidence for spin-phonon coupling

      Pradeep Kumar Surajit Saha C R Serrao A K Sood C N R Rao

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      We have measured near normal incidence far-infrared (FIR) reflectivity spectra of a single crystal of TbMnO3 from 10 K to 300 K in the spectral range of 50 cm-1 –700 cm-1. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identified in the imaginary part of the dielectric function $\varepsilon_{2} (\omega)$ and energy loss function Im($−1/\varepsilon(\omega))$, respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature $T_{N}$ ($\sim 46$ K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the superexchange integral between the Mn3+ spins. The effective charge of oxygen ($Z_{O}$) calculated using the measured LO–TO splitting increases below $T_{N}$.

    • Probing ultrafast carrier dynamics, nonlinear absorption and refraction in core–shell silicon nanowires

      Sunil Kumar M Khorasaninejad M M Adachi K S Karim S S Saini A K Sood

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      We investigate the relaxation dynamics of photogenerated carriers in silicon nanowires consisting of a crystalline core and a surrounding amorphous shell, using femtosecond time resolved differential reflectivity and transmission spectroscopy at 3.15 eV and 1.57 eV photon energies. The complex behaviour of the differential transmission and reflectivity transients is the mixed contributions from the crystalline core and the amorphous silicon on the nanowire surface and the substrate where competing effects of state-filling and photoinduced absorption govern the carrier dynamics. Faster relaxation rates are observed on increasing the photogenerated carrier density. Independent experimental results on crystalline silicon-on-sapphire (SOS) help us in separating the contributions from the carrier dynamics in crystalline core and the amorphous regions in the nanowire samples. Further, single-beam z-scan nonlinear transmission experiments at 1.57 eV in both open- and close-aperture configurations yield two-photon absorption coefficient 𝛽 (∼3 cm/GW) and nonlinear refraction coefficient 𝛾 ($−2.5 × 10^{−4}$ cm2 /GW).

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