• Volume 43, Issue 3

      September 1994,   pages  181-253

    • Second order elastic anomalies in barium titanate from orthorhombic to rhombohedral phase transition

      B Subramanyam

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      The anomalies in second order elastic constants have been derived for barium titanate for the phase transition from orthorhombic to rhombohedral state. The equilibrium values of order parameter, strain variables and fluctuations in order parameter have been derived using stability conditions and Landau-Khalatnikov equations respectively. Expression for shift in specific heat is obtained. All the anomalies in second order elastic constants have been derived and relations among them reported. The numerical values of anomalies in the individual constants are calculated and their variation is represented graphically. Changes in elastic constants occur over a range of temperature of the order 10−2K.

    • High pressure studies on AgI-Ag2O-MoO3 glasses

      B Vaidhyanathan S Asokan K J Rao

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      The effect of pressure on the conductivity of AgI-Ag2O-MoO3 glasses has been reexamined. A conductivity maximum is observed around 0·7 GPa. No variation of the sample temperature is noted under pressure. The results are found to agree well with the cluster-tissue model.

    • Investigation of phonon anomalies in intermediate valence compounds SmS and Sm0·65Y0·25S

      Prafulla Kumar Jha Sankar P Sanyal

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      Phonon anomalies in two intermediate valence compounds (IVC), SmS and Sm0·75Y0·25S have been investigated using breathing shell model (BSM). The BSM includes breathing motion of electron shells of the rare earth atom due tofd hybridization. The phonon dispersion curves of IVC, calculated from the present model, agree well with the measured data. One-phonon density of states calculated from the present model compares well with the Raman spectra.

    • Computation of collective modes and acoustic investigations at different temperatures of vitrous silica

      R V Gopala Rao R Venkatesh

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      This paper is mainly concerned with elastic and acoustic properties of vitrous silica besides the computation of phonon frequencies. Thus the phonon frequencies of vitrous silica have been calculated assuming the electronic bulk modulus,Ke, as equal to zero. New equations have been derived to relate the pressure derivatives of second order elastic constants to the acoustic Gruneisen’s parameters using both Bhatia-Singh’s parameters and Schofield’s equations. The calculated longitudinal and transverse Gruneisen’s parameters and the predicted absorption band spectra from Nagendranath’s equation and Bhatia Singh’s parameters are in good agreement with experiment. The calculated mean acoustic mode Gruneisen’s parameter evaluated from the pressure derivative of Nagendranath’s equation is also in good agreement with experiment.

    • Even harmonics of AC susceptibility in some sintered pellets of high temperature superconductors

      A Das A Bajpai A Banerjee R Srinivasan

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      The real (χ′n) and imaginary (χ″n) parts of even harmonic susceptibility (n⩽6) are measured as a function of external DC field (Hdc) in the field increasing (H↑) and decreasing (H↑) cycle. Hysteresis is observed betweenH↑ andH↓ cycles. In theH↓ cycle, at a field,Hcomp=4·2G, bothχ′ andχ″ of all the even harmonics vanish indicating a true cancellation of fields in the intergrain region (Heff≈0) caused by the balance betweenHdc and the remanent magnetization of the grains. The position of the extrema and the zero of the various harmonics undergo a shift proportional to the remanent magnetization of the grains at that particular field.

    • Electric and dielectric properties of solution-gas interface grown amorphous AgCl films

      P S Nikam K A Pathan

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      Electric and dielectric properties of solution-gas interface grown AgCl thin film capacitors (Al/AgCl/Al) of various thicknesses have been studied in the frequency range 101–106 Hz at various temperatures (303–393 K). I–V characteristics show ohmic, space-charge-limited, and thermionic emission conduction mechanisms to operate at low, intermediate and high voltages respectively. Capacitance decreases with increasing film thickness and applied frequency while it increases with increase of temperature. Loss factor (tanδ), which shows a pronounced minimum with frequency, increases with the rise of temperature and (tanδ)min shifts to a higher frequency. The large values of capacitance and dielectric constant (ɛ) in the low frequency region indicate the possibility of an interfacial polarization mechanism to operate in this region while electronic and ionic polarizations dominate in the high frequency region.

    • Overtone spectroscopy of benzaldehyde

      P K Srivastava G Ullas S B Rai

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      The vibrational overtone spectra of aryl and alkyl C-H stretch vibrations in benzaldehyde have been studied using conventional IR and thermal lensing technique at room temperature. The stretching vibrational frequencyωe, anharmonicity constantωexe and the dissociation energies of the two C-H bonds have been calculated. The bond length of C-H bond in aryl position has been estimated.

    • Fluorescence quenching in dyes under an optically thin approximation

      George C Chennattucherry G Ajithkumar N V Unnikrishnan

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      Kinetic analysis of the energy level scheme of a dye molecule yields expression for the fluorescence quantum yield. The quantum yield is dependent on molecular parameters and laser pulse characteristics. Calculations show quenching of fluorescence quantum yield due to excited state absorption. Earlier studies on quenching of fluorescence have been discussed quantitatively under an optically thin approximation.

    • Photoconductivity studies of (PbCrO4-HgO-ZnO) composites

      Sadhana Devi S G Prakash

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      This paper deals with the photoconductivity studies of (PbCrO4-HgO-ZnO) composites comprising of base materials of three different energy gaps. The response of (10%PbCrO4-80%HgO-10%ZnO) and (20%PbCrO4-60%HgO-20%ZnO) composites is faster than that of other compositions and of base materials. The dark current for (20%PbCrO4-60%HgO-20%ZnO) is trap limited-space charge limited at higher voltages while it shows non-ohmic behaviour at lower voltages. The photocurrent for (20%PbCrO4-60%HgO-20%ZnO) composition shows non-ohmic behaviour at lower voltages and tends to saturation at higher voltages. The photocurrent for the above composition varies sublinearly with the intensity of illumination. The (10% PbCrO4-80%HgO-10%ZnO) composition is the best suitable material for optical switching. The existence of discrete trap depths at 16°C appears plausible for different compositions.

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