• J Philip

      Articles written in Pramana – Journal of Physics

    • Low temperature elastic behaviour of As-Sb-Se and Ge-Sb-Se glasses

      E S R Gopal T S Mukundan J Philip S Sathish

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      The ternary glasses of arsenic and germanium with antimony and selenium can be prepared in large sizes for optical purposes. The elastic behaviour of eight compositions of each glass has been studied down to 4.2 K using a 10 MHz ultrasonic pulse echo interferometer. The glasses have a normal elastic behaviour, with the velocities gradually increasing as the temperature is lowered. An anharmonic solid model of Lakkad satisfactorily explains the temperature variations. The elastic moduli of GexSb10Se90−x glasses increase linearly as the Ge content is increased up to 25 at. % and beyond this the increase is nonlinear. (AsSb)40Se60 glasses show a linear increase in elastic moduli with increasing Sb content. The elastic moduli of AsxSb15Se85−x glasses exhibit a drastic change near the stoichiometric composition As25Sb15Se60. These behaviours have been qualitatively explained on the basis of the structural changes in glasses.

    • Determination of thermal effusivity of solids by a photoacoustic scanning technique

      J Philip A A Sudhakaran

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      A new method is proposed to determine the thermal effusivity of solid samples using a one dimensional photoacoustic scanning technique. The method employs a sample configuration in which the backing for a good light absorber layer is changed from a reference sample to the unknown sample by scanning the absorber surface with an incident modulated light beam. From the measured phase difference or amplitude ratio one can determine the thermal effusivity of the unknown sample, knowing the effusivity of the reference sample. The Rosencwaig-Gersho theory of photoacoustic effect has been extended to the present experimental situation and expressions have been derived for photoacousitc phase difference and amplitude ratio as the backing is changed. Values calculated using these expressions are found to agree well with measured values for different sample combinations except in amplitude ratio values when the thermal effusivities of the samples differ very widely. The reason for this disagreement is discussed.

    • Slowness surfaces, ray velocity surfaces and phonon focussing in highTc superconductor crystals

      J Philip M S Kala

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      This paper reports the results of the study of anisotropy in elastic wave propagation in single crystal superconducting BSCCO. The inverse and group velocities of elastic waves propagating in different directions have been computed and the corresponding slowness and ray velocity surfaces plotted, taking elastic constant data from literature. In addition, the phenomenon of phonon focussing has been investigated in this material by computing the phonon enhancement factor along different directions in spherical polar coordinates. The abnormally high values in phonon enhancement factor exhibited in certain directions for the phonon modes are interpreted as due to caustics occurring in the geometrical acoustics approximation adopted in the computational analysis. The results in LSCO and YBCO are found to be similar to those in BSCCO.

    • Characteristics of photoconductivity in As-Sb-Se glasses

      G Mathew J Philip

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      Characteristic features of photoconductivity in AsxSb15Se85−x and AsxSb10Se90−x bulk glass systems have been measured and reported. Both a.c and d.c techniques have been employed for the measurements. The temperature dependence of photoconductivity has been analysed in terms of the ABFH model and both systems are found to exhibit type I photoconductivity. Photoconductivity shows a clear change in slope corresponding to the stoichiometric composition in both the systems. The composition dependence of photoconductivity is explained on the basis of chemically ordered covalent network (COCN) model. Photoconductivity response time of the two systems have been determined from frequency-resolved photocurrent (FRPC) measurements. Variation of response time with composition exhibits a clear change in slope at the stoichiometric composition in both the systems.

    • Ultrasonic measurement of the elastic properties of benzoyl glycine single crystals

      A V Alex J Philip

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      Certain organic crystals are found to possess high non-linear optical coefficients, often one to two orders of magnitude higher than those of the well-known inorganic non-linear optical materials. Benzoyl glycine is one such crystal whose optical second-harmonic generation efficiency is much higher than that of potassium dihydrogen phosphate. Single crystals of benzoyl glycine are grown by solvent evaporation technique usingN, N-dimethyl formamide as the solvent. All the nine second-order elastic stiffness constants of this orthorhombic crystal are determined from ultrasonic wave velocity measurements employing the pulse echo overlap technique. The anisotropy of elastic wave propagation in this crystal is demonstrated by plotting the phase velocity, slowness, Young’s modulus and linear compressibility surfaces along symmetry planes. The volume compressibility, bulk modulus and relevant Poisson’s ratios are also determined. Variation of the diagonal elastic stiffness constants with temperature over a limited range are measured and reported.

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