• Jacob Philip

      Articles written in Pramana – Journal of Physics

    • Cuspidal edges for elastic wave surfaces for cubic crystals

      Jacob Philip K S Viswanathan

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      The paper deals with a detailed numerical study of the sections of the inverse and ray velocity surfaces for cubic crystals. The figures for the sections of the inverse and ray surfaces by the (001) and (110) planes have been plotted for over 65 crystals and from these, the nature of the cuspidal edges has been discussed. Typical graphs of the inverse and ray surfaces have been given. The parameters characterising the dimensions of the cusps have been tabulated. It is shown that the A-15 compounds exhibit very unusual and interesting wave surfaces at temperatures below superconducting critical temperatures.

    • Interlattice displacements and elastic constants of the A-15 compounds V3Si, V3Ge and Nb3Sn

      Jacob Philip C S Menon

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      General expressions for the interlattice displacements of the A-15 structure compounds are obtained in terms of the strain components making use of the deformation theory. The nature of the interlattice displacements of all the 8 atoms in the unit cell is discussed. It is found that the interlattice displacements occur in such a way that the pair of atoms along any linear chain move in opposite directions with equal magnitudes. Expression for the strain energy of these compounds is developed using deformation theory and this is compared with the strain energy expression from continuum theory to obtain the elastic constants. The theoretical values of the elastic constants fairly agree with the experimental values for V3 Si, V3Ge and Nb3 Sn.

    • Neutron scattering by anharmonic crystals and the effect of sublattice displacements

      K S Viswanathan Jacob Philip

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      A theory has been given for the scattering of neutrons by anharmonic crystals, for which terms of the typeV(3) (k1j1; —k1j1;oj) which contribute to the sublattice displacements are not neglected. Using the standard perturbation theory in the interaction picture or Green’s function method, an expression has been derived for the differential scattering cross-section which brings in the shift and the width of the phonons in one-phonon energy exchange processes. It is shown that the sublattice displacements will modify the phase factor arising from the scattering by any atom in the unit cell, and the Debye-Waller factor also gets altered both by the sublattice displacements as well as by higher order terms arising from anharmonicity. It is shown that the differential scattering cross-section contains a term linearly depending on the third order anharmonicity coefficientV(3) (k1j1;k2j2;k3j3) and neutron scattering by crystals should provide a useful method for evaluating the third order anharmonicity coefficients.

    • Elastic nonlinearity parameters of tetragonal crystals

      Jacob Philip

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      The equations of motion for the propagation of finite amplitude elastic waves in crystals of tetragonal symmetry have been derived starting from the expression for the elastic strain energy. The equations have been solved for a finite amplitude sinusoidal wave propagating along the pure mode directions which are [100], [110] and [001] for the tetragonal group TI. The solutions corresponding to longitudinal wave propagation yield expressions for the amplitudes of the fundamental and generated second harmonic for these directions in terms of certain combinations of second and third order elastic constants of the medium. The results will aid the experimenter to determine these constants using ultrasonic harmonic generation technique.

    • Thermal diffusion near glass transition in Ge-Se glasses measured by photoacoustics

      K N Madhusoodanan Jacob Philip

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      The glass transition in GexSe1−x) (0·1 ⩽x ⩽ 0·25) glasses has been investigated using the photoacoustic (PA) technique. It is found that the PA amplitude and phase undergo anomalous changes at the glass transition temperatureTg. The amplitude has critical minimum and phase has maximum values atTg. The variation of the thermal diffusivity, determined by measuring the frequency dependence of the PA amplitude and phase, with temperature shows sharp decrease near the glass transition temperature. The temperature dependence of the optical energy gap also has been measured and it shows a decrease with temperature for all compositions, the rate of decrease being higher for temperatures greater thanTg.

    • Composition dependence of the energy gap and thermal diffusivity in bulk As-Se glasses

      K N Madhusoodanan Jacob Philip

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      We have measured the composition dependence of the optical energy gap and thermal diffusivity in bulk AsxSe1−x (0.10⩽x⩽0.50) glasses using photoacoustic technique. The energy gap shows a threshold minimum value and thermal diffusivity has a threshold maximum value at the stoichiometric composition As2Se3 corresponding tox=0.40. The decrease in energy gap is explained on the basis of chemical bonding. It is argued that the threshold percolation of rigidity in the random network is responsible for the peaking of the thermal diffusivity at the stoichiometric composition.

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