• Volume 49, Issue 3

      September 1997,   pages  263-322

    • Projection method for 4d non-orthogonal hyperlattices

      S Jayanthi R K Mandal S Lele

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      The Wigner-Seitz cell of a lattice inn-dimensional space displays the complete point group of such a lattice. The vertices of the cell when projected onto pseudo space can serve as the outer shape of acceptance domain or motif. This general procedure leads to acceptance domain or motif identical to those discussed in literature for primitive orthogonal hyperlattices.

      Example of 4d non-orthogonal hyperlattices corresponding to 12-fold symmetry will be considered. It will be shown that the first Wigner-Seitz cell degenerates into more than one shape in 2d pseudo space and can serve as a natural partition of the motif. Following a parallel procedure, the consequence of projection of first 4d Brillouin zone will also be discussed.

    • Four constant potential function of amorphous selenium and evaluation of its thermodynamic and acoustic properties

      R Venkatesh R V Gopala Rao Sumita Bandyopadhyay

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      Litov and Anderson after various considerations suggested a four constant potential function for a-Se as well as a-As2S3. Hence we also used a four constant potential function with the sole purpose of applying this potential function to obtain several acoustic, thermodynamic and other properties. We calculated several acoustic properties of a-Se like second order elastic constants (SOECs), their pressure derivatives, the longitudinal and transverse Grüneisen constant by two different methods, phonon frequencies, absorption band position through the use of Nath-Smith-Delaunay’s equation, and the thermodynamic properties like heat capacity, bulk modulus, thermal Grüneisen constant, the pressure derivative of the bulk modulus (dKT/dP=C1), the pressure derivative ofC1 which is related to Anderson-Grüneisen parameter, pressure derivative of Grüneisen constant namelyγg which is related to second Grüneisen constant, characteristics of phonon frequencies, potential energy function through the use of fitted parameters and third order elastic constants. Finally we calculatedKT at the reduced density ofρ/ρ0=1.1.KT is obtained from the potential function with the fitted parameters. In all the above cases the calculated values are found to be in good agreement with experiment wherever available. In this connection it is important to point out that we eliminated ‘C’ a constant in the potential function using the equilibrium condition as was done by Litovet al in a-Se and Gerlichet al in the case of a-As2S3 as all amorphous substances are isotropic as mentioned by several authors. We contemplate to calculate several other properties for a-Se and a-As2S3 and present them at a later stage.

    • High pressure study of phase transitions inα-FePO4

      Chitra Murli Surinder M Sharma S K Kulshreshtha S K Sikka

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      High pressure behaviour of FePO4 in berlinite form has been investigated up to 10 GPa using vibrational Raman spectroscopy and energy dispersive x-ray diffraction. Combination of these techniques along with studies on pressure quenched samples reveal structural transitions in this material from its room pressure trigonal phase to a disordered and a crystalline phase near 3±0.5 GPa. The latter is the Cmcm phase which is the equilibrium structure at high pressures. These high pressure phases do not revert back to its initial structure after release of pressure. Irreversibility of these transformations indicates that FeO4 tetrahedra do not regain their initial coordination. These high pressure transitions can be rationalized in terms of the three level free energy diagram for such systems.

    • Thermoelectric power of high-Tc superconductors using extended Hubbard model

      S Keshri P K Barhai

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      The thermoelectric power (TEP) for a one dimensional lattice has been studied by using the extended Hubbard model in the limitU≠∞, whereU is the on-site Coulomb interaction. A new expression for TEP, derived in this study, has been found to successfully reproduce the experimental results of the insulating samples of different hole- and electron-doped high temperature superconducting systems.

    • Radially dependent antishielding factor γ(r) of the Sc3+ ion in the solid state

      B B Panigrahi N C Mohapatra S Hafizuddin

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      We report the results of our calculation of γ(r), the radially dependent antishielding factor of Sc3+ ion in the crytalline environment. Watson sphere model is used to approximately represent the crystal potential. Differential equations resulting from non-orthogonal Hartree-Fock perturbation theory are solved to obtain perturbations to core electron states. Contribution to γ(r) from electron self-consistency effect has been evaluated by using the latter wave-functions in the many-body expression of Schmidtet al.

    • Time dependent magnetization of a Zn0.1Fe0.9Fe2O4 fine magnetic particle system

      R V Upadhyay

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      The time dependent magnetization measurements of the fine particles (100Å) of Zn0.1Fe0.9Fe2O4 ferrite system at 68 K are reported. The time decay of magnetization is found to be logarithmic. The time dependent co-efficient varies with the applied field and is maximum at coercive field. The value of anisotropy obtained from the variation of time dependent co-efficient with field agrees well with earlier results.

    • Fine structure in the time of flight distribution of C2 in laser produced plasma from graphite

      S S Harilal Riju C Issac C V Bindhu V P N Nampoori C P G Vallabhan

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      Time resolved optical emission spectroscopy is employed to study the expansion dynamics of C2 species in a graphite plasma produced during the Nd : YAG ablation. At low laser fluences a single peak distribution with low kinetic energy is observed. At higher fluences a twin peak distribution is found. It has been noted that these double peak time of flight distribution splits into a triple peak structure at distances ≥17mm from the target surface. The reason for the occurrence of multiple peak is due to different formation mechanisms of C2 species.

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