• Volume 38, Issue 4

      April 1992,   pages  1-409

    • New forms of quantum statistics

      A K Mishra G Rajasekaran

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      We propose a new two-parameter deformation of the algebra of creation and destruction operators, which allows the construction of a new family of Hillbert spaces with positive definite inner product. This provides a continuous interpolation between two new forms of statistics named orthofermi and orthobose statistics. Positivity of the inner product over the two-parameter region is discussed.

    • Effect of liquid phase on the triple point pressure of argon

      A K Bandyopadhyay J K N Sharma E S R Gopal

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      We report here a systematic data analysis of the vapour pressure of argon at different amounts of the liquid phase to understand the thermodynamic behaviour of this inert gas around triple point. At the triple point plateau, the applied heat pulse melts a certain phase of solid argon into liquid and increases vapour pressure. It is observed that this vapour pressure attains the thermodynamic equilibrium pressure after a certain time interval. The expoential decay of the vapour pressure as a function of time at different fractions of the liquid phase shows two different features. In one region, the relexation time constant (τ) is low and is not varying with the liquid phase, while in the other region the value ofτ increases with the amount of the liquid phase. Further, the peak pressure from the equilibrium pressure (ΔPh), obtained from the fitting parameters, shows a dip at around 50% of the liquid phase. A qualitative physical interpretation has been given to explain these results.

    • Debye-Waller factor of lead nitrate

      R Somashekar U D Prahllad M S Madhava

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      The X-ray Debye-Waller factors and Debye temperatures of lead nitrate single crystals taken in the powder form have been determined by measuring integrated intensities of selected Bragg reflections at different temperatures. The characteristic specific Debye temperature has been compared with the value obtained from elastic constant data.

    • Structural and dielectric properties of Pb5(Ge,Si)O11

      R N P Choudhary N K Misra P S Chidambaram

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      The ferroelectric lead germanate (Pb5Ge3O11) and its isomorphous compounds are important because of their uses as pyroelectric and electro-optic devices. Comparison of inter-planar d-spacings of Pb5Ge3−xSixO11 (x=0, 0.3, 0.7 and 1.00) suggests that there is no change in basic structure of Pb5Ge3−xSixO11 when Si is substituted for Ge in small quantity (x<1). The dielectric properties of the Si-substituted compounds have been studied as a function of temperature (30 to 200°C). The ferroelectric-paraelectric phase transition has been observed at 185°C. The Si doping causes (a) Curie point to shift towards low temperature, (b) peak value of the dielectric constant to decrease and (c) phase transition diffuse. The fast increase in dielectric constant of pure Pb5Ge3O11 with temperature (beyond transition temperature) may be attributed to the development of space charge polarization in the system.

    • Temperature dependence of the linewidth of the first-order Raman spectrum for GeO2 powder

      Tsutomu Sato

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      The first-order Raman spectrum of A1g mode in GeO2 powder was measured at a temperature range of 130–973 K and the linewidths at these temperatures were obtained. The temperature dependence of the linewidths was analysed by the phonon dispersion curves based on the rigid ion model, and the results show that it was caused perhaps by the cubic anharmonic term in crystal potential energy.

    • Hyperfine structure and isotope shift studies of rhenium by laser optogalvanic spectroscopy

      Ranjit Singh G N Rao R K Thareja

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      Doppler-limited high resolution laser optogalvanic spectra of six transitions of rhenium lying in the range 560–620 nm have been recorded. Isotope shifts between the two natural isotopes of rhenium,185Re and187Re, have been calculated in all the six transitions. Hyperfine coupling constants for eight levels belonging to the configurations 5d56s2, 5d66s, 5d46s26p and 5d56s6p are reported. Wherever possible our constants are compared with those given by others.

    • Beam couplings and phase conjugate effects in reflection and transmission in BaTiO3

      Putcha Venkateswarlu M Moghbel P Chandra Sekhar M C George

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      The details of experiments showing the effects of self-pumped phase conjugation on reflection and on transmission in barium titanate crystal are given. The specular reflection and the second-surface reflection of an extraordinary polarized beam, incident on the face of the crystal parallel to itsc-axis, get reduced in intensity as the phase conjugation develops. It has been found that parts of the self-pumped phase conjugate beam emerge out of the crystal as additional transmission beams. They grow in intensity as the phase conjugation develops. Other measurements which combine coherent or incoherent coupling beams are presented and used to explain the observations.

    • Design and operation characteristics of a high power transverse flow pulser sustained cw CO2 laser

      A K Nath L Abhinandan P Choudhary Manoj Kumar

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      A transverse flow, transverse discharge cw CO2 laser in which de discharge is sustained by employing high repetition rate high voltage pulses has been developed. Pulser sustained discharge through electrodes of innovative design provided uniform excitation at electrical input power densities more than 10 W/cc. Laser output power more than 2.5 kW was obtained in a laser gas mixture consisting of 0.5 mbar of CO2, 16 mbar of N2 and 38.5 mbar of He. Design details and operational characteristics of this laser are presented.

    • Preparation and properties of vanadium dioxide thermochromic thin films

      K A Khan M S Rahman Khan

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      Deposition of vanadium dioxide and the study of its electrical and optical properties at varying deposition conditions have been presented. The materials have been deposited by reactive r.f. magnetron sputtering technique in Ar, O2 ambient followed by annealing post-treatment. Electrical conductivity measurements indicate that oxygen pressure plays an important role in obtaining VO2 and atPO2=2.4% stoichiometric VO2 can be obtained. The deposition rate of oxides decreases with increasing O2 pressure and the rate of VO2 was about 130Å/min. Optical studies show that VO2 films exhibit thermochromism and it has the potential application for energy efficient solar energy utilization.

    • Characterization of supersonic beams by time-of-flight technique

      P N Bajaj P K Chakraborti

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      Time-of-flight technique has been used to characterize supersonic beams of polyatomic molecules in terms of translational and vibrational temperatures, various velocity parameters and speed ratio. Collision effectiveness and effective specific heat ratio of polyatomic gases pertinent to jet expansion have also been determined and interpreted.

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