• Volume 11, Issue 6

      December 1978,   pages  673-760

    • Cluster approaches to random alloys: An appraisal

      Vipin Srivastava Meena Chaturvedi S K Joshi

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      Some of the cluster extensions of the coherent potential approximation (CPA) based on the effective medium theory have been critically studied with respect to the decoupling schemes involved in them. Their computational tractability has been examined and it has been found that theself-consistent calculations in three-dimensional systems are immensely difficult to perform. A self-consistent calculation has been reported for simple cubic lattices with diagonal and off-diagonal disorder using a pair-CPA method. A significant finding of the paper is that it has been shown thatnon-analyticities are a general feature of extensions of CPA within multiple scattering framework. The non-analyticities were reported several times but a general proof of their existence was not noticed. It was also believed that the so-called molecular—CPA is analytic, this has been shown to be wrong here. The density of states results with off-diagonal randomness have been qualitatively understood to yield some information about the influence of off-diagonal randomness on Anderson localisation of an electron.

    • Optical absorption spectrum of Ni2+ doped in ammonium zinc sulphate

      S V Lakshman K Janardhanam

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      The optical absorption spectrum of Ni2+ ion doped in ammonium zinc sulphate has been studied at room and liquid air temperatures. From the nature and the positions of the bands a successful interpretation of all the bands could be made assumingOh symmetry for the Ni2+ ion in the crystal. The fine splitting of the3T11 band at liquid air temperature has been successfully interpreted to be due to spin-orbit interaction. The crystal field and spin-orbit parameters derived areDq=1000 cm−1;B=750 cm−1;C=3.45B andξ=600 cm−1.

    • Magnetic and transport properties of some light rare earth tungstates

      Naseeb Dar H B Lal

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      Measurements relating to molar magnetic susceptibility, dc electrical conductivity and thermoelectric power of Nd, Sm, Eu and Gd are reported. The ac electrical conductivity at a few temperature ranges is also given. It is found that it follows the Curie-Weiss law behaviour and this has been attributed to the crystal field effect. The experimental value of Bohr magneton for the magnetic ions has been found to be in good agreement with theory. Thermoelectric power is negative in the measured temperature range suggesting these materials to bep-type semi-conductors and holes as the dominant charge carriers. The results are explained using band theory.

    • Elastic constants of lead-bismuth alloys

      P A Varkey A R K L Padmini

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      A study of ultrasonic velocities and internal friction has been carried out in Pb-Bi alloys in the concentration range of 0 to 49.5 atomic % Bi using the composite oscillator technique. From the velocity and density data a set of elastic constants namely, Young’s modulus, rigidity modulus, bulk modulus and Poisson’s ratio are estimated. The results are interpreted in terms of the phase changes occurring in the alloy system. Internal friction is found to be more sensitive than the elastic constants to the phase changes.

    • Electroluminescence in the phosphor anthracene doped by 9-vinylanthracene

      B R Pandey D R Dubey Padmakar

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      Electroluminescence has been excited in the phosphor anthracene doped by 9-vinylanthracene (10−4 mole %) at room temperature (300°K). Variation of light output with voltage is governed by the relationB=B0 exp [−(C/V1/2)] as in the case of inorganic phosphors. HereB0 andC are constants andV is the rms value of the voltage applied to the phosphor. Frequency dependence of the electroluminescence has also been studied at different voltages of the applied electric field.

    • Interaction of a charged extended body with Magnetic field

      R C Sinha

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      It is shown that the conventional energy expressions need modification for the interaction of a charged extended body with a magnetic field. It is also shown that the correction term arises as a result of the rotation of the charged extended body about its centre of mass in a magnetic field. This can provide us with an additional source of energy that can be tapped with a suitable energy conversion device.

    • A new approach to peratization technique

      S V Kulkarni L K Sharma

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      The usual method of peratization technique is to expand scattering lengthA(a) in Born series in powers of the coupling constantg. In this paper a new approach to the peratization technique has been discussed starting with the standard equation for the scattering length. As an application of the theory developed, the cases of inverse fourth power and a logarithmic singular potentials have been discussed.

    • Classical solutions of a model of quark confinement

      G Rajasekaran V Srinivasan

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      We find the classical solutions of a model of quark confinement defined by the vanishing of colour currents. Both plane-wave type of solutions extending all over space as well as string-type of solutions confined to restricted regions of space are found.

    • Explicit classical solutions to Euclidean Yang-Mills theory with spherical symmetry

      Amitabha Mukherjee Probir Roy

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      All real classical solutions of the SU(2) Yang-Mills theory with spherical symmetry in 4-dimensional Euclidean space are constructed analytically and catalogued. The uniqueness of the solution of Belavinet al in possessing finite action is explicitly demonstrated.

    • Effect of gaunt factor correction in soft x-ray plasma diagnostics

      S V Deshmukh

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      The effect of Gaunt factor correction on temperature estimation over a range 100 to 1500 eV had been studied. Greene’s analytical expression for the quantum mechanical Gaunt factor averaged over Maxwellian distribution is used. Transmission ratios are calculated with and without Gaunt factor for various combinations of beryllium foils, taking into account x-ray emission due to free-free transitions. A significant difference (≳ 15%) is observed between the temperatures estimated from classical and quantum mechanical curves, above 600 eV. Selection of foil combinations useful for estimating higher temperatures is also discussed.

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