• Volume 33, Issue 6

      December 1989,   pages  627-719

    • A PSD-based electronic system for AC response studies of superionic conductors

      P Balaya C S Sunandana

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      An electronic system based on quadrature oscillator, current-to-voltage converter and phase sensitive detector (PSD) has been designed and constructed for measurement of AC electrical conductivity and complex impedance/admittance on ionic and superionic conductors at several frequencies upto 60 kHz. The design incorporates a CMOS FET switch controlled by two anti-square reference signals for rectification and a differential amplifier for summation and impedance matching. The performance of the system has been demonstrated and the measurement possibilities discussed.

    • Interpretation of the recent Kolar events

      A S Joshipura G Rajasekaran V Gupta K V L Sarma

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      We give plausible interpretations of the unusual events seen in the proton decay detector at Kolar Gold Fields indicating the existence of a massive (≳2GeV) long lived (10−8−10−9s) particle. We show that it is possible to accommodate the particle in the standard model as a fourth generation neutrino, or inE6 grand unified theory as a neutral fermion occurring in27 representation or in supersymmetric theory as a scalar neutrino. However, there is a difficulty in explaining the large production rate for the particle.

    • An extended technicolour model for grand unification

      C Premkumar Yesudian

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      An extended technicolour grand unification model based on the gauge groupE6×SU(7) extended technicolour is presented. The symmetry-breaking based on extended technicolour theory is discussed. It is shown that the existing phenomenology is well explained by the model. The strangeness changing neutral currents may not be a problem with this model.

    • Infrared and raman studies on neodymium pentaphosphate and metaphosphates

      V Ramakrishnan J Kumar M Thirumavalavan F D Gnanam P Ramasamy

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      The infrared and Raman spectra of crystalline samples of NdP5O14, NaNdP4O12 and KNdP4O12 are recorded. The spectra are analysed on the basis of vibrations of P=O, P-O, PO2 and P-O-P groups. The P4O124− ion has cyclic structures both in NaNdP4O12 and KNdP4O12 crystals. It has been inferred that the P-O-P bridges are stronger in metaphosphates.

    • Phase transformation and electrical resistivity of tetracyanoethylene under pressure

      P Ch Sahu K Govinda Rajan Mohammad Yousuf R Mukhopadhyay S L Chaplot K R Rao

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      This paper reports the phase transformation behaviour of tetracyanoethylene (TCNE) under pressure as revealed by AC electrical resistivity, its time evolution and X-ray diffraction studies. An irreversible transformation from monoclinic to cubic phase occurs at 2.1±0.1 GPa and is indicated by a sharp resistivity drop at this pressure. The time evolution of resistivity studies indicate that this transformation occurs via an intermediate phase having resistivity higher than either of the two crystalline phases. Finally, the kinetics of phase transformations obtained by time evolution of resistivity is compared with the X-ray studies on the pressure quenched TCNE.

    • Electronic structure of La2CuO4

      G Subramoniam R S Rao S Mathi Jaya R Asokamani

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      The electronic band structure of La2CuO4 is performed using self-consistent linear muffin-tin orbital method. The 17 band complex is found to arise mainly from the overlap between Cu-3d and O-2p wavefunctions. The calculated density of states at the Fermi energy (NEF), the conduction band-width and the electronic specific heat coefficient are given.

    • Quantitative Auger electron spectroscopy analysis of binary alloys with separate matrix correction factors for bulk and surface

      N C Jain

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      A new method which takes into account the separate matrix correction factors for bulk and surface has been tried out for quantitative Auger electron spectroscopy analysis of binary alloys. The calculations use an iteration scheme. It has been applied to the Fe-Cr alloys studied at this Centre and the Auger electron spectroscopy data for the other alloy systems available in literature. The results are now more compatible with the expectation that the surface composition is different from the bulk.

    • Electron paramagnetic resonance and thermally stimulated luminescence studies of uranyl doped calcium sulphate

      T K Seshagiri V Natarajan A G I Dalvi M D Sastry

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      Electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) studies were conducted onγ-irradiated CaSO4:UO22+ to elucidate the role of the electron/hole traps in thermally stimulated reactions and to obtain the trap parameters (trap depth and frequency factor). Intense TSL glow peaks around 140, 375, 400 and 438±2K are observed and their spectral characteristics have shown that UO22+ and UO66− act as luminescent centres. EPR studies have shown the peaks at 140 and 400/438K to be associated with the thermal destruction of O and SO4 radical ion in two stages respectively. The maximum rate of thermal destruction of SO4 ions (as seen by EPR) in various alkaline earth sulphate matrices investigated in our laboratory is also summarized. The activation energy which characterizes the electron transfer reaction between SO4 and the dopant ion lies in the range of (0.95±0.15 eV). This value is independent of the dopant and therefore seems to be characteristic of the binding energy of hole in the SO4 radical ion.

    • Solitons and their resonances on two-dimensional superfluid films

      J Sreekumar V M Nandakumaran

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      The dynamics of saturated two-dimensional superfluid4He films is shown to be governed by the Kadomtsev-Petviashvili equation with negative dispersion. It is established that the phenomena of soliton resonance could be observed in such films. Under the lowest order nonlinearity, such resonance would happen only if two dimensional effects are taken into account. The amplitude and velocity of the resonant soliton are obtained.

    • 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.

    • 1H relaxation study in (NH4)2SbF5

      B Bandyopadhyay A Ghoshray R Mukhopadhyay R M Kadam

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      1H spin-lattice relaxation rate (T1−1) has been measured using inversion recovery technique in polycrystalline (NH4)2SbF5 system in the temperature range 140–400 K. From the plot of log (M0M) againstτ, we have estimated two differentT1 corresponding to two inequivalent ammonium ions in the unit cell. Temperature-dependence ofT1 in each case exhibits features of double minima indicating the influence of different correlation times corresponding to different types of motion. Activation energies at different temperature regions have been estimated. Some features of dynamics of motion of the different groups of ions across the phase transitions have been discussed.

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