• Volume 34, Issue 4

      April 1990,   pages  1-367

    • Detection limits of high temperature superconducting materials on various substrates by energy dispersive X-ray fluorescence and proton induced X-ray emission methods

      M Lal H N Bajpai D Joseph R K Choudhury

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      Application of energy dispersive X-ray fluorescence (EDXRF) and proton induced X-ray emission (PIXE) methods has been demonstrated for determining the elemental composition of thin film superconducting materials. The results of analysis carried out by EDXRF method have been compared with those obtained by PIXE method. Thin films of YBa2Cu3O7 superconducting material were deposited on various substrates such as thin mylar sheet and thick substrates of SrTiO3, MgO and Al2O3. In thin backing the minimum detection limits obtained for Cu, Y, Ba by the PIXE method are 20 ng, 70 ng and 800 ng respectively and the corresponding values by the EDXRF method are 3000 ng, 600 ng and 1000 ng. Detection limits for samples on thick backings deteriorated to a large extent by both methods.

    • Layered superconductors with anisotropic energy gap: specific heat and infrared absorption

      Sudhanshu S Jha

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      New oxide superconductors with layered structure are expected to have anisotropic energy gap in the generalized BCS pairing theory. The gap parameter $$2\Delta (\hat k)$$ can be quite different for $$\hat k$$ perpendicular to the plane of the layers as compared tok parallel to layer planes. Because of short coherence lengths ξ, quite small compared to the normal state carrier meanfree pathl, the effect of these anisotropies do not average out, as in many of the conventional superconductors. For a proper comparison of experimental results with the correct predictions of the pairing theory, a formulation is developed to obtain important physical quantities like specific heat and infrared absorption in the superconducting state of such anisotropic systems. This includes a brief account of the pairing theory generalized to layered crystals with arbitrary number of layers per unit cell, not necessarily equidistant. In an explicit model for the anisotropy of the gap parameter ink-space, with a simple form for the nonspherical Fermi-surface, it is shown that the low-temperature specific heat can have even a linear or a power-law temperature-dependence in the superconducting state. Even if the gap parameter does not vanish anywhere, its smeared-out exponential temperature-dependence may be difficult to be distinguished experimentally from a power-law behaviour. Similarly, it is shown that in the case of appreciable anisotropy, infrared absorption can take place much below the in-plane gap parameter $$2\Delta _t (\hat k_t )$$, wherekt is the wavevector in the plane of the layers.

    • Field and temperature dependence of intergranular hysteresis in YBa2Cu3O7

      Shailendra Kumar G Ravikumar P K Mishra P Chaddah B A Dasannacharya Ram Prasad N C Soni

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      We have studied the variation of low field magnetization hysteresis in YBa2Cu3O7 as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<Hmax<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofHmax andT and the measured dependences are explained using the extended critical state model. The potential of this technique as a contactless method of probing the temperature dependence ofJc is discussed.

    • Electrical resistivity of the alkaline earth metals Ca, Sr and Ba in the 50–300K temperature region

      O Akinlade U Marte L A Hussain

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      The computed values of the temperature dependent electrical resistivity is presented for the alkaline earth metals Ca, Sr and Ba. Numerical values of the mean free paths obtained using a finite mean free path approach is also reported. The structure factor has been evaluated using experimental values for the phonon spectra while local model potentials were used for the form factors. Our numerical results compare favourably with experiments.

    • Elastic behaviour of lead fluoride under pressure

      R K Singh C N Rao

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      The variation of the second-order elastic constants (SOECs) and the longitudinal and shear modulii with hydrostatic pressure for the lead fluoride (PbF2) has been investigated for the first time by means of a three-body force potential (TBP) model. The significance of three-body interactions (TBI) has been clearly demonstrated in reproducing the elastic constant variations and the pressure derivatives of SOECs of PbF2. The equation of state for this crystal has also been reported.

    • Computation of small angle neutron scattering functions for molecules of arbitrary shapes

      F U Ahmed P Martel A M Khan

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      A computer program has been developed to analyze small angle neutron scattering (SANS) data by using the Debye method of spherical modification proposed by Glatter. In the calculational procedure the model shape is emulated with a large number of overlapping small spheres which fill the volume of the model shape. A technique is described for fitting experimental data to a resolution-broadened model scattering function. At each stage of the iterative procedure the radius of gyration is computed. The program is able to calculate the scattering function of the mixture of two different molecules. This facility even allows one to calculate the scattering function of the mixture of monomer and dimer of a particular molecule in aqueous solution. In case a portion of the molecule has a different weight from the rest, the program has a variation to calculate the scattering function of that model as well.

    • NMR investigations of molecular motions inp-n-hexyloxybenzylidene-p′-n-propylaniline

      K Usha Deniz S R Kasturi

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      Proton spin-lattice relaxation times,T1, have been measured in the smectic phases, SG2, SG1 and SA, and in the nematic phase of HxBPA, in the temperature range, 220K<T<360 K. In the SG1 and SG2 phases,T1 has been measured at 15 and 40 MHz. The (SG1→SG2) and (SG2→SG1) transitions are clearly seen as discontinuities inT1. The former transition is seen to be due to possible freezing or change of hydrocarbon chain motions of the molecule. Our data in the SG1 phase have been fitted to a model in which anisotropic rotational diffusion of the end hydrocarbon chains as also that of the rigid part of the molecule are considered. In the nematic phase, at 351 K, the observed behaviour ofT1, measured in the frequency range, 5 to 40 MHz, agrees well with the theoretical predictions of Uklejaet al and Freed, who take into account long range collective order fluctuations and local reorientations. Using these theories, the correlation time for short range reorientations has been calculated from our results to be 4.3 × 10−10 and 1.1 × 10−9 s respectively.

    • Study of electric potentials across a moving solid-liquid interface

      A K Bhuniya N Prasad Lakshman Pandey P Ramachandrarao

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      The electric potential across a melting sample of pure lead was measured as a function of time. A difference of the order of a few hundred microvolts was seen between the measured values of the potential and those calculated using the temperature gradients and the thermopowers of the solid and the liquid phases. This difference persists as long as the solid-liquid interface moves and is attributed to the Galvani potential present at the solid-liquid interface. It is suggested that such measurements can be used to obtain the value of the Fermi energy for liquids.

    • Evaluation of downstream-mixing scheme for 9.4-µm CO2 gasdynamic laser

      P Chakravarty N M Reddy K P J Reddy

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      A theoretical analysis of a downstream-mixing 16-µm CO2 gasdynamic laser revealed the possibility of utilizing the downstream-mixing scheme for the generation of 9.4-µm radiation using a CO2 gasdynamic laser. The flow-field has been analyzed using complete two-dimensional, unsteady laminar form of Navier-Stokes equations coupled with the finite rate vibrational kinetic equations. The analysis showed that integrated small-signal gain of 11.5m−1 for Lorentzian broadening and 4.8m−1 considering Voigt function can be obtained for N2 reservoir temperature of 2000°K and velocity ratio 1:1 between the CO2 and N2 mixing streams. These results (presented in graphs) clearly highlight the large potential of downstream-mixing CO2 gasdynamic laser for 9.4-µm laser generation.

    • Beam propagation method and its application to integrated optic structures and optical fibers

      Shiva Kumar T Srinivas A Selvarajan

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      Practical applications of integrated optics require understanding of light propagation in dielectric waveguides of various geometries and calls for elegant and quick methods of analysis. In this paper, we use beam propagation method to analyse some integrated optic waveguiding elements such as waveguide with bend and branching waveguide. The method is extended to cylindrical co-ordinates, so that structures with circular symmetry can be easily solved. We first present a general beam propagation method algorithm, followed by results taking typical values for various parameters. Our studies show that the efficiency of the method depends on thez-propagation steps and on the number of points chosen for the Fourier transform. The algorithm developed can be used to analyse many other integrated optic structures and to study the effect of other input beam profiles.

    • Differential temperature sensors method for simultaneous determination of thermal conductivity and diffusivity

      L S Verma A K Shrotriya Ramvir Singh D R Chaudhary

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      An attempt has been made to minimize the sources of error involved in the transient probe method for thermal conductivity determination. Two sensors (thermocouples) are mounted parallel to the needle probe at known distances. This modification makes it a device for simultaneous conductivity and diffusivity determination. Thermal conductivity and diffusivity for glycerine, dune sand and mustard seed are determined by this method. Results obtained are compared with those obtained by a calibrated transient probe for conductivity and by a parallel wire method for diffusivity. Analysis of the results prove it to be a better instrument over the traditional ones. The technique can also be used as a direct reading device for conductivity and diffusivity measurements.

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