Volume 34, Issue 4
April 1990, pages 1-367
pp 1- April 1990 Rapid Communications
M Lal H N Bajpai D Joseph R K Choudhury
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 YBa_{2}Cu_{3}O_{7} superconducting material were deposited on various substrates such as thin mylar sheet and thick substrates of SrTiO_{3}, MgO and Al_{2}O_{3}. 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.
pp 259-277 April 1990
Layered superconductors with anisotropic energy gap: specific heat and infrared absorption
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 $$2\Delta _t (\hat k_t )$$, wherek_{t} is the wavevector in the plane of the layers.
pp 279-287 April 1990
Field and temperature dependence of intergranular hysteresis in YBa_{2}Cu_{3}O_{7}
Shailendra Kumar G Ravikumar P K Mishra P Chaddah B A Dasannacharya Ram Prasad N C Soni
We have studied the variation of low field magnetization hysteresis in YBa_{2}Cu_{3}O_{7} as a function of the maximum magnetic field applied during a hysteresis cycle (1 G<H_{max}<7.3 G) and also as a function of temperature (77 K<T<95 K). The remnant magnetization is studied as a function ofH_{max} 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 ofJ_{c} is discussed.
pp 289-295 April 1990
Electrical resistivity of the alkaline earth metals Ca, Sr and Ba in the 50–300K temperature region
O Akinlade U Marte L A Hussain
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.
pp 297-302 April 1990
Elastic behaviour of lead fluoride under pressure
The variation of the second-order elastic constants (SOECs) and the longitudinal and shear modulii with hydrostatic pressure for the lead fluoride (PbF_{2}) 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 PbF_{2}. The equation of state for this crystal has also been reported.
pp 303-312 April 1990
Computation of small angle neutron scattering functions for molecules of arbitrary shapes
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.
pp 313-324 April 1990
NMR investigations of molecular motions inp-n-hexyloxybenzylidene-p′-n-propylaniline
Proton spin-lattice relaxation times,T_{1}, have been measured in the smectic phases, S_{G}^{2}, S_{G}^{1} and S_{A}, and in the nematic phase of HxBPA, in the temperature range, 220K<T<360 K. In the S_{G}^{1} and S_{G}^{2} phases,T_{1} has been measured at 15 and 40 MHz. The (S_{G}^{1}→S_{G}^{2}) and (S_{G}^{2}→S_{G}^{1}) transitions are clearly seen as discontinuities inT_{1}. The former transition is seen to be due to possible freezing or change of hydrocarbon chain motions of the molecule. Our data in the S_{G}^{1} 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 ofT_{1}, 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.
pp 325-331 April 1990
Study of electric potentials across a moving solid-liquid interface
A K Bhuniya N Prasad Lakshman Pandey P Ramachandrarao
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.
pp 333-345 April 1990
Evaluation of downstream-mixing scheme for 9.4-µm CO_{2} gasdynamic laser
P Chakravarty N M Reddy K P J Reddy
A theoretical analysis of a downstream-mixing 16-µm CO_{2} gasdynamic laser revealed the possibility of utilizing the downstream-mixing scheme for the generation of 9.4-µm radiation using a CO_{2} 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 N_{2} reservoir temperature of 2000°K and velocity ratio 1:1 between the CO_{2} and N_{2} mixing streams. These results (presented in graphs) clearly highlight the large potential of downstream-mixing CO_{2} gasdynamic laser for 9.4-µm laser generation.
pp 347-358 April 1990
Beam propagation method and its application to integrated optic structures and optical fibers
Shiva Kumar T Srinivas A Selvarajan
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.
pp 359-367 April 1990
L S Verma A K Shrotriya Ramvir Singh D R Chaudhary
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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