• Volume 38, Issue 2

      February 1992,   pages  1-209

    • Instability of the spiral state of the doped Hubbard model

      A Vinash Singh Zlatko Tešanović Ju H Kim

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      It is shown that the homogeneous spiral state of the doped Hubbard model is unstable for infinitesimal doping concentration. The spin response about the Hartree-Fock spiral state is evaluated, considering both the interband and intraband processes. Driven by long-wavelength, intraband spin fluctuations, the instability towards infinitesimal fluctuations about the mean-field state is inferred from the nature of the static spin response.

    • Multiple small-angle scattering—A review

      S Mazumder A Sequeira

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      Small-angle scattering (SAS) is a powerful experimental technique in condensed matter physics for studying structural features of inhomogeneities of colloidal dimensions. So far the technique has been largely exploited to study thin samples for which the single scattering approximation, for the radiation-matrix interaction, holds good. The single scattering approximation is invalid when the thickness of the sample exceeds the scattering mean free path. This situation calls for a guideline to analyse the scattering data having significant contribution from multiple scattering. Since multiple scattering broadens the scattering profile, the beam broadening nature of multiple scattering can also be exploited, by making the sample suitably thick, to study large size inhomogeneities which are otherwise inaccessible to a small-angle scattering set up because of its resolution constraints. The present article presents a review and extension of the theoretical basis for analysing multiple scattering data from the point of view of a recent formalism on multiple small-angle scattering. The formalism is valid for both monodisperse and polydisperse scattering media characterized by the presence of large size inhomogeneities in the matrix. It is shown that multiple scattering from a polydisperse sample can be described by a system of coupled integrodifferential equation. However, multiple scattering from a monodisperse sample can be described by a Fokker-Planck type of equation. These equations have been analysed with an emphasis laid on the nature of the structural information pertaining to the inhomogeneities which is extractable from the multiple scattering profile. When the linear dimension of inhomogeneities becomes comparable to the scattering mean free path of the radiation in the sample, the statistical nature of the medium becomes pronounced. The statistical nature of the medium modulates the scattering profile. The modulation effect could be broadening or narrowing of the profile depending upon the nature of the inhomogeneities and their population distribution. The limiting regimes of validity and the implications of various approximations, frequently used to analyse the scattering data, have been indicated.

    • Synthesis and dielectric properties of Ba3NaRNb10O30 [R = La and Sm]

      K S Singh R Sati R N P Choudhary

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      Polycrystalline samples of Ba3NaRNb10O30 (R = La and Sm) have been prepared by high temperature solid-state reaction method. The X-ray powder diffraction technique has been used to test the formation of the compounds. Lattice parameters, space groups and basic structures of the compounds have been obtained from X-ray data. The dielectric constant has been measured as a function of frequency (100 Hz–10 kHz) and temperature (−180 to 200°C) to understand transition mechanism in these ferroelectric compounds.

    • Effect of addition of La2O3 in the grain boundaries of a material based on BaTiO3

      A P Biur

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      Results of an impedance study of capacitors of a material based on BaTiO3 with variable concentrations of La are presented. It is observed that variation in tanδ is related to variation in covalency of the added La at the grain boundaries. The tanδ is temperature dependent. The discussion brings out that added La controls the electron density at the grain boundaries.

    • On preparation of fine particles of MnFe2O4 and their magnetic properties

      S N Dolia Anjali Krishnamurthy Bipin K Srivastava Vipasha Ghose

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      Influence of the heat treatment, following a wet chemical process, has been examined on the sizes of the obtained particles of MnFe2O4. X-ray diffraction, magnetization and Mössbauer measurements have been used. The average sizes of the obtained particles are in the range of ∼ 100 Å–300Å with the upper size limit being ∼ 450 Å and more. The average size as well as size distribution is strongly influenced by the nature of the heat treatment followed for preparation of the sample. Prolonged hydrothermal heating resulted in larger average size than did dry heating at 400°C of the precipitate obtained from the initial wet process. Further dry heating following hydrothermal treatment did not lead to any major growth. Chemical nature of the starting materials also affects the sizes.

    • Critical current in Ag/BPSCCO tapes using low purity materials

      S R Shukla Y S Reddy R G Sharma

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      Silver-clad Bi1·7Pb0·4Sr1·8Ca2Cu3·5Ox (BPSCCO) tapes have been fabricated using low purity (98–99%) starting materials and following the powder-in-tube technique. MaximumJc values of 6·14 × 103 A·cm−2 at 77 K and 1·4 × 105 A·cm−2 at 4·2 K have been obtained in tapes subjected to the process of intermediate rolling and sintering. The bulk superconducting material used for the tape-fabrication contains both 2223 and 2212 phases in the ratio 60:40. A pure phase material and the optimization of the sintering parameters are expected to yield much higherJc values at 77 K. It is possible that the copper-rich phase(s) and/or a small amount of iron impurity (60 ppm) present in CuO might be acting as flux pinning sites and could be responsible for highJc values.

    • Fermi surface characteristics and enhancement factors for tantalum

      A K Solanki R Ahuja S Auluck

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      We present calculations of the extremal areas of Fermi surface orbits in the bcc transition metal tantalum usingab initio linear muffin tin orbital method in the atomic sphere approximation. The calculations demonstrate the need to include relativistic corrections for a good representation of the Fermi surface. Self-consistent calculations are performed using various exchange-correlation potentials. The calculations indicate that Barth-Hedin-Janak exchange provides the best agreement with the experiment. Enhancement factors are also calculated using the BHJ exchange-correlation potential. These are compared with experimental results as well as with some of the available theoretical calculations.

    • Surface line and plug flow models governing laser produced vapour from metallic surfaces

      Bekir Sami Yilbas R Davies Z Yilbas A Gorur I Acar

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      In this study a line surface and plug-flow models are examined. The laser beam/target interactions present alternative explanations for the previously noted observations of vapour ejection in the bursts. The importance of choosing between these models rests in the potential difference, in estimates for fluid and vapour variables and in possible inferences for adjacent regions.

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