• R K Singh

      Articles written in Bulletin of Materials Science

    • Temperature dependence of microhardness in mixed crystals

      Sankar P Sanyal R K Singh

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      The temperature dependence of microhardness of mixed KCl-KBr crystals has been studied using the Kataoka and Yamada theory which considers plastic deformation and internal stress field due to elastic interactions in mixed crystals. The microhardness showed systematic variation with temperature in both the components and the mixed crystals and the probable reasons for this variation are indicated.

    • Composition dependence of transition temperature in new superconductors: La2 −x(Ba, Sr)xCuO4

      R K Singh A C Sharma N K Gaur Dinesh Varshney

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      We have investigated the variation of transition-temperature and coupling parameter with the composition concentration in La2 −x(Ba, Sr)xCuO4 superconductors using a formulation developed on the basis of an idea of pairing of charge carriers by exchange of both acoustic plasmons and phonons. Reasonably good agreement is found between the recent experimental results and our theoretical results on the superconducting transition temperature.

    • Composition dependence of transition temperature in some ceramic superconductors

      R K Singh Dinesh Varshney N K Gaur A C Sharma

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      The composition dependence of transition temperature in some ceramic superconductors (La2−x(Ba, Sr)xCuO4) was studied by modifying our earlier approach and developing a Fourier-transformed effective potential which involves the effect of two-dimensional (2D) acoustic plasmons. This potential was used to obtain the pairing (electron-electron attraction) parameter (λ), the averaged Coulomb repulsive parameter (μ*) and the cut-off 2D acoustic plasmon frequency (ωc) required to compute the superconducting transition temperature (Tc) from the strong coupling theory. The variations ofTc with compositions (x) obtained for La2−x(Ba, Sr)xCuO4 show reasonably good agreement with experimental data.

    • Effect of doping concentration on normal state resistivity of La2 −x (Ba, Sr)x CuO4 superconductors

      Dinesh Varshney R K Singh

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      Based on free electron layered electron gas model of quasi two dimensional CuO2 layers in La(Ba/Sr)CuO superconductors a model potentialV(q) is developed earlier with the electron-electron and electron-phonon interactions. The model approach facilitates the dielectric functions and the dispersion relations of 2D acoustic phonon and plasmon modes. We have then worked out the coupling strength (γ) linking electrons to the 2D acoustic phonon mode (ħω_) from the residue at the pole ofV(q). Furthermore, the scattering time (τe−ph) during electron-phonon interaction (EPI) for this simplified system is also estimated. The contribution to the normal state in plane resistivity due to EPI is then evaluated. Finally, the variations ofτ andρ is studied with the doping concentration (x) and temperature (T) and the results obtained by us show reasonably good agreement with the available experimental data.

    • Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors

      R K Singh Dinesh Varshney A K Khaskalam

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      The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO2(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (Tc) is then evaluated by using these coupling parameters and obtainedTc = 95(92)K for Y(Yb)Ba2Cu3O7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofTc withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.

    • Electrical conduction in the system Sr0·90La0·10Ti0·90M′0·10O3 (M′ = Co, Ni or Cr)

      H S Tewari R K Singh

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      We have successfully synthesized the system Sr1 −xLaxTi1 −xM′xO3 where M′ = Cr, Ni and Co by using conventional solid state ceramic method. Powder X-ray diffraction patterns of the different compositions show the formation of single phase materials. Measurement of AC conductivity as a function of frequency at different temperatures in the range 300–550 K show that conduction in these compositions occurs due to hopping of charge carriers between localized transition metal ion sites.

    • Oxygen deficiency dependence of transition temperature in (Sm, Er) Ba2Cu3O7−δ superconductors

      R K Singh Dinesh Varshney Amit K Khaskalam

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      We have investigated the effects of oxygen deficiency (δ) on the transition temperature (Tc) of (Sm, Er)Ba2Cu3O7−δ superconductors by incorporating the effects of the two dimensional (2D) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO2 plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation ofTc withδ in Y Ba2Cu3O7−δ system. The coupling strength (λ), the screening parameter (μ*) and the two dimensional acoustic phonon (plasmon) energyħω+) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium cuprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er)Ba2Cu3O7−δ superconductors.

    • Effect of oxygen deficiency (δ) on normal state resistivity of YBa2Cu3O7-δ superconductors

      Dinesh Varshney R K Singh Amit K Khaskalam

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      We have investigated theoretically the effect of oxygen deficiency (δ) on normal state resistivity (ρ) as well as its temperature dependence in YBa2Cu3O7-δ superconductors. This has been based on a potential which incorporates the structure factors and various interactions for double two-dimensional (2-D) conducting CuO2 plane. Using the Coulomb and electron-phonon terms of the interaction potential, we have then worked out the coupling strength (γ) for neighbouring electrons linked via 2-D acoustic phonons (_). Furthermore, the scattering time (τe-ph) due to electron-phonon interaction is deduced. The variations inτe-ph andρe-ph are studied with oxygen deficiency (δ) which is in the range of 0·0≤δ≤1·0, and the results thus obtained are found to be consistent with the earlier reported data. The residual resistivityρ0 obtained by extrapolation from experimental data together withρe-ph will predict the nearly-linear behaviour of normal state resistivity at temperatures (T) [90≤T≤300 K] in YBa2Cu3O7-δ superconductors.

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