S.P. Tewari

Theory of long wavelength collective motion in liquids

Abstract The theory of collective motion of liquids, recently suggested by the authors, has been found to explain successfully the three-peak structure for small momentum transfers, observed experimentally and confirmed by molecular dynamical calculations, in liquid rubidium. The theory happens to be the first microscopic theory of liquids which joins smoothly the zero-sound and hydrodynamical regions of density fluctuations in liquids.

Dynamics of a supercooled liquid

Abstract The theory of collective motion in liquids suggested by the authors has been found to explain successfully the recent experimental results of temperature dependent disorder in supercooled liquid gallium. Metastability limit is exhibited through a singularity of S ( k , ω = 0) and corresponds to a critical value of correlation between different particles beyond which the supercooled liquid goes to the thermodynamically stable solid state.

Effect of plasmons on the critical temperature of lanthanum and yttrium based high temperature superconductors

Abstract An expression for superconducting critical temperature, T c has been derived from a generalized BCS integral gap equation for a strongly coupled superconductor, when electron-phonon, electron-electron and electron-plasmon mechanisms are simultaneously present. The expression has been used to calculate the variation of T c with electron-plasmon coupling constant, λ pl , for lanthanum and yttrium based high- T c superconductors. It is found that the effect of plasmon on T c is significant. The study also indicates as to how one can raise T c - up to room temperature - in YBa 2 Cu 3 O 7 .

Superconductivity in zinc at high pressure

The variation of the superconducting critical temperature, Tc, with pressure in 67Zn has been studied making use of a recent experimental study on the pressure-dependent Lamb-Mossbauer factor in the range 0-5.8 GPa at 4.2 K. It is found that the critical temperature decreases with pressure. At 5.8 GPa the value of Tc ( approximately=0.123 K) is about 1/6.8 of that at ambient pressure. The variation of Tc with pressure can easily be measured.

Effect of thermal phonons and two-dimensional acoustic plasmons on superconductivity in YBa2CuO7

Abstract The superconducting transition temperature, T c , has been studied using the strongly coupled theory of superconductors in high- T c YBa 2 Cu 3 O 7 in the presence of realistic physical parameters, thermal phonons and two-dimensional acoustic plasmons. The calculated value of T c , using a realistic phonon distribution function, the electron-electron Coulomb interaction and the Stoner factor, comes out to be ≈ 65 K. When the finite temperature effect is taken into account through the modification of the temperature dependence of the electron-phonon mass enhancement factor, the value of T c increases significantly by ≈ 18 K to ≈ 83 K. Finally, when the two-dimensional acoustic plasmons are considered, a further increase of ≈ 17 K results, making the calculated T c ≈ 102 K, which is in reasonable agreement with the experimental results. The superconductivity in YBa 2 Cu 3 O 7 is, thus, marked by strong coupling effects, electron-phonon interaction, the finite temperature effect and electron-plasmon interaction, which contribute significantly towards its high T c .

Effect of anisotropic phonons and planar acoustic plasmons on superconductivity in lanthanum cuprates

Abstract A realistic anisitropic phonon frequency distribution function for superconducting lanthanum cuprates, which takes account of (i) the presence of planar modes, (ii) is quite close to the experimental phonon frequency distribution function and (iii) yields the specific heat values in the temperature range 5 to 50 K in reasonable agreement with the various measured values, has been used to calculate the superconducting critical temperature, T c , making use of the expression based on the strongly coupled theory of superconductivity and other realistic parameters. The calculated value of T c ≅34.8 K is in reasonable agreement with the observed results. The effect of thermal phonons and two-dimensional acoustic plasmons has been studied (when both electron-phonon and electron-plasmon mechanisms are present) which results in an increase of ~3 K and ~9 K, respectively. Finally the variation of T c with the change of phonon anisotropy has been reported and it is found that the T c is noticeably dependent on the degree of phonon anisotropy. A significant result is that for superconductivity to occur in these compounds, coupling between the Cu-O planes is essential.

The effect of crystal anisotropy on the Lamb Mossbauer recoilless fraction and second-order Doppler shift in zinc

The recently observed highly anisotropic Lamb Mossbauer recoilless fractions along the c axis and in the basal plane in a zinc crystal at temperatures of 4.2, 20.8 and 47 K are successfully explained using a dynamical model which takes into account the presence of planer modes in its dynamics. The experimental temperature-dependent second-order Doppler shifts and specific heat are also in agreement with the calculated values based on the suggested dynamical model.

The absorption spectra of optical bistability with dispersion

Abstract The absorption spectra of a weak field (Ω) probing the bistable behaviour of a homogeneously broadened medium (ω 0 ) detuned with respect to the exciting laser light (ω L ) has been investigated. A dispersion like structure superimposed on an inverted peak near the critical threshold is predicted. The frequency response of the differential gain amplifier is also obtained.

Effect of thallium concentration on Tl-2201 superconducting phase by flash sintering process

Abstract Samples with nominal composition TlxBa2Cu2O6 where x = 0.5, 1.0, 1.5, 1.75, 2.0, 2.5 and 3.0 were calcined at 890°C for 10 h and sintered at various temperatures for 3 1 2 min. X-ray powder diffraction patterns of all the samples show the characteristics of the 2201 phase which suggests the transformation of all the nominal compositions to 2201 phase. In some cases extra peaks due to BaCuO2 and Tl2O3 are also present. The samples having Tl contents 1.5, 1.75 and 2.0 are monophase material with unit cell parameters a = 3.86 A , c = 23.20±0.02 A . The overall symmetry is tetragonal. All the lines are indexible on the I4/mmm space group. Samples with Tl content 1.5-2.0, sintered at 930–950°C, show the best superconducting properties. The highest Tc is obtained in Tl2.0 and the highest Meissner fraction in Tl1.5. Chemical analysis indicates Tl deficiency in all the samples, which suggests substitution of Tl3+ by Cu1+ or Cu2+ and oxidation of copper to Cu3+, which increases monotonically with Tc. Tl contents in optimum samples lie in the range 1.14-1.82 and oxygen content varies from 5.6-6.1

Effect of anisotropic phonons on superconductivity

Abstract Effect of crystal anisotropy which results in the anisotropic phonon frequency distribution function has been studied on superconductivity in a strongly coupled superconductor. The dependence of the superconducting critical temperature, Tc, on the degree of anisotropy is quite involved. In highly anisotropic systems superconductivity is suppressed. The expression for Tc is related to the total mean square displacement of the vibrating ion, yielding the result that, for superconductivity, the phonon frequency distribution function from mean square displacement is preferred to that obtained from other studies for consistent physical parameters. Explicit calculations for 67Zn are reported.