S. P. Tewari

Density fluctuation spectra of expanded liquid 85Rb at different temperatures

Abstract Density fluctuation spectra in expanded liquid 85 Rb measured at 1373 and 1873 K have been successfully explained using the microscopic theory of density fluctuations in a monatomic liquid. The self-diffusion coefficient occurring in the intermediate self-correlation function which is inextricably linked with the collective dynamics of the system, turns out to be frequency dependent and such a consideration also leads to the reproduction of the observed peak in dynamical structure factor near 4–5 ps −1 .

Collective dynamics of liquid alkali metals Cs and Rb

The modified microscopic theory of collective dynamics in a simple liquid has been used to explain the observed dynamical structure factor of liquid alkali metals Cs and Rb. The presence of collective modes is indicated by the observation of three-peak structure in the dynamical structure factor. Remarkable improvement, in agreement with the experimental results, is seen when the diffusion coefficient is made frequency dependent.

Effect of cubic and planar collective and localized modes on the specific heat of C60 fullerite for 0.2≤ T≤ 300 K

A dynamical model for C60 polycrystalline fullerite is suggested to explain successfully the recent experimentally observed seven orders of variation in its specific heat for 0.2 ≤ T ≤ 300 K. The collective translational modes have both cubic and planar characters which yield correct T3 dependence of the observed specific heat for T < 1 K and over 60% contribution of the total specific heat for T ≥ 100 K. Other localized modes of the buckyballs: librational, orientational diffusive, tunneling and intramolecular vibrational, contribute significantly in different ranges of temperatures. It appears that surface modes persist in curled up graphite sheets that form fullerenes.

ANISOTROPIC TEMPERATURE DEPENDENT RAYLEIGH-MOSSBAUER RECOILLESS FRACTION IN FULLERITE

Abstract The Rayleigh-Mossbauer recoilless fraction, f , for 57 Fe Mossbauer radiation in the temperature range 0–300 K, has been computed for different dynamical models of fullerite and compared with the corresponding anisotropic f in graphite. The realistic dynamical model incorporating acoustic modes, librational, orientational diffusive, tunneling and intramolecular modes, which explains the measured temperature variation of the specific heat in the range 0.2–300 K, yields an anisotropic f which can be easily observed by performing a Rayleigh-Mossbauer scattering experiment in fullerite.

COLLECTIVE DYNAMICS OF LIQUID Al

The recently suggested modified microscopic theory of liquids has been used successfully to explain the collective dynamics of liquid Al near its melting point. Considering the frequency-dependent self-diffusion coefficient occurring in the intermediate self-correlation function, which is inextricably linked with the collective dynamics of the system, the observed variation in dynamical structure factor of Al can be quantitatively reproduced for wave vector range 0.42 A-1 to 1.38 A-1 taking into account single relaxation time.

The effect of two dimensions on the temperature variation of displacement-displacement autocorrelation function, second-order Doppler shift and specific heat in graphite

A model for the frequency-distribution function of phonons in graphite that explicitly takes into account the presence of two-dimensional modes has been used to explain successfully (i) the recently observed temperature-dependent displacement-displacement autocorrelation function perpendicular to the basal plane (uz2) in the temperature range 5-300 K, (ii) the measured specific heat in the temperature range 10-300 K and (iii) the observed second-order Doppler shift in the temperature range 293-743 K. It is found that the two dimensional modes play a dominant role over the entire temperature range. Calculations based on other anisotropic models for (uz2) have also been made and compared.

DIFFERENTIAL AND TOTAL PHONON FREQUENCY DISTRIBUTION FUNCTIONS FROM THE CORRESPONDING MEASURED TEMPERATURE DEPENDENT MEAN SQUARE DISPLACEMENTS IN AN ANISOTROPIC CRYSTAL USING UNFOLDING TECHNIQUE

It is shown, for the first time, that phonon frequency distribution function (FDF) can be successfully extracted from the measured temperature dependent mean square displacement (MSD) of a particle in a condensed system using unfolding technique. In particular, the observed MSD of a Zinc atom in the basal plane, along the z direction and the total have been unfolded to obtain corresponding phonon FDF which yield much better values of the temperature dependent MSD than the trial FDF. The method can be fruitfully utilized to obtain FDF of elastic vibrations both for ordered and disordered solids from the readily available experimental MSD.

Formation of Cooper pair and superconducting transition temperature in a heavy fermion material: CeCu2Si2

Abstract It is shown that two heavy electrons in the heavy fermion material form a Cooper pair whose binding energy is significantly dependent on different physical parameters like the range of attractive interaction, the position of the conduction electron band with respect to the fermi surface the hybridisation potential, etc. Further, superconducting critical temperatures for different physical conditions were evaluated. It is therefore, possible to show consistently the formation of Cooper pair and occurrence of superconductivity in heavy fermion system: CeCu 2 Si 2 .

Phonon frequency distribution function and temperature variation of Rayleigh Mössbauer scattering fraction from aligned multiwalled carbon nanotubes

The phonon frequency distribution function (FDF) of aligned multiwalled carbon nanotubes (MWNTs) was obtained by unfolding its observed temperature variation of specific heat in the temperature range 1.8 to 250 K. An anisotropic dynamical model which incorporates the presence of two-dimensional modes on the surface of the tube and the intertube coupling was used as a trial function to carry out the unfolding process numerically. Temperature variation of Rayleigh Mossbauer scattering fraction of Sn-119 gamma-photons was computed using the trial and final FDF of MWNT. The temperature variation of Rayleigh Mossbauer scattering fraction in each case is quite marked and can be observed experimentally.

COLLECTIVE DYNAMICS OF A NANO-FLUID: FULLERENE, C60

Collective dynamics of a strongly correlated nano-fluid of fullerenes, C60 having number density 0.945 particles/nm3 at 1850 K has been predicted using the sphericalized inter-fullerene interaction and the self-consistent microscopic theory of fluids. The dynamical structure factors have been computed to yield much different dispersion relation of the collective excitations from that of the Lennard–Jones type fluid. The wave-vector dependent longitudinal viscosity of the nano-fluid has also been reported.