Kakoli Bera

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.

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.

Role of Collective and Localized Modes on the Temperature-Dependent Thermal Conductivity in Polycrystalline C60 Fullerite Compacts

Recently observed thermal conductivity of polycrystalline C60 fullerite compacts has been explained on the basis of a suggested dynamical model of the fullerites which takes into account the collective acoustic phonon modes with frequency dependent relaxation time and localized libronic and orientational diffusive modes with constant relaxation times, in the temperature range 0.7–300 K. Though the bulk of the conduction is via collective modes, the localized modes, too, contribute significantly to the total thermal conductivity.

Wavevector- and frequency-dependent collective modes in one-component rare hot quantum and classical plasmas

An analytical expression for the complex dielectric function epsilon Q(k, omega )= epsilon 1Q(k, omega )+i epsilon 2Q(k, omega ) for a one-component quantum hot rare plasma, valid for all values of the wavevector k and angular frequency omega , has been obtained using the two-particle quantum distribution function. It contains terms of all orders in h2 and reduces to epsilon ct(k, omega ), suggested earlier by us, when Plancks constant h=0. epsilon 1Q(k, omega ) yields a convergent value for ( omega /k nu )>>1, unlike that given by the Green function formalism, when a large number of terms in the series are summed (V being the thermal velocity). Expressions for the dynamic structure factors SQ(k, omega ) and SQ(k, omega =0) and the static structure factor SQ(k) have been obtained which reduce to the corresponding classical expressions when h=0. Computations for one-component quantum and classical plasmas in an n-doped semiconductor having an electron density of 3.9*1018 cm-3 at 297 K have been made. s1Q(k, omega ), epsilon 2Q(k, omega ), SQ(k, omega ), etc., turn out to be quite different from the corresponding classical functions when hk/mV>1 (m being the mass of the mobile component). SQ(k, omega ), the singularity of which represents the collective mode, shows a smaller peak and its width is much broader in comparison with that of the corresponding Scl(k, omega ). The quantum-mechanical nature of the plasma thus introduces a sort of disorder in the plasma.

Energy-dependent total thermal-neutron-scattering cross-section and transport of a neutron pulse in Fullerite at 300 K

Crystalline fullerine-fullerite- is a molecular crystal and therefore possesses a number of dynamical modes: librational, orientational-diffusive, tunnelling and optical in addition to the usual anisotropic translational modes. Making use of these dynamical modes a thermal neutron scattering kernel for fullerite has been developed. The total neutron scattering cross-section of thermal neutrons have been computed in the energy range 10-4 - 0.3 eV. For inelastic scattering while one phonon and two phonon processes are sufficient, for libron exchanges as many as thirteen libron quanta exchanges have to be considered. Using the thermal neutron scattering kernel the multigroup Boltzmann transport equation has been diagonalized, to obtain its eigenvalue and corresponding eigenfunctions of a neutron pulse propagating in the medium. The variation of lowest eigenvalue with the size of the assembly is compared with the corresponding measured values in graphite and it turns out to be slower in smaller assemblies of fullerite. The time dependent thermalization of the neutron pulse, introduced in the assembly at time t=0, has also been computed. While for large assembly (B2=0.0006 /cm-2) the high energy neutron pulse gets thermalised in about 1500 μs, it takes as long as 5000 μs for smaller assemblies ( B2=0.005 /cm-2).

WAVEVECTOR AND FREQUENCY DEPENDENT DIELECTRIC FUNCTION, DYNAMIC STRUCTURE FACTOR AND THE INSTABILITY OF PLASMA WAVES IN TWO COMPONENT HOT RARE QUANTUM AND CLASSICAL PLASMAS

The full wavevector and frequency dependent complex dielectric function for two component classical and quantum rare hot plasmas have been derived. The real part of dielectric function is obtained in the form of a series. Difference between quantum and classical real and imaginary parts of dielectric function have been brought out by making explicit calculations. The quantum nature of the plasma brings about significant changes in both parts depending upon the magnitude of quantum parameter,R (= 8.93(λth)/λ).Expressions for the dynamic structure factors for both two component classical and quantum plasma have been evaluated for different values of the mass of the positive componentm+, temperature T+ and wavevector k. It is found that the plasma exhibits well defined collective modes for certain values of ¦k¦ accompanied by varying disorder which depends upon the values of m+ as well as on ¦k¦ and T+. For the quantum case the collective modes are less well defined as compared to the corresponding classical case, thus proving that quantum nature introduces inherent disorder in the system. But for both the cases, increase in temperature destroys collective modes. Another feature is the appearance of a hump near Ω = 0 which becomes smaller and vanishes as the quantum parameter is decreased.Instability of plasma modes in the presence of constant electric field has also been worked out for the quantum case.

Positron annihilation in one component quantum and classical rare hot plasmas

Abstract Temperature dependent positron annihilation rates λ, which are intimately related to the static pair correlation function of the electrons at the site of the positron g ± (0), have been worked out for electron densities that can be obtained in a laboratory plasma, making use of the recently suggested frequency and wavevector dependent dielectric function for one component quantum and classical rare hot plasmas. For a given density of electrons, the temperature dependence of g ± (0) in the two cases is quite different for low temperatures, T less than ∼100 K and can be detected experimentally.