Aditya Saxena

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.

On the excess specific-heat of single-walled carbon-nanotube ropes due to the adsorption of helium atoms in the temperature range 2-20 K

Experimental specific-heat measurements in single-walled carbon-nanotube (SWNT) ropes show marked increase when He-4 is allowed to be adsorbed, the increase in the values varying from 2 to 2.5 times in the temperature range 2-20 K. Beyond 20 K, the effect of adsorption on the specific heat of SWNT ropes is negligible. The phonon frequency distribution function (FDF) of the dynamical modes was extracted by employing the unfolding technique from the observed experimental temperature variation of the specific heat and a trial phonon FDF. The agreement between the observed experimental values of specific heat and the values obtained using the above method is quite good, the error being less than 10% in the temperature range 2-20 K and within 6% for 20 T 300 K at most of the temperature values. The change in the values of the specific heat can be attributed to the fact that the phonon FDF of He-4 adsorbed SWNT ropes shows marked difference from that when there is no adsorption for energies less than 200 K. The sensitivity of specific heat to adsorption at low temperatures by SWNT ropes is highlighted, particularly, by the appearance of large number of dynamical modes at 15 K. This can be experimentally checked using Rayleigh recoilless fraction of Mossbauer -ray photons from the carbon nanotube ropes sample.

Phonon frequency distribution function in aligned multi-walled carbon nanotubes

It is shown that it is possible to obtain the phonon frequency distribution function (FDF) of aligned multi-walled carbon nanotubes (MWNT) by unfolding its temperature variation of specific heat in the temperature range 1.8-250 K. An anisotropic dynamical model which takes into account the presence of two-dimensional modes on the surface of the tube and the intertube coupling has been used as a trial function to carry out the unfolding process numerically. The aligned MWNT sample turns out to be somewhat similar to graphite.

Variation of the ground state properties of trapped Bose-Einstein condensate due to localized impurity

Bose-Einstein condensates have been, by now, observed in as many as nine different atomic assemblies of bosons. Such a condensate is quantum mechanical interacting system whose ground state properties can be studied theoretically by solving the appropriate non-linear Gross-Pitaevskii-Ginzburg GPG equation. One can now study the change in the behavior of Bose-Einstein condensate by introducing a localized impurity which interacts with the condensate as a function of position of impurity in the condensate. The introduction of such an impurity can be mimicked by simply allowing an intensely focused laser beam to interact with the condensate. This would lead to alteration of ground state properties of the condensate as it would now interact with a potential of type V Sech2(r/w) where, V and w are amplitude and width of the impurity potential, respectively. The modified GPG equation in the presence of localized impurity potential as function of position in the condensate, has been numerically solved to obtain its various ground state properties as function of position, such as total energy per particle, chemical potential, kinetic, harmonic trap potential and two-body interaction energies per particle in addition to energy associated with impurity potential, correlation length, healing length etc. We have studied the behavior of the above-mentioned ground state properties as the position of localized impurity is changed in the condensate from core to peripheral position. While the total, harmonic oscillator potential and impurity energies decrease as the position of localized impurity is displaced from core of the condensate to its periphery, the value of two-body inter-particle interaction energy increases. Further, the values of chemical potential and total energy per particle shows decrease by ~ 9% and ~ 17% respectively, leading to the inference that the stability of condensate increases as the localized impurity is moved away from the core of the condensate.

General expression of chemical potential for Bose Einstein condensate in an anisotropic magnetic trap

A general expression for the chemical potential, μ, of Bose Einstein condensate in any form of the magnetic trap has been derived without resorting to any approximation regarding the number of particles in the condensate. Specific expression for μ has been deduced when the form of the magnetic trap corresponds to a right circular cylinder. Computations on the chemical potential, the speed of sound, healing length, interparticle separation in the core of the condensate, energy and the correlation length of the condensate have been made for different number of 87 Rb atoms ranging from hundred to ten million particles in the Bose Einstein condensate and compared with the corresponding results of numerically evaluated GPG equation and other approximate results wherever possible.

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).