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

Abstract

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.