Specific heats of dilute neon inside long single-walled carbon nanotube and related problems

An elegant formula for coordinates of carbon atoms in a unit cell of a single-walled nanotube (SWNT) is presented and the potential of neon (Ne) inside an infinitely long SWNT is analytically derived out under the condition of the Lennard-Jones potential between Ne and carbon atoms. Specific heats of dilute Ne inside long (20, 20) SWNT are calculated at different temperatures. It is found that Ne exhibits 3-dimensional (3D) gas behavior at high temperature but behaves as 2D gas at low temperature. Especially, at ultra low temperature, Ne inside (20, 20) nanotubes behaves as lattice gas. A coarse method to determine the characteristic temperature ${\mathcal{T}}_c$ for low density gas in a potential is put forward. If $\mathcal{T}\gg {\mathcal{T}}_c$, we just need to use the classical statistical mechanics without solving the Shr"{o}dinger equation to consider the thermal behavior of gas in the potential. But if $\mathcal{T}\sim {\mathcal{T}}_c$, we must solve the Shr"{o}dinger equation. For Ne in (20,20) nanotube, we obtain ${\mathcal{T}}_c\approx 60$ K.