  An elegant formula for coordinates of carbon atoms in a unit cell of a
single-walled nanotube (SWNT) is presented and a new molecular motor of
double-walled carbon nanotube whose inner tube is a long (8,4) SWNT and outer
tube a short (14,8) SWNT is constructed. The interaction between inner an outer
tubes is analytically derived by summing the Lennard-Jones potentials between
atoms in inner and outer tubes. It is proved that the molecular motor in a
thermal bath exhibits a directional motion with the temperature variation of
the bath.
