  Energy exchanges between orderly intertube axial motion and vibrational modes
are studied for isolated systems of two coaxial carbon nanotubes at
temperatures ranging from 300 K to 500 K. It is found that the excess intertube
van der Waals energy, depleted from the intertube axial motion, is primarily
stored in low-frequency mechanical modes of the oscillator for an extended
period of time, and furthermore, such an energy exchange may be reversible. A
new nanoscale tribological phenomenon of negative friction is reported for the
first time in concentric cylindrical carbon nanotube oscillators.
