Quantum effect in the radial thermal expansion of bundles of single-walled carbon nanotubes doped with 4He

The radial thermal expansion ar of bundles of single-walled carbon nanotubes saturated with 4He impurities to the molar concentration 9.4% has been investigated in the interval 2.5-9.5 K using the dilatometric method. In the interval 2.1-3.7 K ar is negative and is several times higher than the negative ar for pure nanotube bundles. This most likely points to 4He atom tunneling between different positions in the nanotube bundle system. The excess expansion was reduced with decreasing 4He concentration.