Evidence of 1D behavior of He4 confined within carbon-nanotube bundles

We present the first low-temperature thermodynamic investigation of the controlled physisorption of He4 gas in carbon single-wall nanotube (SWNT) samples. The vibrational specific heat measured between 100 mK and 6 K demonstrates an extreme sensitivity to outgassing conditions. For bundles with a few number of nanotubes the extra contribution to the specific heat, C(ads), originating from adsorbed He4 at very low density displays 1D behavior, typical for He atoms localized within linear channels as grooves and interstitials, for the first time evidenced. For larger bundles, C(ads) recovers the 2D behavior akin to the case of He4 films on planar substrates (grafoil).