He atoms in carbon nanotubes and nanotube bundles

We have calculated the energetics of He-4 and He-3 atoms in one, two, and three carbon nanotubes. The binding energy and low-lying excited states are obtained for three characteristic sites: inside a tube in the presence of other tubes, within an interstitial channel, and in the outside grooves. Calculations have been performed using both an averaged helium-carbon potential that depends only on the distance to the nanotube axis, and a potential constructed as a sum of individual carbon-helium interactions. That way, we assess the effect of corrugation on the binding properties of helium atoms. We have also studied the energetics and structure of helium dimers, trimers, and tetramers in some of these geometries and found strong dependence of the binding energy on the geometry. From the single-particle spectrum, we obtain the thermodynamic properties of adsorbed helium atoms in carbon nanonotube bundles. As a consequence of corrugation a departure from the one-dimensional ideal gas behavior is observed; this deviation depends on the temperature, density, and bundle model.