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He atoms in the space of carbon nanotubes

We have calculated the energetics of ^4He and ^3He atoms in one, two and three carbon nanotubes, using a fourth order real-space algorithm for solving local Schroedinger equations. The spectrum is obtained for three characteristic sites: inside a tube in the presence of other tubes, within an interstitial channel, and in a groove. We have also employed variational and diffusion Monte Carlo techniques for calculating the ground state energies of a helium atom in the same situations. 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 determine the effect of corrugation on the binding properties of helium. Monte Carlo methods are also used to study energetics and structure of helium dimers, trimers and tetramers in some of these spaces. Strong dependence of the binding energy on the geometry is shown. The single--particle spectrum is then used to obtain thermodynamics of adsorbed helium atoms in carbon nanonotubes bundles. In particular, we have calculated the chemical potential, energy and the specific heat. Depending on the temperature, density and the geometry a range of 1D to 2D behavior is observed.

helium; carbon nanotubes; specific heat

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