Phases of 4He and H2 adsorbed on a single carbon nanotube

Abstract

Using a diffusion Monte Carlo technique, we calculated the phase diagrams of 4He and H2 adsorbed on a single (5,5) carbon nanotube, one of the narrowest that can be obtained experimentally. For a single monolayer, when the adsorbate density increases, both species undergo a series of first-order solid-solid phase transitions between incommensurate arrangements. Remarkably, the 4He lowest-density solid phase shows supersolid behavior, in contrast to the normal solid that we found for H2. The nature of the second layer is also different for both adsorbates. Contrary to what happens on graphite, the second layer of 4He on the tube is a liquid, at least up to the density corresponding to a third-layer promotion on a flat substrate. However, the second layer of H2 is a solid that, at its lowest stable density, has a small, but observable, superfluid fraction.