Phases of He4 and H2 adsorbed on a single carbon nanotube

Abstract

Using a diffusion Monte Carlo technique, we calculated the phase diagrams of $^{4}\mathrm{He}$ and ${\mathrm{H}}_{2}$ 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 $^{4}\mathrm{He}$ lowest-density solid phase shows supersolid behavior, in contrast to the normal solid that we found for ${\mathrm{H}}_{2}$. The nature of the second layer is also different for both adsorbates. Contrary to what happens on graphite, the second layer of $^{4}\mathrm{He}$ 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 ${\mathrm{H}}_{2}$ is a solid that, at its lowest stable density, has a small, but observable, superfluid fraction.