Phases of ${}^4$He and H${}_2$ adsorbed on a single carbon nanotube

Using a diffusion Monte Carlo (DMC) technique, we calculated the phase diagrams of ${}^4$He and 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 ${}^4$He lowest-density solid phase shows supersolid behavior in contrast with the normal solid that we found for H${}_2$. The nature of the second-layer is also different for both adsorbates. Contrarily to what happens on graphite, the second-layer of ${}^4$He on that 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 H${}_2$ is a solid that, at its lowest stable density, has a small but observable superfluid fraction.