Helium in one-dimensional nanopores: Free dispersion, localization, and commensurate/incommensurate transitions with nonrigid orbitals

The single-particle states of helium within a bundle of carbon nanotubes can range fn,m nearly free-particle dispersion to localization, even within a single bundle. At intermediate effective masses, the corrugation in the external potential can be comparable to the intrasite He-He hard-core interaction. This results in a commensurate/incommensurate: transition, where the mobility of the doubly occupied domain-wall solitons at high density greatly exceeds the corresponding hole mobility below the transition.