M. Boninsegni et al., Helium in one-dimensional nanopores: Free dispersion, localization, and commensurate/incommensurate transitions with nonrigid orbitals, PHYS REV L, 86(15), 2001, pp. 3360-3363
The single-particle states of helium within a bundle of carbon nanotubes ca
n range fn,m nearly free-particle dispersion to localization, even within a
single bundle. At intermediate effective masses, the corrugation in the ex
ternal potential can be comparable to the intrasite He-He hard-core interac
tion. This results in a commensurate/incommensurate: transition, where the
mobility of the doubly occupied domain-wall solitons at high density greatl
y exceeds the corresponding hole mobility below the transition.