Superfluidity ofHe4in nanosize channels

Abstract

Superfluid properties of $^{4}\mathrm{He}$ adsorbed in nanometer-size channels have been studied using torsional oscillators for several channel diameters ranging from $1.5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}4.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Clear evidence that $^{4}\mathrm{He}$ exhibits superfluidity in channels larger than $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ is obtained. The superfluid transition in channels larger than $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ is well understood in terms of the finite-size Kosterlitz-Thouless transition, where the vortex unbinding mechanism plays an essential role. This vortex mechanism, however, should break down for channels narrower than the vortex core size $(2.5\ifmmode\pm\else\textpm\fi{}1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. Indeed, features that are not expected on the basis of the finite-size Kosterlitz-Thouless theory are observed for channels smaller than $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$: strong suppression of the observed superfluid density and disappearance of the dissipation peak associated with the diffusive motion of the vortices.