Adsorption properties and third sound propagation in superfluid $^4$He films on carbon nanotubes

TL;DR

This paper investigates how superfluid helium films behave on carbon nanotubes, showing that surface tension and geometry limit superfluidity on very small nanotubes but allow it on larger structures, with experimental detection of third sound on nanotube bundles.

Contribution

It provides new insights into superfluid helium films on nanotubes, highlighting the effects of surface tension and geometry on film thickness and superfluidity, and reports experimental observation of third sound signals.

Findings

Surface tension keeps helium films very thin on nanotubes.

Superfluidity unlikely on single-walled nanotubes of ~1 nm diameter.

Superfluidity possible on larger nanotube bundles and multi-walled nanotubes.

Abstract

We consider the adsorption properties of superfluid $^4$He films on carbon nanotubes. One major factor in the adsorption is the surface tension force arising from the very small diameter of the nanotubes. Calculations show that surface tension keeps the film thickness on the tubes very thin even when the helium vapor is increased to the saturated pressure. The weakened Van der Waals force due to the cylindrical geometry also contributes to this. Both of these effects act to lower the predicted velocity of third sound propagation along the tubes. It does not appear that superfluidity will be possible on single-walled nanotubes of diameter about one nm, since the film thickness is less than 3 atomic layers even at saturation. Superfluidity is possible on larger-diameter nanotube bundles and multi-walled nanotubes, however. We have observed third sound signals on nanotube bundles of average diameter 5 nm which are sprayed onto a Plexiglass surface, forming a network of tubes.