Superfluid Onset and Compressibility of $^4$He Films Adsorbed on Carbon Nanotubes

TL;DR

This study uses third sound measurements to investigate superfluid onset and compressibility of helium-4 films on carbon nanotubes, revealing layer-by-layer growth, oscillating compressibility, and deviations from theoretical predictions.

Contribution

It provides detailed experimental insights into superfluid transition and film compressibility on nanotubes, highlighting deviations from Kosterlitz-Thouless theory predictions.

Findings

Layer-by-layer film growth observed via oscillations in sound velocity

Temperature dependence of compressibility shows linear decrease with temperature

Superfluid onset temperature scales linearly with film thickness, but with an anomalous slope.

Abstract

Third sound measurements of superfluid $^4$He thin films adsorbed on 10 nm diameter multiwall carbon nanotubes are used to probe the superfluid onset temperature as a function of the film thickness, and to study the temperature dependence of the film compressibility. The nanotubes provide a highly ordered carbon surface, with layer-by-layer growth of the adsorbed film as shown by oscillation peaks in the third sound velocity at the completion of the third, fourth, and fifth atomic layers, arising from oscillations in the compressibility. In temperature sweeps the third sound velocity at very low temperatures is found to be linear with temperature, but oscillating between positive and negative slope depending on the film thickness. Analysis shows that this can be attributed to a linearly decreasing compressibility of the film with temperature that appears to hold even near zero temperature. The superfluid onset temperature is found to be linear in the film thickness, as predicted by the Kosterlitz-Thouless theory, but the slope is anomalous, a factor of three smaller than the predicted universal value.