Third sound measurements of superfluid $^4$He films on multiwall carbon nanotubes below 1K

TL;DR

This study investigates third sound in superfluid helium films on carbon nanotubes at sub-Kelvin temperatures, revealing layer-dependent oscillations, dissipation effects, and deviations from theoretical predictions near layer completions.

Contribution

It provides new experimental insights into third sound behavior and KT transition modifications in helium films on nanotubes at ultra-low temperatures.

Findings

Oscillations in third sound velocity at layer completions

Sharp changes in Q factor at partial layer fillings

Broadening of KT transition and reduced $T_{KT}$ near layer completion

Abstract

Third sound is studied for superfluid films of 4He adsorbed on multiwall carbon nanotubes packed into an annular resonator. The third sound is generated with mechanical oscillation of the cell, and detected with carbon bolometers. A filling curve at temperatures near 250 mK shows oscillations in the third sound velocity, with maxima at the completion of the 4th and 5th atomic layers. Sharp changes in the Q factor of the third sound are found at partial layer fillings. Temperature sweeps at a number of fill points show strong broadening effects on the Kosterlitz-Thouless (KT) transition, and rapidly increasing dissipation, in qualitative agreement with the predictions of Machta and Guyer. At the 4th layer completion there is a sudden reduction of the transition temperature $T_{KT}$, and then a recovery back to linear variation with temperature, although the slope is considerably smaller than the KT prediction. Some of these effects may be related to changes in the gas-liquid coexistence regions.