Third Sound Analysis of Superfluid 4He Films Adsorbed on Multiwall Carbon Nanotubes

Third sound is known to depend on the chemistry and geometry of the substrate on which a superfluid film is adsorbed. We have studied third sound in films adsorbed on multiwall carbon nanotubes to determine the effects of the cylindrical nanotube geometry. The Van der Waals potential for a single nanotube and the theoretical propagation velocity on the outer surface of a nanotube are presented. The nanotube geometry is found to lower both the potential and third sound velocity, and strongly limit the film thickness. Preliminary third sound measurements are presented for films on multiwall nanotubes 10 – 20 nm in diameter at 1.3 K. In addition to possible signs of capillary condensation, a novel re-entrant behavior of the superfluidity is observed: as the film thickness is increased near 3 atomic layers a superfluid signal is observed, but it disappears with the addition of further helium, and then reappears when the film thickness is further increased. This unusual behavior may be due to quasi-solidification of the helium when the third layer is just completed. The use of nanotubes is motivated by the possibility of a two-dimensional to one-dimensional crossover at low enough temperatures.