Evidence of 1D behaviour of He$^4$ confined within carbon-nanotube bundles

TL;DR

This study provides the first thermodynamic evidence that helium-4 confined in small carbon nanotube bundles exhibits one-dimensional behavior at low temperatures, while larger bundles show two-dimensional characteristics.

Contribution

It demonstrates the first experimental observation of 1D He$^{4}$ behavior within nanotube bundles through specific heat measurements.

Findings

He$^{4}$ shows 1D behavior in small nanotube bundles.

Larger bundles exhibit 2D He$^{4}$ behavior.

Specific heat is highly sensitive to outgassing conditions.

Abstract

We present the first low-temperature thermodynamic investigation of the controlled physisorption of He$^{4}$ gas in carbon single-wall nanotube (SWNT) samples. The vibrational specific heat measured between 100 mK and 6 K demonstrates an extreme sensitivity to outgassing conditions. For bundles with a few number of NTs the extra contribution to the specific heat, C$_{ads}$, originating from adsorbed He$^{4}$ at very low density displays 1D behavior, typical for He atoms localized within linear channels as grooves and interstitials, for the first time evidenced. For larger bundles, C$_{ads}$ recovers the 2D behaviour akin to the case of He$^{4}$ films on planar substrates (grafoil).