Kinetics of 4He gas sorption by fullerite C60. Quantum effects

TL;DR

This study investigates helium gas sorption kinetics in fullerite C60 across a wide temperature range, revealing thermally activated diffusion at higher temperatures and quantum tunneling effects at very low temperatures.

Contribution

It provides new insights into the temperature-dependent diffusion mechanisms of helium in C60, highlighting a transition from thermal activation to quantum tunneling.

Findings

Two-stage sorption and desorption processes identified.

Characteristic times increase then decrease with temperature, indicating different diffusion regimes.

Evidence of quantum tunneling of helium at temperatures below 8 K.

Abstract

The kinetics of helium gas sorption by a C60 powder and subsequent desorption of the 4He impurity from the saturated powder has been investigated in the temperature interval T = 2-292 K. Evidence is obtained that supports the existence of two stages in the temperature dependences of sorption and desorption. The stages account for the different times taken by helium to occupy the octahedral and tetrahedral interstices in the C60 lattice. The characteristic times of sorption and desorption coincide. It is found that the temperature dependences of the characteristic times of occupying the octahedral and tetrahedral interstices are nonmonotonic. As the temperature is lowered from 292 K to 79.3 K, the characteristic times increase, which indicates a predominance of thermally activated diffusion of helium in C60. On a further decrease to T = 10 K the characteristic times reduce over an order of magnitude. Below 8 K the characteristic times of sorption and desorption are temperature-independent. This suggests a tunnel character of 4He diffusion in C60.