Low temperature thermal expansion of pure and inert gas-doped Fullerite C60

TL;DR

This study investigates the low temperature thermal expansion of pure and inert gas-doped C60, revealing complex positive and negative contributions, impurity effects, and hysteresis linked to molecular reorientation and quantum phenomena.

Contribution

It provides new insights into how inert gas impurities influence the quantum reorientation and thermal expansion behavior of C60 at cryogenic temperatures.

Findings

Negative thermal expansion dominates at helium temperatures.

Impurities significantly affect molecular reorientation.

Hysteresis indicates orientational polyamorphous transformation.

Abstract

The low temperature (2-24 K) thermal expansion of pure (single crystal and polycrystalline) C60 and polycrystalline C60 intercalated with He, Ne, Ar, and Kr has been investigated using high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations on heating shows that the thermal expansion is determined by the sum of positive and negative contributions, which have different relaxation times. The negative thermal expansion usually prevails at helium temperatures. The positive expansion is connected with the phonon thermalization of the system. The negative expansion is caused by reorientation of the C60 molecules. It is assumed that the reorientation is of quantum character. The inert gas impurities affect very strongly the reorientation of the C60 molecules especially at liquid helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr- and He- C60 solutions has been revealed. The hysteresis is attributed to orientational polyamorphous transformation in these systems.