Quantum effects in the kinetics of thermal expansion of C60 fullerite doped with 4He

The kinetics of thermal expansion of a fullerite C60 sample saturated with helium impurity (4He–C60) was studied in the temperature range of 2–20 K using the low-temperature precision capacitance dilatometry method. Two processes with different characteristic times were identified in the thermal expansion of the sample: the faster process is associated with temperature equalization throughout the sample volume, while the slower one corresponds to relaxation phenomena in the orientational subsystem of the C60 crystal lattice, possibly of a tunneling nature. Based on the comparison of the relaxation times in the orientational subsystem with the previously obtained diffusion times of helium atoms in the octahedral voids of the fullerite crystal lattice, a hypothesis was made regarding the possible influence of the diffusion process on the low-temperature thermal expansion of the 4He–C60 system.