Phonon linewidths as measures of anharmonicity in different materials

Phonons are central to most thermophysical properties of materials. Understanding broad trends in thermophysical properties help inform our current understanding of types of anharmonicity in materials and guide research avenues. We report a comparative analysis of experimental phonon linewidths from literature and our own inelastic neutron scattering results for Si, NaBr, FeGe₂, and Cu₂O. The linewidths are indicative of phonon lifetimes owing to 3-phonon processes, and often vary substantially for different branches in the same material. Even though comparisons between materials are impaired by differences in experimental technique, many trends are large enough to see easily. The phonon linewidths of UO₂ are approximately four times broader than those of Si [1]. Similar trends are seen when comparing Al and Si [2, 3]. In materials where the overall anharmonicity is not large, like Si, the anharmonicity can still dominate over the volume dependence of phonon frequencies for thermophysical properties such as thermal expansion [4, 5].

[1] Pang et al. PRL 110, 157401 (2013)
[2] Glensk et al. PRL 123, 235501 (2019)
[3] F. Knoop et al. PRM 4, 083809 (2020)
[4] D.S. Kim et al. PNAS 115, 1992 (2018)
[5] Y. Shen et al. PRL 125, 085504 (2020)