Effect of higher-order anharmonicity on the phonon lineshapes in weakly-bonded solids from first principles

Recently, it has been shown computationally that the inclusion of higher-order anharmonic phonon renormalization and higher-order four-phonon scattering is crucial to accurately represent the thermal conductivities of weakly-bonded solids like sodium chloride (NaCl) [1]. Here we show, using our recently-developed unified first-principles framework, that four-phonon scattering also critically affects the phonon lineshapes of NaCl, typically observed in inelastic neutron and Raman scattering experiments. To capture these higher-order effects accurately, our calculations include the lowest-order three-phonon and higher-order four-phonon scattering processes, and a many-body self-consistent anharmonic phonon renormalization step to address the ill-defined nature of phonon quasiparticles. By performing these calculations over a broad range of temperatures, we show that four-phonon processes significantly broaden the phonon lineshapes compared to their three-phonon counterparts - particularly at high temperatures, and their inclusion into the calculations is pivotal to explain the experimental data.

[1] Navaneetha K. Ravichandran and David Broido, Phys. Rev. B 98 (8), 085205 (2018).