Effect of fourth-order anharmonicity on the phonon lineshapes in strongly anharmonic insulating crystals from first principles*

Our recent work shows that the inclusion of four-phonon scattering and phonon renormalization driven by the fourth-order anharmonicity of the crystal potential is important to accurately represent the thermal conductivities of strongly anharmonic insulating crystals 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 that are typically observed in inelastic neutron scattering measurements. 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. Using this approach, we show that these higher-order anharmonic processes significantly broaden the phonon lineshapes compared to their three-phonon counterparts even around room temperature, and their inclusion into the calculations is crucial to achieve agreement with the experimental measurements.

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