Four-phonon scattering-dominated linewidth of optical phonons

Optical phonon linewidth is crucial to infrared optical properties of polar materials and thermal conductivity of certain materials that have a large number of optical phonon branches. The current understanding is, however, still limited to the lowest order phonon scattering, and the predicted linewidths often have large discrepancy with experiment. In this work we show the significance of four-phonon scattering in determining infrared optical properties for a range of important materials, including cubic boron arsenide (BAs), cubic silicon carbide (3C-SiC), and α-quartz. Strikingly, We find that four-phonon scattering generally dominates optical phonon linewidths at elevated temperatures over three-phonon scattering. In large band gap-materials, e.g., BAs and AlSb, four-phonon scattering rates are found to be orders of magnitude higher than three-phonon scattering rates even at room temperature. The predicted infrared optical properties of α-quartz
after including four-phonon scattering show good agreements with experimental measurements.