Phonon scattering plays a central role in the quantum theory of phonon linewidth, which in turn governs important properties including infrared spectra, Raman spectra, lattice thermal conductivity, thermal radiative properties, and also significantly affects other important processes such as hot electron relaxation. Since Maradudin and Fein's classic work in 1962, three-phonon scattering had been considered as the dominant intrinsic phonon scattering mechanism and has seen tremendous advances. However, the role of four-phonon scattering had been persistently unclear and so was ignored. The tremendous complexity of the formalism and computational challenges stood in the way, prohibiting the direct and quantitative treatment of four-phonon scattering. In 2016, a rigorous four-phonon scattering formalism was developed, and the prediction was realized using empirical potentials. In 2017, the method was extended using first-principles calculated force constants, and the thermal conductivities of boron arsenides (BAs), Si and diamond were predicted. The predictions for BAs were later confirmed by several independent experiments. Four-phonon scattering has since been investigated in a range of materials and established as an important intrinsic scattering mechanism for thermal transport and radiative properties.
After this tutorial, the audience is expected to (1) be familiar with the background of three- and four-phonon scattering and their roles in thermal and radiative properties of materials, (2) understand the various characteristics of four-phonon scattering mechanism in different systems and scenarios, (3) understand the broad impact of four-phonon scattering on thermal transport and radiative properties in various materials, (4) be able to tell in which types of materials and scenarios will four-phonon scattering be critical and (5) be able to use the open-source code Four-Phonon together with Sheng BTE to calculate four-phonon scattering rates (linewidth) and thermal conductivities for materials.
1:30 - 1:50 pm Overview, background, and formalism of four-phonon scattering
1:50 - 2:10 pm Strong four-phonon scattering at high temperatures or in strongly an harmonic materials
2:10 - 2:30 pm Strong four-phonon scattering in materials with large acoustic–optical phonon band gaps
3:30 - 3:45 pm Strong four-phonon scattering for optical phonons and its impact on radiative properties
2:45 - 3:15 pm Break
3:15 - 3:35 pm Strong four-phonon scattering in two-dimensional materials with reflection symmetry
3:35 - 3:55 pm Discussion on (i) scaling with frequency, (ii) strong Umklapp scattering, (iii) negligible three-phonon scattering to the second order
3:55 - 5:00 pm Usage of Four-Phonon with Sheng BTE