Recently, the team led by Jiaqing He (Chair Professor of Physics, SUSTech) reported novel thermal transport mechanisms in thermoelectric materials. The results, entitled “First-Principles Study of Anharmonic Lattice Dynamics in Low Thermal Conductivity AgCrSe₂: Evidence for a Large Resonant Four-Phonon Scattering”, have been published in one of the most famous international journals Physical Review Letters.

According to classical thermal transport theories, the lattice thermal conductivity of a semiconductor material is dominated by the so-called three-phonon scattering process. The phonon scattering rate τ^-1 is thus proportional to the temperature and the square of phonon frequencies as . As a result, it’s commonly believed that the low-frequency acoustic phonons are the main heat carriers in semiconductors. Recently, however, abnormal thermal transport behaviors have been discovered in many materials with extremely low lattice thermal conductivities. These behaviors cannot be simply explained by the classical three-phonon scattering theory and they stimulate the research of novel thermal transport mechanisms such as high-order phonon interactions or off-diagonal heat transport.

In this work, the team developed the many-body perturbation theories for phonons and calculated the anharmonic lattice dynamics of layered structure AgCrSe₂ (as shown in Figure 1a) from 100 K to 300 K. According to their calculations, it was found that (1) Ag atoms have giant fourth-order anharmonic interactions with neighboring atoms and (2) the transverse acoustic phonon branches have very unique flat phonon dispersion relation in Brillouin zone (Figure 1b). These gigantic anharmonic interactions as well as dispersion-less phonon dispersions lead to strong four-phonon Fermi resonance and drastically increase the four-phonon scattering rates (as shown in Figure 2). Further calculations showed that at the temperature range from 100 K to 300 K, the four-phonon scattering rate is larger than the three-phonon scattering rates by an order. Thus, the thermal transport by acoustic phonons is dominated by four-phonon interactions, instead of the commonly believed three-phonon interactions.

The team had also demonstrated that their four-phonon Fermi resonance model is universal and can be readily applied to a variety of material systems, such as cuprous halides, SnSe, PbTiO₃, and filled skutterudites.

Figure 1. (a) Atomic structure of AgCrSe2; (b) Phonon dispersion of AgCrSe₂. The low-lying transverse acoustic phonons are clearly flat throughout the Brillouin zone.