Experimental evidence for the significance of optical phonons in thermal transport of tin monosulfide

Wu, P.*; Murai, Naoki  ; Li, T.*; Kajimoto, Ryoichi  ; Nakamura, Mitsutaka   ; Kofu, Maiko   ; Nakajima, Kenji  ; Xia, K.*; Peng, K.*; Zhang, Y.*; Zhao, W.*

The understanding of the lattice dynamics is essential for engineering the thermal transport properties in quantum materials. Based on the canonical point of view, acoustic phonons are believed to be the principal thermal carriers in heat flow. Here, in this work, optical phonons are elucidated to play a pivotal role in determining the lattice thermal conductivity in thermoelectric material SnS by using the state-of-the-art inelastic neutron scattering technique combined with first-principles calculations. Additionally, in contrast to acoustic phonons, optical phonons are observed to exhibit pronounced softening and broadening with temperature. Our observations not only shed light on the significance of the optical phonons in thermal transport but also provide a vital clue to suppress the propagation of optical phonons to optimize the thermoelectric performance of SnS.