Akhiezer mechanism dominates relaxation of propagons in amorphous material at room temperature
YX Liao and J Shiomi, JOURNAL OF APPLIED PHYSICS, 130, 035101 (2021).
Propagons play an important role in tuning the thermal conductivity of nanostructured amorphous materials. Although advances have been made to quantitatively evaluate the relaxation time of propagons with molecular dynamics, the underlying relaxation mechanism remains unexplored. Here, we investigate the relaxation process of propagons in amorphous silicon, amorphous silica, and amorphous silicon nitride at room temperature in terms of the Akhiezer model, the parameters of which were evaluated by performing lattice dynamics and molecular dynamics analysis. The results show that the Akhiezer model can well reproduce experimental results obtained by various types of measurement methods, indicating that the Akhiezer mechanism dominates the relaxation process of propagons at room temperature. Moreover, we show that the appropriate sound speed of propagons is around 80% of the Debye sound speed and comparable to the transversal sound speed. We also reveal that diffusons' contribution to thermal conductivity of these amorphous materials is similar, which is around 1 W/m K, whereas propagons' contribution to the overall heat conduction varies significantly depending on the materials: it is 30% in amorphous silicon and silica but as high as 70% in amorphous silicon nitride. The Akhiezer mechanism offers a deeper understanding of propagon heat transport and sound attenuation in amorphous materials.
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