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Propagons play an important role in tuning the thermal conductivity ofnanostructured amorphous materials. Although advances have been made toquantitatively evaluate the relaxation time of propagons with moleculardynamics, the underlying relaxation mechanism remains unexplored. Here, weinvestigate the relaxation process of propagons in amorphous silicon, amorphoussilica, and amorphous silicon nitride at room temperature in terms of Akhiezermodel, the parameters of which were evaluated by performing lattice dynamicsand molecular dynamics analysis. The results show that the Akhiezer model canwell reproduce experimental results obtained by various kinds of measurementmethods, indicating that Akhiezer mechanism dominates the relaxation process ofpropagons at room temperature. Moreover, we show that the appropriate soundspeed of propagons is around 80% of the Debye sound speed and comparable tothat of the sound speed of the transversal modes. We also reveal that thecontribution of diffusons to the total thermal conductivity of these amorphousis similar, which is around 1 W/m K, while the contribution of propagons variessignificantly depending on the materials, which is 30% in amorphous silicon andsilica but can be as high as 70% in amorphous silicon nitride.

Akhiezer Mechanism Dominates Relaxation of Propagons in Amorphous at Room Temperature