Phonon transmission at crystalline-amorphous interfaces studied using mode-resolved atomistic Green's functions

The transmission and reflection processes of THz phonons at solid interfaces are of fundamental interest and of importance to thermal conduction in nanocrystalline solids. The processes are challenging to investigate, however, because typical experiments and many computational approaches do not provide transmission coefficients resolved by phonon mode. Here, we examine the modal transmission and reflection processes of THz phonons across an amorphous Si region connected to two crystalline Si leads, a model interface for those that occur in nanocrystalline solids, using mode-resolved atomistic Green's functions. We find that the interface acts as a low-pass filter, reflecting modes of frequency greater than around 3 THz while transmitting those below this frequency, in agreement with a recent experimental report [C. Hua et al., Phys. Rev. B 95, 205423 (2017)]. Further, we find that these low frequency modes travel nearly unimpeded through the interface, maintaining their wave vectors on each side of the interface. Our work shows that even completely disordered regions may not be effective at reflecting THz phonons, with implications for efforts to alter thermal conductivity in nanocrystalline solids.