Phonon interference in crystalline and amorphous confined nanoscopic films

摘要

Using molecular dynamics phonon wave packet simulations, we study phonon transmission across hexagonal (h)-BN and amorphous silica (a-SiO_2) nanoscopic thin films sandwiched by two crystalline leads. Due to the phonon interference effect, the frequency-dependent phonon transmission coefficient in the case of the crystalline film (Si|h-BN|Al heterostructure) exhibits a strongly oscillatory behavior. In the case of the amorphous film (Si|a-SiO_2|Al and Si|a-SiO_2|Si heterostruc-tures), in spite of structural disorder, the phonon transmission coefficient also exhibits oscillatory behavior at low frequencies (up to ~ 1.2 THz), with a period of oscillation consistent with the prediction from the two-beam interference equation. Above 1.2 THz, however, the phonon interference effect is greatly weakened by the diffuse scattering of higher-frequency phonons within an a-SiO_2 thin film and at the two interfaces confining the a-SiO_2 thin film.