Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations

TL;DR

This study uses non-equilibrium molecular dynamics to predict vibrational mean free paths and thermal conductivity in amorphous silicon, revealing frequency-dependent behaviors and quantum effects that align with experimental data.

Contribution

It introduces a method to predict vibrational mean free paths and thermal conductivity in amorphous silicon using spectrally decomposed heat current from NEMD simulations, incorporating quantum statistics.

Findings

MFPs scale as (frequency)^-2 below 5 THz

MFPs peak at 8 THz and drop below 1 nm above 10 THz

Predicted thermal conductivity matches experimental trends

Abstract

The frequency-dependent mean free paths (MFPs) of vibrational heat carriers in amorphous silicon are predicted from the length dependence of the spectrally decomposed heat current (SDHC) obtained from non-equilibrium molecular dynamics simulations. The results suggest a (frequency)$^{-2}$ scaling of the room-temperature MFPs below 5 THz. The MFPs exhibit a local maximum at a frequency of 8 THz and fall below 1 nm at frequencies greater than 10 THz, indicating localized vibrations. The MFPs extracted from sub-10 nm system-size simulations are used to predict the length-dependence of thermal conductivity up to system sizes of 100 nm and good agreement is found with separate molecular dynamics simulations. Weighting the SDHC by the frequency-dependent quantum occupation function provides a simple and convenient method to account for quantum statistics and provides reasonable agreement with the experimentally-measured trend and magnitude.