The specific heat of amorphous silica within the harmonic approximation

TL;DR

This study assesses the harmonic approximation's effectiveness in calculating the specific heat of amorphous silica, finding good agreement at intermediate temperatures but discrepancies at very low temperatures.

Contribution

It demonstrates the conditions under which the harmonic approximation accurately predicts specific heat in amorphous silica, emphasizing the importance of system size and cooling rates.

Findings

Harmonic approximation valid below 300K

Accurate low-frequency density of states requires large systems and slow quenching

Calculated specific heat matches experiments between 200K and T_g

Abstract

We investigate to what extent the specific heat of amorphous silica can be calculated within the harmonic approximation. For this we use molecular dynamics computer simulations to calculate, for a simple silica model (the BKS potential), the velocity autocorrelation function and hence an effective density of states g(nu). We find that the harmonic approximation is valid for temperatures below 300K but starts to break down at higher temperatures. We show that in order to get a reliable description of the low frequency part of g(nu), i.e. where the boson peak is observed, it is essential to use large systems for the simulations and small cooling rates with which the samples are quenched. We find that the calculated specific heat is, at low temperatures (below 50K), about a factor of two smaller than the experimental one. In the temperature range 200K <= T <= T_g, where T_g=1450K is the glass transition temperature, we find a very good agreement between the theoretical specific heat and the experimental one.