Low-temperature anomalous specific heat without tunneling modes: a simulation for a-Si with voids

TL;DR

This study uses molecular dynamics simulations to analyze low-temperature specific heat in amorphous silicon with voids, revealing localized vibrational states that explain experimental observations without invoking tunneling modes.

Contribution

First numerical simulation demonstrating low-temperature specific heat behavior in disordered systems aligns with experiments without tunneling states, using harmonic approximation.

Findings

Good agreement with experimental specific heat data

Localized low-energy vibrational states identified

Sharp peak in C(T)/T^3 at T < 3K due to voids

Abstract

Using empirical potential molecular dynamics we compute dynamical matrix eigenvalues and eigenvectors for a 4096 atom model of amorphous silicon and a set of models with voids of different size based on it. This information is then employed to study the localization properties of the low-energy vibrational states, calculate the specific heat C(T) and examine the low-temperature properties of our models usually attributed to the presence of tunneling states in amorphous silicon. The results of our calculations for C(T) and "excess specific heat bulge" in the C(T)/T^3 vs. T graph for voidless a-Si appear to be in good agreement with experiment; moreover our investigation shows that the presence of localized low-energy excitations in the vibrational spectrum of our models with voids strongly manifests itself as a sharp peak in C(T)/T^3 dependence at T < 3K. To our knowledge this is the first numerical simulation that provides adequate agreement with experiment for the very low-temperature properties of specific heat in disordered systems within the limits of harmonic approximation.