Boson peak and Ioffe-Regel criterion in amorphous silicon-like materials: the effect of bond directionality

TL;DR

This study investigates how bond directionality and local bending rigidity influence vibrational properties, including the Boson peak and Ioffe-Regel criterion, in amorphous silicon-like materials using large-scale numerical methods.

Contribution

It demonstrates that the Boson peak's upper limit aligns with the Ioffe-Regel criterion and explores how local bending rigidity affects vibrational density of states and phonon transport.

Findings

Boson peak corresponds to Ioffe-Regel limit for transverse waves

Mean-free path for transverse waves is 5-7 Å

Vibrational density of states is sensitive to bending rigidity

Abstract

The vibrational properties of model amorphous materials are studied by combining complete analysis of the vibration modes, dynamical structure factor and energy diffusivity with exact diagonalization of the dynamical matrix and the Kernel Polynomial Method which allows a study of very large system sizes. Different materials are studied that differ only by the bending rigidity of the interactions in a Stillinger-Weber modelization used to describe amorphous silicon. The local bending rigidity can thus be used as a control parameter, to tune the sound velocity together with local bonds directionality. It is shown that for all the systems studied, the upper limit of the Boson peak corresponds to the Ioffe-Regel criterion for transverse waves, as well as to a minimum of the diffusivity. The Boson peak is followed by a diffusivity's increase supported by longitudinal phonons. The Ioffe-Regel criterion for transverse waves corresponds to a common characteristic mean-free path of 5-7 \AA{} (which is slightly bigger for longitudinal phonons), while the fine structure of the vibrational density of states is shown to be sensitive to the local bending rigidity.