Atomic Vibrations in Glasses

TL;DR

This paper explores how atomic disorder in glasses leads to unique vibrational modes, including boson-peak modes, affecting thermal and spectroscopic properties, with a focus on simple glasses and ongoing debates in complex systems.

Contribution

It demonstrates how disorder generates quasi-localized vibrational modes and explains their role in glass anomalies using a simple model.

Findings

Disorder creates quasi-local modes on both optic and acoustic branches.

Low-frequency modes contribute to the boson peak in glasses.

Vibrational behavior in simple glasses is well described, but complex systems remain debated.

Abstract

In glasses, atomic disorder combined with atomic connectivity makes understanding of the nature of the vibrations much more complex than in crystals or molecules. With a simple model, however, it is possible to show how disorder generates quasi-local modes on optic branches as well as on acoustic branches at low-frequency. The latter modes, possibly hybridizing with low-lying optic modes in real glasses, lead to the excess, low-frequency excitations known as {\it boson-peak modes}, which are lacking in crystals. The spatially quasi-localized vibrations also explain anomalies in thermal conductivity and the end of the acoustic branches, two other specific features of glasses. Together with the quasi-localization of the modes at the nanometric scale, structural disorder lifts the crystalline or molecular spectroscopic selection rules and makes interpretation of experiments difficult. Nevertheless, vibrations in simple glasses such as vitreous silica or vitreous boron oxide are nowadays rather well described. But a comprehensive understanding of the boson peak modes remains a highly debated issue as illustrated by three archetypal glass systems, vitreous SiO$_2$ and B$_2$O$_3$ and amorphous silicon.