The boson peak in the vibrational spectra of glasses

TL;DR

This paper provides a comprehensive understanding of the boson peak in glasses by analyzing experimental data, simulations, and models, revealing its origin in quasi-localized vibrations and their collective behavior.

Contribution

It introduces a unified physical explanation for the boson peak, linking it to quasi-localized vibrations and their interactions, advancing the understanding of vibrational spectra in glasses.

Findings

The nonphononic VDoS has a universal power-law tail and a peak.

The boson peak arises from quasi-localized vibrations.

Modes near the peak involve coupled quasi-localized vibrations.

Abstract

A hallmark of glasses is an excess of low-frequency, nonphononic vibrations, in addition to phonons. It is associated with the intrinsically nonequilibrium and disordered nature of glasses, and is generically manifested as a THz peak -- the boson peak -- in the ratio of the vibrational density of state (VDoS) and Debye's VDoS of phonons. Yet, the excess vibrations and the boson peak are not fully understood. Here, using reanalysis of experimental data, extensive computer simulations and a mean-field model, we show that the nonphononic part of the VDoS itself features both a universal power-law tail and a peak, entirely accounted for by quasi-localized nonphononic vibrations, whose existence was recently established. We explain the mild variation of the peak's frequency and magnitude with glasses' thermal history, along with the strong variation of the power-law tail. We also show that modes that populate the peak's region feature many coupled quasi-localized nonphononic vibrations, when their spatial structure is considered. Our results provide a unified physical picture of the low-frequency vibrational spectra of glasses, and in particular elucidate the origin, nature and properties of the boson peak.