Revealing the Geometrical and Vibrational Properties of the Defects Driving the Boson Peak

TL;DR

This paper introduces a method to distinguish hybridized phonons from localized defects in amorphous solids, revealing that boson peak vibrations are associated with two-dimensional shear defects, thereby clarifying the nature of these vibrational anomalies.

Contribution

A novel technique is developed to identify and characterize localized defects at the boson peak, resolving previous ambiguities about their geometry and vibrational properties.

Findings

Boson peak phonons hybridize with 2D shear defects

Localized defects are present at the boson peak frequency

Defects are not one-dimensional stringlets but compact 2D structures

Abstract

In amorphous solids, the vibrational density of states shows an excess of modes over the Debye model, known as the boson peak, whose origin remains unclear. Studies suggest a link to quasi-localized nonphononic vibrations or 'defects,' but identifying them is challenging due to hybridization with phonons that renders methods based on localization properties, such as the participation ratio, unreliable. We introduce a practical method to separate hybridized phonons from localized vibrations and find that boson peak phonons hybridize with compact, two-dimensional defects exhibiting oscillatory pure shear deformations. These two-dimensional defects are also exposed by the procedure recently employed to identify stringlets (Nature Physics volume 18, pages 669-677 (2022)), suggesting that these may not be one-dimensional objects as speculated. Our work demonstrates the presence of localized defects at the boson peak frequency and provides a comprehensive characterization of their vibrational and geometric properties, resolving the tension between the concepts of quasi-localized quadrupolar defects and stringlets.