Structural Origin of Boson Peak in Glasses

TL;DR

This study uses ultrafast electron diffraction to reveal that the boson peak in glasses originates from collective transverse vibrational modes, linking atomic dynamics to the structural origin of this feature.

Contribution

It provides real-time, real-space experimental evidence connecting the boson peak to transverse acoustic vibrations in metallic glasses.

Findings

Boson peak frequency range identified as 1.0-1.8 THz.

Oscillation frequency inversely related to interatomic distances.

Transverse acoustic wave nature confirmed for boson peak.

Abstract

Boson peak, the excess low energy excitations in the terahertz regime, is one of the most unique features of disordered systems and has been linked to many anomalous properties of glass materials. The nature and structural origin of the boson peak remain elusive and have been debated for more than a half century mainly due to the lack of real-time and real-space experimental insights of the dynamic phenomenon. In this work we employed femtosecond MeV ultrafast electron diffraction to characterize the atomic dynamics of metallic glasses in real time. The experiment reveals collective atomic oscillations, presented in elastic electron scattering and atomic pair distribution functions, within the boson peak frequency range of 1.0-1.8 THz in both reciprocal and real space. It was found that the oscillation frequency has reciprocal dependence on interatomic pair distances and the corresponding wave velocity experimentally affirms the transverse acoustic wave nature of the boson peak. The observed strong correlation between THz acoustic vibrations and coherent electron scattering provides compelling evidence that the boson peak originates from the collective transverse vibrational modes of structurally ordered atoms in the disordered system.