Boson peak in the dynamical structure factor of network- and packing-type glasses

TL;DR

This study investigates the microscopic origin of the boson peak in glasses by analyzing the dynamical structure factor, revealing it as a consequence of a dispersionless excitation band, using molecular dynamics simulations and effective-medium theory.

Contribution

The paper introduces a method to extract the vibrational density of states from the dynamical structure factor and links the boson peak to a dispersionless excitation band in glasses.

Findings

Boson peak arises from a dispersionless excitation band in $S(q,)$.

Two methods to extract vDOS from $S(q,)$ are compared.

Effective-medium theory captures the same mechanism in spring networks.

Abstract

Glasses are structurally disordered solids that host, in addition to crystalline-like phonons, vibrational excitations with no direct phononic counterpart. A long-standing universal signature is the excess vibrational density of states~(vDOS) over the Debye prediction, known as the boson peak~(BP), which has been extensively reported via inelastic neutron and X-ray scattering measurements of the dynamical structure factor $S(q,\omega)$. Here we quantify the vDOS directly from dynamical-structure-factor data and clarify the microscopic origin of the BP. We contrast two routes to extract the vDOS from $S(q,\omega)$: (i) using high-wavenumber $q$ data beyond the Debye wavenumber $q_D$ to access predominantly incoherent scattering and recover the vDOS in a manner analogous to velocity-autocorrelation-based approaches; and (ii) integrating $S(q,\omega)$ over the low-$q$ regime below $q_D$, which enables a decomposition of the vDOS into contributions from distinct wavenumber sectors and thereby provides direct access to the spatial character of vibrational modes. Focusing on the second route, we demonstrate that the BP in the vDOS emerges as the spectral consequence of a dispersionless excitation band in $S(q,\omega)$. Our main results are obtained from molecular-dynamics simulations, and we further show that the same mechanism is captured by an effective-medium theory for random spring networks, providing a unified interpretation that connects the excess vDOS to the wavenumber-resolved structure of vibrational excitations in glasses.