  The Boson peak (BP), an excess of vibrational density of states, is
ubiquitous for amorphous materials and is believed to hold the key to
understanding the dynamics of glass and glass transition. Previous studies have
established an energy scale for the BP, which is ~1-10 meV or ~THz in
frequency. However, so far, little is known about the momentum dependence or
spatial correlation of the BP. Here, we report the observation of the BP in
model Zr-Cu-Al metallic glasses over a wide range of momentum transfer, using
inelastic neutron scattering, heat capacity, Raman scattering measurements, and
molecular dynamics (MD) simulations. The BP energy is largely dispersionless;
however, the BP intensity was found to scale with the static structure factor.
Additional MD simulations with a generic Lennard-Jones potential confirmed the
same. Based on these results, an analytical expression for the dynamic
structure factor was formulated for the BP excitation. Further analysis of the
simulated disordered structures suggests that the BP is related to local
structure fluctuations (e.g., in shear strain). Our results offered insights
into the nature of the BP and provide guidance for the development of theories
of amorphous materials.
