Merging of the acoustic branch with the boson peak in densified silica glass

Both high-frequency acoustic modes and the boson peak related to SiO₄ librations are observed in a single inelastic x-ray scattering experiment. The experimental data are consistent with a picture where the acoustic modes experience a crossover at a frequency Ωco beyond which plane waves cease to exist. The spectra evolve with the scattering vector to merge into a broad boson peak at ΩBP≈Ωco. The latter, although essentially optic in nature, might hybridize with the resonant acousticlike modes, which can be crucial to their strong scattering.