Quasilocalized Vibrations in Vitreous Silica

Abstract

The properties of quasilocalized atomic vibrations in numerical models of vitreous silica are studied by means of a demixing technique. The low‐frequency vibrational modes are decomposed into bare non‐localized (plane‐wave‐like) modes and bare localized modes (BLMs). The BLMs are shown to exhibit three distinct spatial regions, characterized by different decay behaviors of the displacement field amplitude: the core, tail, and crossover regions. In the core region near the center of localization, the displacement amplitude decays exponentially with a typical localization length being on the order of the interatomic spacing. In the tail region, the decay of the displacement amplitude follows approximately a power law characterized by a decay exponent α, found to be in the range . The crossover between the core and tail regions takes place at about 10 Å. By means of a local symmetry‐mode comparative projection technique, a distinct similarity in the local vibrational motion for quasilocalized modes in vitreous silica and optic phonons in α‐cristobalite is demonstrated.