Quasilocalized Vibrations in Vitreous Silica

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 a, found to be in the range 1 less than or similar to alpha less than or similar to 2. The crossover between the core and tail regions takes place at about 10 angstrom. 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 a- cristobalite is demonstrated.