The low energy excess of vibrational states in v-SiO2: The role of transverse dynamics

A numerical simulation study of the density dependence (rho = 2.2-4.0 g cm(-3) of the high energy collective dynamics in vitreous silica at mesoscopic wavevectors (Q = 1-18 nm(-1)) is reported. The dynamic structure factor, S(Q, w), and the density of states, rho (E), have been determined in the harmonic approximation via the system eigenvalues and the eigenvectors, in turn obtained by the direct diagonalization of the dynamical matrix. The BKS interaction potential employed is capable of reproducing the experimentally observed excess of states (boson peak), and its density dependence. The numerical simulation also indicates a strong density dependence of the transverse excitation dispersion relation, Omega(T)(Q), at large Q. Specifically, Omega(T)(Q) is found to: flatten at high Q to a value that increases with increasing density. The parallel between the density dependent flattening of Omega(T) (Q) and the density dependence of the boson peak suggests that the latter feature arises from the high Q portion of the transverse branch. This hypothesis is in line with both the interpretation by Elliott and co-workers (Taraskin et al 2001 Phys. Rev. Lett. 86 1255), who assign the boson peak to a phenomenon in glass reminiscent of the lowest energy Van Hove singularity in the companion crystal, and the Buchenau et al (1986 Phys. Rev. B 34 5665) assignment of the boson peak to the localized hindered rotation of SiO2 tetrahedra.