Dynamic structure factor and vibrational properties of SiO2 glass.

Abstract

Molecular-dynamics simulations are performed to determine dynamic correlations in ${\mathrm{SiO}}_{2}$ glass. The frequency and eigenvectors of vibrational normal modes are obtained by diagonalizing the dynamical matrix. Dynamic structure factors, partial and total vibrational density of states (DOS), and participation ratios are calculated. The neutron-weighted dynamic structure factor, ${\mathit{S}}_{\mathit{n}}$(q,\ensuremath{\omega}), exhibits all the important features observed in the inelastic-neutron-scattering experiments on ${\mathrm{SiO}}_{2}$ glass. As a function of \ensuremath{\omega}, ${\mathit{S}}_{\mathit{n}}$(q,\ensuremath{\omega}) has two regions separated by a gap near 120 meV. The dominant features in ${\mathit{S}}_{\mathit{n}}$(q,\ensuremath{\omega}) are the peaks around 10--20 meV, an almost monotonic decrease between 20 and 100 meV, the gap near 120 meV, and a broad peak between 130 and 160 meV. The dynamic structure factor oscillates with variations in q. The total phonon DOS show two well-delineated bands, a broad band between 5 and 110 meV and a narrow band between 120 and 180 meV. The modes below 100 meV are spatially extended, whereas the high-frequency modes are localized. Calculations of the generalized neutron-weighted effective density of states are compared with neutron-scattering experiments.