DYNAMIC STRUCTURE FACTOR AND VIBRATIONAL PROPERTIES OF SIO2 GLASS

Molecular-dynamics simulations are performed to determine dynamic corr elations in SiO2 glass. The frequency and eigenvectors of vibrational normal modes are obtained by diagonalizing the dynamical matrix. Dynam ic structure factors, partial and total vibrational density of states (DOS), and participation ratios are calculated. The neutron-weighted d ynamic structure factor, S(n)(q, omega), exhibits all the important fe atures observed in the inelastic-neutron-scattering experiments on SiO 2 glass. As a function of omega, S(n)(q, omega) has two regions separa ted by a gap near 120 meV. The dominant features in S(n)(q, omega) are the peaks around 10-20 meV, an almost monotonic decrease between 20 a nd 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. Th e total phonon DOS show two well-delineated bands, a broad band betwee n 5 and 110 meV and a narrow band between 120 and 180 meV. The modes b elow 100 meV are spatially extended, whereas the high-frequency modes are localized. Calculations of the generalized neutron-weighted effect ive density of states are compared with neutron-scattering experiments .