High frequency sound and the boson peak in amorphous silica

We present the results: of extensive molecular dynamics computer simulation s in which tile high frequency dynamics of silica, i.e. for frequencies nu > 0.5 THz, is investigated in the viscous liquid state as well as in the gl ass state. We characterize the properties of high frequency sound modes by analyzing J(1) (q, v) and J(t) (q, v), the longitudinal and transverse curr ent correlation function, respectively. For wave-vectors q > 0.4 Angstrom ( -1) the spectra are sitting on top of a flat background. The dynamic struct ure factor S(q, v) exhibits for q > 0.23 Angstrom (-1) a boson peak which i s located nearly independent of q around 1.7 THz and for which the intensit y scales approximately linearly with temperature. We show that the low freq uency part of the boson peak is mainly due to the elastic scattering of tra nsverse acoustic modes with frequencies around 1 THz. The strength of this scattering depends on q and is largest around q = 1.7 Angstrom (-1), the lo cation of the first sharp diffraction peak in the static structure factor. By studying S(q, v) for different system sizes we show that strong finite s ize effects are present in the low frequency part of tire boson peak in tha t for small systems part of its intensity is: missing. We discuss tile cons equences of these finite size effects for the structural relaxation.