Eur. Phys. J. B 19, 531-543 (2001)
https://doi.org/10.1007/s100510170299

High frequency sound and the boson peak in amorphous silica

Institut für Physik, Johannes Gutenberg-Universität, Staudinger Weg 7, 55099 Mainz, Germany

Corresponding author: ^a kob@ldv.univ-montp2.fr

Received: 27 June 2000
Revised: 9 January 2001
Published online: 15 February 2001

Abstract

We present the results of extensive molecular dynamics computer simulations in which the high frequency dynamics of silica, i.e. for frequencies THz, is investigated in the viscous liquid state as well as in the glass state. We characterize the properties of high frequency sound modes by analyzing and , the longitudinal and transverse current correlation function, respectively. For wave-vectors q> 0.4 Å^-1 the spectra are sitting on top of a flat background. The dynamic structure factor exhibits for q> 0.23 Å^-1 a boson peak which is located nearly independent of q around 1.7 THz and for which the intensity scales approximately linearly with temperature. We show that the low frequency part of the boson peak is mainly due to the elastic scattering of transverse acoustic modes with frequencies around 1 THz. The strength of this scattering depends on q and is largest around q=1.7 Å^-1, the location of the first sharp diffraction peak in the static structure factor. By studying for different system sizes we show that strong finite size effects are present in the low frequency part of the boson peak in that for small systems part of its intensity is missing. We discuss the consequences of these finite size effects for the structural relaxation.