Neutron scattering from low-frequency excitations in a-sio2

The temperature dependence of the inelastic neutron scattering intensity from vitreous silica has been studied between 50 and 300 K down to frequencies of 150 GHz. Above 500 GHz one finds essentially harmonic behaviour. The anharmonic behaviour at low frequencies can be described in a relaxational model using parameters determined from mechanical measurements. From their dynamic structure factor, the relaxations are coupled rotational jumps of SiO4 tetrahedra. The jump width of the atoms in these relaxations is shown to be smaller than 0.8 Å. The results support the hypothesis of a common origin of the low temperatures glass anomalies.