Neutron Scattering Study of the Low-Frequency Vibrations in Vitreous Silica

The vibrational excitations in vitreous silica above 150 GHz were investigated at 50, 100, and 290 K by inelastic scattering of cold neutrons. A detailed study of the frequency and wave-vector dependence shows that even at the lowest frequencies only a small part of the scattering is due to acoustic phonons. The type of motion of the dominating additional low-frequency modes is identified as a coupled rotation of Si${\mathrm{O}}_4$ tetrahedra. Possible connections to the low-temperature glass anomalies are discussed.