Measurements of the elastic and inelastic neutron scattering from vitreous silica in the frequency range 0.3 to 4 THz and with scattering vectors in the range 0.2 to 5.3 A${̊}^{\mathrm{-}1}$ are analyzed in conjunction with heat-capacity measurements on the same samples to provide a microscopic description of low-frequency vibrational modes. The results show that additional harmonic excitations coexist with sound waves below 1 THz, and that these excitations correspond to relative rotation of ${\mathrm{SiO}}_4$ tetrahedra.

• Received 9 June 1986

DOI:https://doi.org/10.1103/PhysRevB.34.5665