Structural relaxation in vitreous silica

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 behavior. Low-frequency anharmonic behavior can be described by a relaxational model using parameters determined from ultrasonic measurements. The dynamic structure factor shows that relaxation involves coupled rotational jumps of ${\mathrm{SiO}}_4$ tetrahedra, with a jump width of the atoms smaller than 0.8 Å. These results support the hypothesis of a common origin of low-temperature glass anomalies.