Novel dynamics of vitreous silica and metallic glass

The dynamical structure factors S(Q, E) of SiO(2) glass and of Ni(67)Zr(33) metallic glass were measured by neutron inelastic scattering, over wide momentum hQ-energy E transfer range. The collective character of the excitations, that is whole dispersion relation, is observed for the first time in quasi-Brillouin zones, whose periodicity is defined by the static structure factor S(Q). The dispersions are quite analogous for both systems, but there are large differences. For sig, glass, it behaves like an acoustic dispersion, while it becomes optical with a broad feature at larger Q for Ni(67)Zr(33) metallic glass. The difference may be attributed to the difference in the degrees of short-range order. We also confirmed that the dispersion coexists with the low-lying boson peak, strongly localized mode. We suggest a change in common lore, namely that a wave whose wavelength is shorter than the local density fluctuation, that is a high-energy acoustic phonon, cannot go beyond this region and dampens; this is known as the Ioffe-Regel criteria.