Dispersive excitation in different forms of SiO2

The dynamical structure factors of vitreous silica (v-SiO2), polycrystalline α-cristobalite and polycrystalline α-quartz are measured by inelastic neutron scattering. Clear dispersive behavior extending up to 55 meV is observed for all samples, irrespective of structural order. Especially, the similarity of dynamical behavior between v-SiO2 and polycrystalline α-cristobalite is found to be extremely remarkable in the range of E⩾7 meV. We consider that the essential difference between a glassy system and a crystalline one should be in the low-energy region below the Boson peak energy. The quantum excitation between a two-level system based on buckling motion may contribute to the anomalous low-energy excitation in glasses.