Structure and Dynamics of Excited States in Glasses Studied by Pulsed Neutron Scattering

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The dynamical structure factors S(Q,E) of SiOィイD22ィエD2 glass and NiィイD267ィエD2ZrィイD233ィエD2 glass were measured by inelastic neutron scattering. The advance of pulsed neutron scattering technique enabled us to obtain the S(Q,E) in a wide momentum-energy space with high resolution and high neutron flux.The dispersive excitations in these glasses are clearly observed for the first time, whose periodicity is defined by the respective static structure factors S(Q). This feature suggests the existence of quasi-Brillouine zone which originate from short range structure. However, the first sharp diffraction peak in S(Q), which correlate with the medium range order of glass structure, has no relation with dispersive behavior. The dispersive excitations in SiOィイD22ィエD2 glass is acoustic-like behavior, while that of NiィイD267ィエD2ZrィイD233ィエD2 glass optic-like one. This difference may be attributed to the rigidity of short range structure. It is considered that NiィイD267ィエD2ZrィイD233ィエD2 glass has no rigid structural units which surely appear in SiOィイD22ィエD2 glass. The Boson peak, which is a universal feature of glassy materials, is also proved to be strongly localized mode.It must be noted that the dispersive behavior exists above the energy range of Boson peak. Our result is contrary to the Ioffe-Regel criteria which argue that no collective excitations should exist above an energy determined from correlation length of local density fluctuation. We can say that the observations of dynamic structure factor S(Q,E) over a wide range of momentum-energy space bring us fruitful information on structure and dynamics of glassy materials.