Low energy excitations in glasses and melts

Glasses and amorphous materials show, coexisting with the sound waves, a variety of low energy excitations: tunneling, quasi-localized vibrations and relaxations. The latter two are observed well into the liquid state. Using molecular dynamics both were shown to be centered, at low temperatures, typically on more than ten atoms or molecular units, which form chainlike structures. With increasing frequency the interaction of the quasi-localized modes with the sound waves and with each other increases, they de-localize. However, even at the so-called boson peak frequency, where the sound waves become over-damped due to the interaction, the vibrations can be decomposed into local and extended modes. Closely correlated with the local vibrations are the local relaxations, which can be envisaged as collective jumps of groups of atoms. With rising temperature both the total jump length and the number of atoms participating increases. In the melt when single jumps are no longer resolved one still observes a collective motion of chains of atoms.