Origins of the two-step relaxation and the boson peak in an alkali silicate glass studied by molecular-dynamics simulation

A molecular-dynamics simulation of lithium metasilicate (${\mathrm{Li}}_2$ ${\mathrm{SiO}}_3$) has been performed. A clear two-step relaxation was observed at 700 K in a density correlation function (self-part) for Li ions following an exponential decay by vibrational motion. Oscillation is found in the second (β relaxation) region of the function, which is attributed to the so-called ‘‘boson peak.’’ The oscillation is clearer for O and Si than for Li. T he slowest relaxation (α relaxation) can be well fitted to a stretched exponential form, and an origin of this type of decay is confirmed to be a waiting time distribution of jump motions. Both the β relaxation and the boson peak are found to be due to correlated motion.