Vibrational behavior of a realistic amorphous-silicon model

The vibrational properties of a high-quality realistic model of amorphous silicon are examined. The longitudinal and transverse dynamical structure factors are calculated, and fitted to a damped harmonic oscillator (DHO) function. The width $Γ$ of the best-fit DHO to the longitudinal dynamical structure factor scales approximately as $k²$ for wavevectors $k\lesssim0.55\textrmÅ^-1$, which is above the Ioffe-Regel crossover frequency separating the propagating and diffusing regimes, occurring at $k=0.38\pm0.03\textrmÅ^-1$. Using the DHO function as a fitting function for the transverse dynamical structure factor (without theoretical justification), gives a dependence of $Γ\propto k^α$ with $α\sim2.5$ for wavevectors $k\lesssim0.7\textrmÅ^-1$. There was no evidence for $Γ\propto k⁴$ behaviour for either polarization.