Calculations of the generalized dynamic structure factor for amorphous silicon

Abstract In an extension of our previous studies, a normal mode analysis is performed for a 4096 atom structural model, of cubic cell size L=44 A, of amorphous silicon within the Stillinger Weber potential. The peaks in dynamic structure factor quantities SL(Q,ω) and ST(Q,ω), where L and T refer to projections of eigenvectors onto vectors parallel and perpendicular to Q , are studied. An attempt is made to estimate Ioffe–Regel crossover frequencies. In agreement with a previous estimate for the commencement of diffusons a frequency corresponding to 17 meV is found for the transverse-like Ioffe–Regel crossover. SL for fixed Q, is found to be reasonably Lorentzian up to Q=3 (2π/L). SL functions which have peak-height frequencies above the transverse acoustic maximum frequency ωT,max are strongly skewed towards higher frequencies. The Lorentzian breadths below and above ωT,max are roughly of the forms Q2 and Q4, respectively. Finally, a small positive dispersion describes the peak positions of SL, in qualitative agreement with related studies on other model glassy systems.