Diffusons, Locons, Propagons: Character of Atomic Vibrations in Amorphous Si

TL;DR

This study classifies vibrational modes in amorphous silicon into propagons, diffusons, and locons, revealing that diffusons are the most prevalent and characterizing their properties to understand atomic vibrations.

Contribution

It introduces a detailed classification of vibrational modes in amorphous silicon and characterizes diffusons using various properties, which was not previously done.

Findings

Propagons are the lowest 4% of modes, locons are the highest 3%, and diffusons comprise the rest.

Diffusons are the most numerous vibrational modes in amorphous silicon.

Different properties like phase quotient and spatial polarization memory are used to characterize diffusons.

Abstract

Numerical studies of amorphous silicon show that the lowest 4% of vibrational modes are plane-wave like ("propagons") and the highest 3% of modes are localized ("locons"). The rest are neither plane-wave like nor localized. We call them "diffusons." Since diffusons are by far the most numerous, we try to characterize them by calculating such properties as wavevector and polarization (which seem not to be useful), "phase quotient" (a measure of the change of vibrational phase on between first neighbor atoms), spatial polarization memory, and diffusivity. Localized states are characterized by finding decay lengths, inverse participation ratios, and coordination numbers of the atoms participating.