Internal mechanical dissipation mechanisms in amorphous silicon

Using activation-relaxation technique nouveau (artn), we search for two-level systems (TLSs) in models of amorphous silicon ($a$-Si). The TLSs are mechanisms related to internal mechanical dissipation and represent the main source of noise in the most sensitive frequency range of the largest gravitational wave detectors as well as one of the main sources of decoherence in many quantum computers. We show that in $a$-Si, the majority of the TLSs of interest fall into two main categories: bond-defect hopping, where neighbors exchange a topological defect, and Wooten-Winer-Weaire bond exchange. The distribution of these categories depends heavily on the preparation schedule of the $a$-Si. We use our results to compute the mechanical loss in amorphous silicon, leading to a loss angle of ${10}^{-3}$ at room temperature, decreasing to ${10}^{-4}$ at 150 K in some configurations. Our modeling results indicate that multiple classes of events can cause experimentally relevant TLSs in disordered materials and therefore multiple attenuation strategies might be needed to reduce their impact.