Decoupling between propagating acoustic waves and two-level systems in hydrogenated amorphous silicon*

Tunneling two-level systems are the dominant energy loss mechanisms for amorphous solids at low temperatures. Hydrogenated amorphous silicon is one of the few materials that shows mechanical loss  values below the “glassy range,” which indicates a low density of tunneling two-level systems  at low temperature. We have measured the specific heat  of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition, which shows a large density of two-level systems. Annealing at 200 °C, well below the growth temperature, does not significantly affect the already-low mechanical loss, but irreversibly reduces the specific heat by an order of magnitude at 2 K, indicating a large reduction in the density of two-level systems. We compare the specific heat with the internal friction results, which suggest that the two-level systems are uncharacteristically decoupled from acoustic waves, both before and after annealing. Our analysis yields an anomalously low value of the coupling constant , which increases upon annealing but remains anomalously low. The results suggest that the coupling constant value is lowered by the presence of hydrogen.