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

TL;DR

This study investigates the relationship between two-level systems and acoustic wave propagation in hydrogenated amorphous silicon, revealing that hydrogen presence significantly reduces the coupling between them, especially after annealing.

Contribution

It demonstrates that hydrogenation in amorphous silicon leads to a decoupling of two-level systems from acoustic waves, with annealing further reducing this coupling and the density of two-level systems.

Findings

Large density of two-level systems at low temperature.

Annealing reduces the density of two-level systems.

Coupling constant remains anomalously low, influenced by hydrogen.

Abstract

Specific heat measurements of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large density of two-level systems at low temperature. Annealing at 200 {\deg}C, well below the growth temperature, does not significantly affect the already-low internal friction or the sound velocity, but irreversibly reduces the non-Debye specific heat by an order of magnitude at 2 K, indicating a large reduction of the density of two-level systems. Comparison of the specific heat to the internal friction suggests that the two-level systems are uncharacteristically decoupled from acoustic waves, both before and after annealing. Analysis yields an anomalously low value of the coupling constant, which increases upon annealing but still remains anomalously low. The results suggest that the coupling constant value is lowered by the presence of hydrogen.