Elastic measurements of amorphous silicon films at mK temperatures

TL;DR

This study measures the elastic properties of amorphous silicon films at millikelvin temperatures to investigate the low-energy excitations responsible for glassy behavior and their impact on mechanical dissipation.

Contribution

It provides the first elastic measurements of amorphous silicon films at very low temperatures, enabling analysis of the origins of reduced mechanical dissipation.

Findings

Measurements conducted down to 20 mK.

Potential to distinguish between TLS density and coupling effects.

Foundation for understanding dissipation reduction mechanisms.

Abstract

The low temperature properties of glass are distinct from those of crystals due to the presence of poorly understood low-energy excitations. The tunneling model proposes that these are atoms tunneling between nearby equilibria, forming tunneling two level systems (TLSs). This model is rather successful, but it does not explain the remarkably universal value of the mechanical dissipation $Q^{-1}$ near 1 kelvin. The only known exceptions to this universality are the $Q^{-1}$ of certain thin films of amorphous silicon, carbon and germanium. Recently, it was found that $Q^{-1}$ of amorphous silicon (a-Si) films can be reduced by two orders of magnitude by increasing the temperature of the substrate during deposition. According to the tunneling model, the reduction in $Q^{-1}$ at 1 kelvin implies a reduction in $P_{0}\gamma ^{2}$, where $P_{0}$ is the density of TLSs and $\gamma $ is their coupling to phonons. In this preliminary report, we demonstrate elastic measurements of a-Si films down to 20 mK. This will allow us, in future work, to determine whether $P_{0}$ or $\gamma $ is responsible for the reduction in $Q^{-1}$ with deposition temperature.