  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.
