Elastic Measurements of Amorphous Silicon Films at mK Temperatures

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 K. 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 K implies a reduction in P-0 gamma(2), where P-0 is the density of TLSs and. 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.