Two-Level Systems Observed in the Mechanical Properties of Single-Crystal Silicon at Low Temperatures

Using the high-Q mechanical-oscillator technique we have measured the sound velocity and mechanical dissipation of high-purity single-crystal silcon as function of temperature (0.005-4.2K), frequency (0.6-6.0kHz), and strain amplitude (10 sup -5 - 10 sup -8) . In the mechanical properties we find a surprising strong temperature dependence with the same qualitative behavior for silicon as for vitreous silica. This implies a density of two-level systems only 2 orders of magnitude lower for silicon than amorphous silica. In silicon we find evidence for a new dissipation mechanism at low temperatures and report the first observation of a saturation with strain of the resonant contribution to the sound velocity.