Anomalous Sound Velocity in Vitreous Silica at Very Low Temperatures

The temperature dependence of the longitudinal sound velocity has been measured in vitreous silica between 0.28 and 4.2 K and at frequencies between 30 and 150 MHz. With decreasing temperature the sound velocity does not increase monotonically towards its value at $T=0$ K, but reaches a maximum around 2 K and decreases steadily at lower temperatures. This new result, unique for a pure dielectric solid, gives further evidence for the existence of two-level systems in glasses.