  We report low-temperature measurements of dissipation in megahertz-range,
suspended, single-crystal nanomechanical oscillators. At millikelvin
temperatures, both dissipation (inverse quality factor) and shift in the
resonance frequency display reproducible features, similar to those observed in
sound attenuation experiments in disordered glasses and consistent with
measurements in larger micromechanical oscillators fabricated from
single-crystal silicon. Dissipation in our single-crystal nanomechanical
structures is dominated by internal quantum friction due to an estimated number
of roughly 50 two-level systems, which represent both dangling bonds on the
surface and bulk defects.
