Evidence of universality in the dynamical response of micromechanical diamond resonators at millikelvin temperatures

We report kelvin- to millikelvin-temperature measurements of dissipation and frequency shift in megahertz-range resonators fabricated from ultrananocrystalline diamond. Frequency shift δf/f₀ and dissipation Q^-1 demonstrate temperature dependence in the millikelvin range similar to that predicted by the glass model of tunneling two-level systems. The logarithmic temperature dependence of δf/f₀ is in good agreement with such models, which include phonon relaxation and phonon resonant absorption. Dissipation shows a weak power law, Q^-1∝T1/3, followed by saturation at low temperature. A comparison of both the scaled frequency shift and dissipation in equivalent micromechanical structures made of single-crystal silicon and gallium arsenide indicates universality in the dynamical response.