Quantum friction of micromechanical resonators at low temperatures

Dissipation of micro- and nanoscale mechanical structures is dominated by quantum-mechanical tunneling of two-level defects intrinsically present in the system. We find that at high frequencies-usually, for smaller, micron-scale structures-a novel mechanism of phonon pumping of two-level defects gives rise to weakly temperature-dependent internal friction, Q-1, concomitant to the effects observed in recent experiments. Because of their size, comparable to or shorter than the emitted phonon wavelength, these structures suffer from superradiance-enhanced dissipation by the collective relaxation of a large number of two-level defects contained within the wavelength.