The damping rates of high quality factor nanomechanical resonators are well beyond intrinsic limits. Here,we explore the underlying microscopic loss mechanisms by investigating the temperature-dependent dampingof the fundamental and third harmonic transverse flexural mode of a doubly clamped silicon nitride string. Itexhibits characteristic maxima reminiscent of two-level defects typical for amorphous materials. Coupling tothose defects relaxes the momentum selection rules, allowing energy transfer from discrete long-wavelengthresonator modes to the high frequency phonon environment.     «

The damping rates of high quality factor nanomechanical resonators are well beyond intrinsic limits. Here,we explore the underlying microscopic loss mechanisms by investigating the temperature-dependent dampingof the fundamental and third harmonic transverse flexural mode of a doubly clamped silicon nitride string. Itexhibits characteristic maxima reminiscent of two-level defects typical for amorphous materials. Coupling tothose defects relaxes the momentum selection rules, allowing energy trans...     »