We present measurements of the dissipation and frequency shift in gold nanomechanical resonators at temperatures down to 10 mK. The resonators were fabricated as doubly clamped beams above a GaAs substrate and actuated magnetomotively. Measurements on beams with frequencies 7.95 and 3.87 MHz revealed that from 30 to 500 mK the dissipation increases with temperature as T 0.5 , with saturation occurring at higher temperatures. The relative frequency shift of the resonators increases logarithmically with temperature up to at least 400 mK. Similarities with the behavior of bulk amorphous solids suggest that the dissipation in our resonators is dominated by two-level systems.

We report on experiments performed at low temperatures on aluminum covered silicon nanoelectromechanical resonators. The substantial difference observed between the mechanical dissipation in the normal and superconducting states measured within the same device unambiguously demonstrates the importance of normal-state electrons in the damping mechanism. The dissipative component becomes vanishingly small at very low temperatures in the superconducting state, leading to exceptional values for the quality factor of such small silicon structures. A critical discussion is given within the framework of the standard tunneling model. PACS numbers: 85.85.+j, 62.30.+d, 62.40.+i, Micro and nanomechanical devices are under intense investigation for both their promising instrumental applications and their implication in fundamental issues of physics. These devices are ultra-sensitive mass [1] and force detectors [2], they can be used in their linear [3] or nonlinear regimes [4] to implement various signal processing schemes [5,6]. In a more fundamental realm, they can be thought of as probes for non-newtonian deviations to gravity at small scales [7], for refined studies of the Casimir force [8], and for the study of quantum fluids [9]. Moreover, nanoresonators themselves cooled to their quantum ground state tackle problems that have been around quantum mechanics since the early beginning, with the possibility of controlling a mechanical collective macroscopic degree of freedom at the quantum level [10][11][12][13].Having high quality devices is desirable in many of these fields. However, it is well known that the quality factor Q of mechanical structures becomes worse as their size is reduced [14], while internal stresses have been found to drastically increase the Q in silicon-nitride nanobeams [15]. Although it is clear that the surfaceto-volume ratio is a key ingredient for the understanding of mechanical dissipation, a proper theoretical explanation covering all experiments remains elusive [16][17][18][19]. Nanomechanical friction mechanisms thus deserve to be understood from both an engineering and a fundamental condensed matter physics point of view.Almost all nanoresonators used in dissipation experiments possess a metallic coating used to actuate and detect the motion. This layer has an essential impact on the mechanical properties, since it adds mass and surface stresses which significantly modify the dissipation characteristics [16,20]. Most experiments are performed with normal conducting metals; only little is known about superconductor-covered nanodevices [13,21,22].Addressing dissipation mechanisms requires a broad temperature range to be explored, within the Kelvin and sub-Kelvin range. Common features are observed: the dissipation follows a power law T n below a certain temperature T * , with a crossover to a rather flat high temperature region that depends on the nature and size of the object. The resonance frequency shifts logarithmically at the lowest temperatures, and reveals a maximum around the same cr...

We present results from a study of the nonlinear inter-modal coupling between different flexural vibrational modes of a single high-stress, doubly-clamped silicon nitride nanomechanical beam. Using the magnetomotive technique and working at 100 mK we explored the nonlinear behaviour and modal couplings of the first, third and fifth modes of a 25.5 µm long beam. We find very good agreement between our results and a simple analytical model which assumes that the different modes of the resonator are coupled to each other by displacement induced tension in the beam. The small size of our resonator leads to relatively strong nonlinear couplings, for example we find a shift of about 7 Hz in the third mode for a 1 nm displacement in the first mode and frequency shifts ∼20 times larger than the linewidth (130 Hz) are readily observed.

Emerging quantum technologies require mastering thermal management, especially at the nanoscale. It is now accepted that thermal metamaterial-based phonon manipulation is possible, especially at sub-kelvin temperatures. In these extreme limits of low temperatures and dimensions, heat conduction enters a quantum regime where phonon exchange obeys the Landauer formalism. Phonon transport is then governed by the transmission coefficients between the ballistic conductor and the thermal reservoirs. Here we report on ultra-sensitive thermal experiments made on ballistic 1D phonon conductors using a micro-platform suspended sensor. Our thermal conductance measurements attain a power sensitivity of 15 attoWatts  around 100 mK. Ballistic thermal transport is dominated by non-ideal transmission coefficients and not by the quantized thermal conductance of the nanowire itself. This limitation of heat transport in the quantum regime may have a significant impact on modern thermal management and thermal circuit design.