Evidence for the Role of Normal-State Electrons in Nanoelectromechanical Damping Mechanisms at Very Low Temperatures

Abstract

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...

A macroscopic object passively cooled into its quantum ground state of motion beyond single-mode cooling

“…On the other hand, the frequency shift follows a Logarithmic temperature dependence in the whole range, as is commonly observed with low-temperature nanomechanics 36 . This is interpreted as the signature of two-level systems (TLS) defects present in the structure, that couple to the macroscopic mode via the strain field 37,38 . The exact fit expression from theory is shown as the dashed line in Fig.…”

Tunable low-temperature dissipation scenarios in palladium nanomechanical resonators

“…For example, in Refs. [25] and [26], aluminum beams in normal and superconducting states show similar behavior as opposed to silicon devices with aluminium electrodes [22]. Metallic thin films and deformed bulk aluminium have been extensively studied in the context of TLS dissipation in Ref.…”

“…One often ignores the effect of the electrodes like gold as negligible due to its smaller Young's modulus compared to the material under study. Recently, Al electrodes on silicon structures showed a profound difference when measured in the superconducting and nonsuperconducting state [22].…”

Nonlinear damping and dephasing in nanomechanical systems

“…The relaxation mechanisms include radiation into the bulk modes of the medium surrounding the nanosystem [19][20][21][22][23], nonlinear coupling between the modes localized inside the system [10,[24][25][26][27][28], and coupling with electronic excitations [29][30][31][32] and two-level defects [33][34][35].…”