We measured the thermal conductivity kappa of an 80 microm thick hydrogenated amorphous silicon film prepared by hot-wire chemical-vapor deposition with the 3omega (80-300 K) and the time-domain thermo-reflectance (300 K) methods. The kappa is higher than any of the previous temperature dependent measurements and shows a strong phonon mean free path dependence. We also applied a Kubo based theory using a tight-binding method on three 1000 atom continuous random network models. The theory gives higher kappa for more ordered models, but not high enough to explain our results, even after extrapolating to lower frequencies with a Boltzmann approach. Our results show that this material is more ordered than any amorphous silicon previously studied.

Attachment losses can play a role in limiting the quality factors of micro/nanomechanical oscillators. The existing theoretical results in this regard are applicable to highly idealized scenarios. The theory has been extended in two important directions: the width of the cantilever is considered to be small relative to a wavelength as opposed to large, and the base is allowed to have finite thickness. These extensions result in significant, in many cases order of magnitude, changes in the estimates of attachment loss. Simple formulas for Q−1 covering most of the parameter range are given.

Analytical expressions are provided for the energy loss from vibrating mechanical resonators into their support structures for two limiting cases: supports that can be treated as plates, and supports that act as semi-infinite elastic media, with effectively infinite thickness. The former case is applicable to many microscale resonators, while the latter is appropriate for nanoscale devices. General formulations are given, applicable to a wide range of resonator geometries. These formulations are then applied to two geometries commonly used in microelectromechanical systems and nanelectromechanical systems applications: cantilevered beams and doubly fixed beams. Experimental data are presented to validate the finite-thickness support theory, and the predictions of the theory are also compared to data from existing literature for a microscale rectangular paddle oscillator.

A simple model of thermoelastic dissipation is proposed for general, free standing microelectromechanical ͑MEMS͒ and nanoelectromechanical ͑NEMS͒ oscillators. The theory defines a flexural modal participation factor, the fraction of potential energy stored in flexure, and approximates the internal friction by assuming the energy loss to occur solely via classical thermoelastic dissipation of this component of the motion. The theory is compared to the measured internal friction of a high Q mode of a single-crystal silicon double paddle oscillator. The loss at high temperature ͑above 150 K͒ is found to be in good agreement with the theoretical prediction. The importance of this dissipation mechanism as a function of scale is briefly discussed. We find that the relative importance of this mechanism scales with the size of the structure, and that for nanoscale structures it is less important than intrinsic phonon-phonon scattering. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1449534͔Obtaining high Q performance in Si based oscillators in microelectromechanical systems ͑MEMS͒ and nanoelectromechanical systems ͑NEMS͒ is desirable for applications that include charge detectors, 1 radio-frequency filters, 2 magnetic resonance force microscopes, 3 and torque magnetometers. 4 Achieving this goal has proven to be difficult however, and to date, the achieved Qs in MEMS/NEMS oscillators have been orders of magnitude smaller than expected from considerations of fundamental loss mechanisms.5 Explanations of the observed behavior include attachment losses, 6,7 bulk defects, 4 losses associated with the electrical contacts, 8 and surface effects. 9 Unfortunately, untangling the various potential mechanisms is not a straightforward exercise because of the difficulty of controlling and measuring the properties of microscale and nanoscale devices.The MEMS oscillators are typically free standing, planar samples vibrating predominantly either in flexure or torsion. In the case of a simple beam in flexure, thermoelastic ͑TE͒ dissipation is known to be a significant loss mechanism near room temperature. 10 The loss mechanisms in other oscillators involving torsional vibration, or in general possessing a nontrivial mode shape, are however still unknown. We have developed a simple model which predicts the internal friction arising from TE dissipation in these types of oscillators. We have found the internal friction can be quite significant even for nominally torsional vibration modes which one might conclude have no TE loss. Analysis of the experimental results for a macroscopic single-crystal silicon oscillator yields good agreement with the prediction.TE dissipation arises from thermal currents generated by compression/decompression in elastic media. Typically, the TE dissipation arising from longitudinal waves in solids is very small 11 owing to the large distance between the compressed and rarefied regions. In contrast, the compression associated with flexural vibration can give rise to TE dissipation orders of magnitude l...

The room-temperature quality factors of silicon micromechanical oscillators have been investigated by scanning laser vibrometry. One of the flexural modes has very little attachment loss to its environment, which enables us to study internal loss mechanisms. After several consecutive annealing steps up to 800°C, the quality factor Q has increased from 8×104 to 6.0×105. However, the Q decays to 1.4×105 over six months in air. We conclude that near-surface lattice defects caused by reactive-ion etching and surface adsorbates are the main source of internal loss while surface adsorbates are responsible for the time dependence. We also discuss the thermoelastic limit in terms of Zener’s theory and flexural modal components of thin plates with vibratory volume change, and compare it with our results.

Singular value decomposition (SVD) has recently been suggested as a filtering tool to analyze acoustic radiation. The relationship between this filtering technique and the more familiar wave vector filtering is examined. For a geometry with a preferred axis, the source ‘‘modes’’ are found to consist approximately of a single wave number, and the accompanying radiation modes are thus beams. Some simple examples are worked out to illustrate the general behavior of the filtering, and some of the properties of the beams are given.

Two-dimensional (2D) materials offer unique opportunities in engineering the ultrafast spatiotemporal response of composite nanomechanical structures. In this work, we report on high frequency, high quality factor (Q) 2D acoustic cavities operating in the 50–600 GHz frequency (f) range with f × Q up to 1 × 1014. Monolayer steps and material interfaces expand cavity functionality, as demonstrated by building adjacent cavities that are isolated or strongly-coupled, as well as a frequency comb generator in MoS2/h-BN systems. Energy dissipation measurements in 2D cavities are compared with attenuation derived from phonon-phonon scattering rates calculated using a fully microscopic ab initio approach. Phonon lifetime calculations extended to low frequencies (<1 THz) and combined with sound propagation analysis in ultrathin plates provide a framework for designing acoustic cavities that approach their fundamental performance limit. These results provide a pathway for developing platforms employing phonon-based signal processing and for exploring the quantum nature of phonons.

We study composite operators in lattice gauge theory that reduce to operators of the form ~T B B .. . 1C, in the continuum limit; such operators arise in perturbative analyses of quantum chromodynamics. Using our results and the data of a numerical simulation one could normalize exclusive processes and predict moments of deep-inelastic scattering structure functions. To initiate the program we construct and renormalize lattice operators to the one-loop level. We are encouraged that the hadronic matrix elements of the simpler operators are within reach of numerical simulations.