Excess Damping Research Articles

Nanomechanical damping via electron-assisted relaxation of two-level systems

We report on measurements of dissipation and frequency noise at millikelvin temperatures of nanomechanical devices covered with aluminum. A clear excess damping is observed after switching the metallic layer from superconducting to the normal state with a magnetic field. Beyond the standard model of internal tunneling systems coupled to the phonon bath, here we consider the relaxation to the conduction electrons together with the nature of the mechanical dispersion laws for stressed/unstressed devices. With these key ingredients, a model describing the relaxation of two-level systems inside the structure due to interactions with electrons and phonons with well-separated timescales captures the data. In addition, we measure an excess $1/f$-type frequency noise in the normal state, which further emphasizes the impact of conduction electrons.

Sensitivity of a simple atmospheric model to changing surface friction with implications for seasonal prediction

AbstractAn ensemble of idealized experiments with the simplified general circulation model PUMA is used to analyze the response to reduced surface friction, that is a strengthening of the eddy‐driven jet, a weakening of the Eulerian mean overturning, and a suppression of baroclinic instability. The suppression of baroclinic instability is caused by an effect called the barotropic governor by which increased horizontal shear restricts the ability of baroclinic disturbances to convert available potential energy into kinetic energy. This governor effect ensures that the residual circulation and Eliassen–Palm flux (EP flux) divergence are largely invariant to the surface friction parameter despite the connection between surface friction, the Eulerian mean overturning, and the eddy‐momentum flux. The suppression of instability leads to an increase in persistence measured by the period of peak variance on synoptic time‐scales and a strengthened signal‐to‐noise ratio on seasonal time‐scales. These findings suggest that the signal‐to‐noise paradox seen in the context of seasonal prediction can be caused by excess mechanical damping in atmospheric prediction systems inhibiting the barotropic governor effect.

Optimal viscous damping of vibrating porous cylinders

We theoretically study small-amplitude oscillations of permeable cylinders immersed in an unbounded fluid. Specifically, we examine the effects of oscillation frequency, permeability and shape on the effective mass and damping coefficients, the latter of which is proportional to the power required to sustain the vibrations. Cylinders of circular and elliptical cross-sections undergoing transverse and rotational vibrations are considered. The dynamics of the fluid flow through porous cylinders is assumed to obey the unsteady Brinkman–Debye–Bueche equations. We use a singularity method to analytically calculate the flow field within and around circular cylinders, whereas we introduce a Fourier-pseudospectral method to numerically solve the governing equations for elliptical cylinders. We find that, if rescaled properly, the analytical results for circular cylinders provide very good estimates for the behaviour of elliptical ones over a wide range of conditions. More importantly, our calculations indicate that, at sufficiently high frequencies, the damping coefficient of oscillations varies non-monotonically with the permeability, in which case it maximizes when the diffusion length scale for the vorticity is comparable to the penetration length scale for the flow within the porous material. Depending on the oscillation period, the maximum damping of a permeable cylinder can be many times greater than that of an otherwise impermeable one. This might seem counter-intuitive at first, since generally the power it takes to steadily drag a permeable object through a fluid is less than the power needed to drive the steady motion of the same, but impermeable, object. However, the driving power (or damping coefficient) for oscillating bodies is determined not only by the amplitude of the cyclic fluid load experienced by them but also by the phase shift between the load and their periodic motion. An increase in the latter is responsible for the excess damping coefficient of vibrating porous cylinders.

Glass Transition Process of Amorphous Quinidine Studied by Using Brillouin Light Scattering

The acoustic properties of amorphous quinidine were investigated by using Brillouin spectroscopy as a function of temperature. The longitudinal sound velocity and the acoustic absorption coefficient showed drastic changes in their slopes at the glass transition temperature of 328 K, which was nearly the same as the thermodynamic and the dielectric glass transition temperatures. The clear difference between the crystalline state and the amorphous state of quinidine, as well as the crystallization process, could be revealed by observing the change in the Brillouin spectrum. Analysis of the lowtemperature absorption coefficient showed that the excess acoustic damping could be attributed to intramolecular relaxation motions, such as side-group rotations.

Characterization of low-frequency acoustic wave propagation through a periodic corrugated waveguide

In this paper, a periodic corrugated waveguide structure is proposed, and its unit-cell is analyzed by the wave finite element method. In low-frequency range, the unit-cell is treated as an equivalent fluid through a homogenization process, and the equivalent acoustic parameters are obtained, which are validated by finite structure simulations and experiments. The proposed structure is shown to add tortuosity to the waveguide, hence higher equivalent fluid density is achieved, while the system elastic modulus remains unchanged. As a result, the equivalent speed of sound is smaller than normal air. The application of such change of speed of sound is demonstrated in the classic quarter-wavelength resonator based on the corrugated waveguide, which gives a lower resonance frequency with the same side branch length. When the waveguide is filled with porous materials, the added tortuosity enhances the broadband, low-frequency sound absorption by increasing the equivalent mass without bringing in excess damping, the latter being partly responsible for the poor performance of usual porous materials in the low-frequency region. Therefore, the proposed structure provides another dimension for the design and optimization of porous sound absorption materials.

Mode-dependent mechanical losses in disc resonators

Mechanical spectroscopy gives information on the structure of solids and their relaxation mechanisms through the measurements of the elastic constants and the mechanical loss angle of materials. One common way to estimate these quantities is the resonant method where the frequency and the characteristic decay time of oscillations are measured. Since many solid materials can be easily found in the shape of thin disc we have investigated the mechanical loss of these resonators and we have found experimentally that the loss angle dependence on the mode is not trivial but rather follow a distribution of modes into families. We give a model that is able to justify the existence of these families and to predict the level of losses in silicon, silica and brass discs. The model considers the thermoelastic effect and the excess damping caused by the condition of the disc edge. The results of this research are relevant to the research on thin films that are deposited on thin discs like the optical coatings used on the mirrors for the gravitational wave detectors.

Ultimate modulation bandwidth of 850 nm oxide-confined vertical-cavity surface-emitting lasers

Complex influence of photon lifetime (controlled by the mirror loss) and aperture size on the performance of 850 nm InGaAlAs oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with fully doped AlGaAs-based distributed Bragg reflectors (DBR) was investigated. We find a tradeoff between photon lifetime and gain nonlinearity for maximizing the optical bandwidth, leading to the optimum aperture size close to 4-6 μm. In spite of the reduced photon lifetime (from 4 ps to 1 ps), the excess damping caused by the current-induced self-heating limits the ultimate modulation bandwidth for the given VCSELs design at 24-25 GHz. Further improvement in high frequency characteristics can be facilitated by decrease of the heat generation and improvement of the heat removal from the active region as well as by proper engineering of the scattering loss at the oxide aperture while keeping the low capacitance optimizing design of the oxide aperture.

Power particles

This paper introduces a new particle-based approach to incompressible fluid simulation. We depart from previous Lagrangian methods by considering fluid particles no longer purely as material points, but also as volumetric parcels that partition the fluid domain. The fluid motion is described as a time series of well-shaped power diagrams (hence the name power particles ), offering evenly spaced particles and accurate pressure computations. As a result, we circumvent the typical excess damping arising from kernel-based evaluations of internal forces or density without having recourse to auxiliary Eulerian grids. The versatility of our solver is demonstrated by the simulation of multiphase flows and free surfaces.

Anomaly of the internal friction in helium-4 crystals prepared with the burst-like growth method

Abstract The internal friction is measured in the 4 He crystals prepared with the conventional and burst-like methods at temperature 0.65 K and frequency 75 kHz. It is observed that the crystals, grown too rapidly, have an excess damping and smaller dynamic elastic modulus. The magnitude of the effect enhances as the initial overpressure increases. The contribution to the excess damping vanishes at the critical overpressure 3.5 mbar separating the regions of the normal and anomalous crystal growth at the corresponding temperature. Corrections to the modulus and decrement relax with the time constant of about 3 ms. The regime of the burst-like growth may result in the crystals with the internal state different from that of equilibrium crystals.

Tenfold reduction of Brownian noise in high-reflectivity optical coatings

Thermally induced fluctuations impose a fundamental limit on precision measurement. In optical interferometry, the current bounds of stability and sensitivity are dictated by the excess mechanical damping of the high-reflectivity coatings that comprise the cavity end mirrors. Over the last decade, the dissipation of these amorphous multilayer reflectors has at best been reduced by a factor of two. Here, we demonstrate a new paradigm in optical coating technology based on directbonded monocrystalline multilayers, which exhibit both intrinsically low mechanical loss and high optical quality.

Fabrication of superficial neuromast inspired capacitive flow sensors**This paper is an extension of Izadi et al, ‘Fabrication of dense flow sensor arrays on flexible membranes’ presented at IEEE Transducer 2009, Denver, Colorado.

A fabrication process is presented which allows realizing the dense arrays of fully supported flexible SU-8 membranes with integrated electrodes underneath that support cylindrical hair-like structures on the top. Electrodes have been insulated from liquid to prevent short circuit or electrolysis. The process allows a controllable distance between counter electrodes. The excess squeeze film damping is eliminated with a closed membrane configuration with backside openings. A low-power differential capacitive measurement mechanism is employed as a sensor readout. These flow sensors provide a distributed sensing mechanism inspired by the superficial neuromast as found in the lateral line system of fish. This paper focuses on the fabrication of the sensors with preliminary mechanical results demonstrating the functionality of the fabricated devices.

Measurement of an anisotropic superconducting gap parameter resolved to a single Fermi surface sheet: YNi2B2C

We have used the de Haas–van Alphen technique in the normal and superconducting states of YNi2B2C to measure the (angularly resolved) orbitally averaged superconducting gap parameter. The results show that the gap parameter is highly anisotropic. This is the first time that superconducting gap anisotropies have been explored and resolved to individual sheets of the Fermi surface. In addition we have observed for the first time an excess damping due to screening by vortex motion in the peak effect region.

Time-Accurate Simulations and Acoustic Analysis of Slat Free Shear Layer

Unsteady computational simulations of a multi-element, high-lift configuration are performed. Emphasis is placed on accurate spatiotemporal resolution of the free shear layer in the slat-cove region. The excessive dissipative effects of the turbulence model, so prevalent in previous simulations, are circumvented by switching off the turbulence-production term in the slat cove region. The justifications and physical arguments for taking such a step are explained in detail. The removal of this excess damping allows the shear layer to amplify large-scale structures, to achieve a proper non-linear saturation state, and to permit vortex merging. The large-scale disturbances are self-excited, and unlike our prior fully turbulent simulations, no external forcing of the shear layer is required. To obtain the farfield acoustics, the Ffowcs Williams and Hawkings equation is evaluated numerically using the simulated time-accurate flow data. The present comparison between the computed and measured farfield acoustic spectra shows much better agreement for the amplitude and frequency content than past calculations. The effect of the angle-of-attack on the slat's flow features radiated acoustic field are also simulated presented.

Critical Hysteresis in a Tilted Washboard Potential

The Brownian motion in a tilted washboard potential subjected to a low-frequency periodic drive develops large hysteresis loops, which attain their maximum at the locked-to-running transition in the underdamped regime. The dependence of such a critical phenomenon on the drive parameters and on the temperature is investigated numerically. Moreover, this hysteresis mechanism is related to the occurrence of random multiple jumps, whose length and time duration distributions appear to decay according to a universal power law with exponent $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.5$. This model may explain various instances of low-frequency excess damping in material science.

Calming the capillary waves on the surface of a bubble using an insoluble surfactant

The excess damping of capillary waves caused by a surfactant monolayer was demonstrated for the quadrupole shape mode of an isolated acoustically trapped bubble in water. A known amount of the water-insoluble surfactant stearic acid was deposited on the bubble surface from a microliter of ethanol solution. As the bubble dissolves, its surface area decreases thus compressing the monolayer. Compression rates ranged from 0.09 to 0.18 Å2 per molecule per minute for different bubbles and surfactant coverage ranged from 20 to 37 Å2 per molecule specific area. This technique was tested for accuracy and reproducibility by application to pendant bubbles and subsequent measurements of the static surface tension. Shape oscillations were induced by modulation of the ultrasonic radiation pressure and the free decay was recorded by an optical extinction technique. Local maxima in the normalized damping and frequency were observed near 26 Å2 per molecule coverage in agreement with similar studies of capillary waves on flat interfaces. An unexpected increase in the frequency was observed at high coverage and may be associated with the solidification or collapse of the monolayer. Any effects due to the presence of ethanol were shown to be negligible. [Work supported by ONR.]

Free decay of shape oscillations of bubbles acoustically trapped in water and sea water

Asymptotic results for the free decay of shape oscillations of viscous liquid spheres have been extended to include higher-order terms in the ratios of the inner and outer viscous penetration lengths to the radius. The new expressions are shown to be important for studies in which the two fluids have dissimilar densities and viscosities such as air/liquid systems. The analysis also includes an expansion for the frequency of maximum response of driven oscillations. The calculations are supported by measurements of the small-amplitude quadrupole mode free decay of nearly spherical bubbles acoustically levitated in clean water. The bubble radii ranged from 400 μm to 1400 μm. The results are interpreted in light of the initial-value problem. The lack of excess damping suggests that the interface behaves ideally for times up to two hours after bubble injection. Measurements were also carried out on bubbles in 0.5 m NaCl solution and in sea water. Larger bubbles (radius > 800 μm) in clean water exhibit damping two to four times larger than predicted by theory. The transition from this anomalous damping to theoretical damping is a very abrupt function of radius. All observations were carried out with similar acoustic fields for counteracting buoyancy. The excess damping appears to be associated with some nonlinear response of the bubble.

Effect of an insoluble surfactant on capillary oscillations of bubbles in water: Observation of a maximum in the damping.

The excess damping of capillary waves caused by a surfactant monolayer is demonstrated to be present as well for the quadrupole shape mode of an isolated acoustically trapped bubble in water. To facilitate measurements of damping as a function of surface concentration, a method was developed for depositing a known amount of insoluble surfactant (stearic acid) on the surface of the bubble. As the bubble dissolves, the stearic acid concentration increases, and the excess damping has a pronounced local maximum near 0.26 ${\mathrm{nm}}^{2}$ per molecule specific area in agreement with capillary wave data for a flat surface. The method of depositing insoluble surfactant should be applicable to the characterization of other surface-limited processes in isolated bubbles.

Quantum oscillation in the vortex state in unconventional superconductors

We extend an early analysis of magnetic quantum oscillation in clean type II superconductors to unconventional superconductors. Since the excess damping in the vortex state is proportional to 〈Δ 2( p)〉 (i.e. average over the quasi-particle trajectory on the Fermi surface perpendicular to the field), this will provide unique information on the anisotropy of the superconducting order parameter Δ( p) .

Phonon States in Sic Small Particles

AbstractSize dependence of optical phonon frequencies and that of phonon dampings of SiC small particles have been studied by analysing their Raman data. The particle size ranges from 30 nm to 1000 nm. Decrease in the TO-phonon frequency as well as the LT-splitting (the splitting between the LO- and TO-phonon) with decrease in the particle size are much larger than that expected from the spatial correlation model or that from the phonon confinement model. The phonon damping for the small particle consists of the usual temperature-dependent term and an excess damping term, which is independent of temperature. These results suggest that the scattering of the phonon at the particle surface plays a major role in determining the phonon states of the small particle. Our experimental data suggest that the surface phonon-polariton mode at the interface between the crystallites plays a minor role in the Raman spectrum of particles consisting of a number of crystallites.

Behavioural adaptations of ground living bushcrickets to the properties of sound propagation in low grassland.

The acoustic behaviour of the closely related tetigoniid species Psorodonotus illyricus and Decticus verrucivorus have been invectigated by bioacoustical and behavioural methods. Both species show adaptations concerning the acoustic behaviour with respect to the biotope and the properties of sound propagation. These insects inhabit low grassland with an average vegetation height of about 20 cm which is also the general height for the song perches. Difficulties arise for efficient acoustic communication in such habitats. Sound propagation is influenced and limited by the strong ground attenuation and the excess damping by grass vegetation. Other limiting factors are the microclimatic conditions in the biotope. The two species counteract these difficulties by moving around in the biotope during stridulation. Both species mainly stridulate in the morning, avoiding problems of reduced sound transmission which often appear in the afternoon due to negative temperature gradients and resulting shadow zones. From the high mobility of these insects, it follows that individuals have no fixed territory and consequently no rivalry against conspecifics, which is very common among Orthopterans with a high degree of territoriality. It can be concluded that the preferred biotope influences and creates behavioural patterns in Orthopterans, especially here in the two investigated species of bushcrickets Psorodonotus illyricus and Decticus verrucivorus.