The attenuation and phase velocity of ultrasonic third sound for optically smooth and roughened substrates

The attenuation and phase velocity of third sound have been measured for propagation on optically smooth and roughened substrates and for ultrasonic frequencies in the 20–200 kHz range. The attenuation for both kinds of substrates is found to be proportional to the frequency and have a magnitude that increases with the amount of roughening. The data are found to be inconsistent with a number of proposed attenuation mechanisms. The phase velocity is nearly independent of the frequency. Pulse wave shapes change substantially as the third sound propagates along a substrate surface.     

Capacitive Generation and Detection of Third Sound Resonances in Saturated Superfluid 4He Films

We report on the operation of a third sound resonator for use with saturated superfluid helium films. Measurements of superfluid ⁴ He third sound velocities as a function of film thickness are described for films ranging in thickness from 26 nm to 85 nm. The film resides on the gold-covered surface of a 38 mm diameter copper disk, which has been machined with a diamond-tipped cutting tool, and has an rms roughness of about 10 nm.     

On the Possibility of Observing Third Sound on 3He

This paper focuses on the hydrodynamics of third sound on a superfluid ³ He film. We solve the hydrodynamical equations in the limit of thick films with weak interaction with the substrate. The surface tension at the free interface is shown to have a large effect on the third sound velocity and on the attenuation for frequencies larger than 1Hz. In the case of a diffusely scattering substrate a ripplon-like dispersion relation is found for this frequency range.     

Excitation and Relaxation of Film Flow Induced by Third Sound

High amplitude third sound waves are observed to create and to destroy persistent flow states within a circular resonator. These changes are necessarily a result modifying the underlying distribution of pinned vortices responsible for the flow. At low temperatures, large oscillatory flows associated with the third sound wave agitation are required to both increase and decrease the flow. At higher temperatures, thermally assisted de-pinning enhances the destructive aspects of lower amplitude third sound agitation. The constructive, or swirling tendency of the third sound agitation is also enhanced, but only in the presence of the wave excitation. At the higher temperatures, the ability of the induced flow to be trapped as a persistent current is diminished.     

Excitations of inhomogeneous liquid He4: A density functional study

Excitations of the free surface, films and bounded channels of liquid⁴He are investigated with a non-local density functional theory at zero temperature in the Feynman approximation. The nature of the various branches is discussed. At the free surface, for large momentum (k2 Å^–1), a hybridization occurs between ripplons and bulk rotons. For low momentum, the ripplon dispersion relation in thick films and the third sound mode in thin films are recovered, including the expected oscillations of the third sound velocity as a function of film thickness due to the layering of the film close to the substrate. Low energy 2-dimensional (2D) rotons confined at the liquid-wall interface are found on most substrates, except on the weakest binding surfaces such as Cs, for which we consider a channel geometry. In thin films, non trivial coverage dependence of the heat capacity may result from the interplay between the contribution of the low momentum part of the spectrum, characterized by the third sound velocity oscillations, and that of the large momentum part, dominated by the 2D rotons.Unité de Recherche des Universités Paris XI et Paris VI associée au CNRS.     

Tunable 4He Third-Sound Index of Refraction Using Patterned Calcium Flouride

No Heading We present the results of experiments that utilize deposited CaF₂ of various nominal thickness, ranging from 50 nm to 520 nm, as a substrate for pulsed third sound propagation with a tunable index of refraction, n. At low ⁴He film thicknesses on samples with CaF₂ depositions of 230 nm and greater, hysteresis is seen in the third sound velocity, and thus n, which results in a history-dependent index of refraction. The porosity and rough surface structure of the deposited CaF₂ cause capillary condensation of fluid on the surface, which is the source of the hysteresis.PACS numbers: 67.40.Pm, 67.70+n, 68.15.+e, 47.55.Mh     

Models for Third Sound in Thin 3He-4He Films

Third sound speeds in ³ He- ⁴ He thin films are sensitive to the transverse single-particle state occupied by the ³ He. The third sound speed in the low coverage region with the ³ He occupying the transverse ground-state can be understood quantitatively. The onset of occupation of the first excited transverse state is signaled by a high coverage feature in the third sound speed. Three third sound models for the high coverage region, differing in their assumptions about the spatial distribution of the excited states, are introduced. Using the experimental third sound data, these models can also be used to infer the fractional population of the ³ He in the first excited state as a function of coverage. It is found that the third sound analyses each predict a larger fractional population in the first excited state at monolayer completion than a recent analysis of magnetization step data taken on the same system.     

Third Sound on Disordered Substrates

We are investigating the properties of third sound on disordered two-dimensional substrates that have position-dependent index of refraction values. To do this we have created a disordered array of third sound scatterers by depositing regions of CaF₂ onto a glass slide. A second slide is uniformly coated with CaF₂ for comparison. We present preliminary results for the third sound transmission spectra across the disordered and uniform regions. The disordered sample exhibits a frequency dependence which is not seen in the uniform sample. The possibility that this is due to Anderson localization is considered.     

A Model for Third Sound Attenuation in Thick 4He Films

Third sound attenuation in thick ⁴He films has been observed to be much greater than predictions based on known mechanisms. We propose a possible mechanism for this observed high attenuation. Pinned vortices, possibly created when the superfluid transition is traversed, undergo driven oscillations in the third sound wave flow field. The dissipation is caused by two related effects. The first is due to the mutual friction between the vortex cores and the normal component. The second, larger contribution, is due to the drag experienced by a vortex-induced surface dimple. Variations in vortex density explain quite naturally the observed lack of reproducibility in attenuation measurements. A vortex density on the order of 10¹⁷m^–2 is required to account for dissipation reported in several experiments. We discuss the temperature, frequency and thickness dependence of the dissipation. The proposed model is also applicable to a vortex contribution to fourth sound attenuation. If third sound attenuation is indeed a signature of a very dense array of pinned vorticity, then our conception of a homogeneous superfluid film needs considerable alteration.     

A Third Sound Resonator with Cesium Boundaries

We have created a two dimensional third sound resonator with cesium boundaries on a PMMA-coated glass substrate. At chemical potential values below those necessary for pre-wetting, the cesium boundaries act as non-wet edges, effectively confining the helium to the region enclosed by the cesium. Aluminum transition-edge bolometers and silver electrodes are used as detectors and thermal sources of third sound, respectively. A first-generation resonator is described and preliminary results of third sound attenuation and ⁴ He - Cs reflection coefficient measurements made in the resonator are presented.     

多盲孔缺陷的超声兰姆波时域拓扑能量成像方法研究

传统超声成像方法受瑞利准则的约束，难以对缺陷间距小于成像分辨率阈值的多缺陷进行成像。提出了一种基于时域拓扑能量的超声兰姆波成像方法，将逆散射拓扑成像方法中的拓扑渐进过程转换成求解直接声场和伴随声场。然后通过将伴随声场进行时间反转，两个声场将具有在缺陷处聚焦，在非缺陷处不聚焦的特性。将直接声场和伴随声场进行融合，以时域拓扑能量值作为像素值进行成像，从而使表征缺陷的精度较高。建立了缺陷间距小于分辨率阈值的多盲孔缺陷有限元模型，通过“一发多收”的方式激发S0模式和采集缺陷散射信号，并进行时域拓扑能量成像。仿真结果表明：对于多盲孔缺陷，时域拓扑能量成像法能够获得比延时叠加法和时间反转成像法更高的分辨率，并且能在缺陷间距小于成像分辨率阈值时进行成像。     

Doppler-Based Airborne Ultrasound for Detecting Surface Discontinuities on a Moving Target

Abstract

This paper presents a novel concept based on the acoustic Doppler effect for detecting defects on the surface of a rapidly moving object. The proof-of-concept tests show the feasibility of this approach. By impinging and detecting airborne ultrasound on a rotating target with surface notches, the Doppler effect was clearly observed in the spectral domain at the time when the transducer passed over the flawed zone. For continuous monitoring, the gated spectral magnitude is used to discriminate the signals returned from a flawed region against those from sound regions. In addition, two real-time signal processing techniques are proposed.     

4He Third Sound and AFM Characterization of Rough Calcium Fluoride Substrates

We present resugts from experiments utilizing third sound and atomic force microscopy (AFM) that are designed to test the reproducibility of the surface roughness obtained by CaF2 thermal deposition onto borosilicate glass substrates. Third sound speed vs. ⁴He film thickness data show reasonable agreement for all samples. Statistical analysis of the AFM measurements confirm that surfaces with similar roughness characteristics (～10 nm) can be fabricated using a common deposition protocol.     

Thin-film profile on a heterogeneous surface

We derive the shape of the interface between vapor and a physisorbed film on a substrate composed of two media. This represents a simple example of adsorption on a heterogeneous surface. The results are explained in terms of competition between the surface energy and potential energy due to the substrate. Applications to superfluid onset and third sound attenuation are considered.     

On Third Sound Amplification by Stimulated Condensation

We describe the process by which condensing ⁴ He vapor atoms are expected to contribute coherently to the macroscopic superfluid motion of a third sound resonance. This phenomenon is similar to the experimentally observed thickening of a film in the fixed velocity state of a persistent current, though with an interesting and subtle difference. As before, the condensing atoms contribute to the kinetic energy of the macroscopic state by the coherent normal-to-superfluid conversion. In the present case, however, the macroscopic state is an oscillatory third sound mode. This allows the energy to remain in the third sound mode even if superfluid is removed by film flow in order to keep the film thickness constant. Thus, a continuous process of third sound amplification by stimulated condensation should accompany a condensing vapor flux under the right conditions.     

Measurements of Third Sound Attenuation at T=1.35 K

The results of measurements of ordinary third sound attenuation at T=1.35 K are reported. The attenuation is measured by using a continuous wave technique for frequencies up to 40 kHz. We find that for helium film thicknesses in the regime 5.09≤d≤9.90 layers the attenuation of third sound increases both with frequency and film thickness. We discuss our results in the context of previous work in this area.     

一种扬声器异常音的时域特征检测方法

扬声器的异常音故障常见于生产过程中,通常可由有经验的听音员凭人耳听音检测出来。与人耳听测异常音的机理类似,一种在时域检测异常音的方法被提出。通过对声响应信号进行经验模态分解得到本征模态函数,再根据本征模态函数与激励信号的瞬时频率差对各模态去混淆处理,可得到包含异常振动信息的模态函数,据此可判断是否存在异常音。算法验证分为仿真和实验两部分。仿真验证中分析了模拟的合格扬声器与故障扬声器的声响应,结果表明算法对于摩擦和碰触导致的异常音是灵敏的。实验验证中检测了20只扬声器,测量得到的故障频率与用高阶谐波失真法得到的结果一致。且对于不同异常音故障,可在经验模态函数的局部特征中观察到不同的时域特征。     

Quantum-Damping and Nonlinear Ultrasonic Excitations in 3He

Novel attenuation effects and the generation of the third harmonic frequency in nonlinear zero and first sound in ³ He were found in pulsed sound-transmission experiments at various pressures (0, 10 bar), frequencies (10-50 MHz), sound-pulse energies (0.01-110 nJ) and temperatures ranging from 0.8 mK to 200 mK. The nonlinear damping of zero sound increases with the square of the frequency and demonstrates that quantum-damping exists in a Fermi liquid even for low frequency phonons at temperatures as high as 1 mK due to multiple phonon absorption processes. The origin of the additional damping found in first sound at high temperatures and sound pulse powers is unclear, but part of the apparent attenuation is caused by third harmonic generation which was also found in superfluid ³ He-B. In zero- as well as in first sound the third harmonic generation takes place in the leading edge of the propagating sound pulse at times when the fundamental frequency has not even developed to its full amplitude.