Imaging of surface acoustic waves

A new experimental method has been devised that directly determines the group velocities of surface acoustic waves. A point source and a point detector are employed to measure the ultrasonic transmission across a solid surface as a continuous function of the propagation direction. Results for single pulses give the times-of-flight for both Rayleigh surface waves (RSW's) and pseudo-surface-waves (PSW's). Calculations and measurements of the group velocities of the surface waves on silicon show some unanticipated behavior: fluid loading qualitiatively changes the group velocity curves for both RSW and PSW. In particular, the RSW branch gains an additional component which we denote here as an induced Rayleigh wave (IRW). If a wave train is employed in the experiment, the analog of phonon focusing is observed for the ultrasonic waves, modified by internal-diffraction effects. Systematic measurements of the wave intensities on silicon as a function of propagation distance are consistent with expected acoustic losses into the surrounding water: the attenuation length of a wave depends on the mode and frequency. A survey of surface-wave images on other crystals is included in this study.     

Additional signals due to negative refraction in acoustic microscopy of anisotropic plates

The additional V(z) oscillations and pulses are predicted in the case of positive defocusing (focus above the sample surface) in acoustic microscopy of anisotropic plates exhibiting negative refraction of acoustic rays. The relationship between these additional signals and separate points on the acoustic slowness surface of the plate material is elucidated.     

Measurement of the elastic properties of evaporated C60 films by surface acoustic waves

Surface Acoustic Wave (SAW) pulses were excited in C₆₀ films deposited on quartz and silicon substrates using pulses from excimer lasers with wavelengths of 248 nm and 308 nm for excitation. An optical beam-deflection technique and polymer electret transducers were utilized to detect the propagation of the SAW pulse with high spatial and temporal resolution, allowing an accuracy of better than 0.1% for SAW velocity measurements. With this technique the frequency dependence of the SAW velocity was determined for a number of fullerite films and density, as well as elastic bulk properties of the films were derived by a theoretical analysis of the dispersion effect.     

Frequency dependence of the acoustic radiation force acting on absorbing cylindrical shells

The frequency dependence of the radiation force function Y(p) for absorbing cylindrical shells suspended in an inviscid fluid in a plane incident sound field is analysed, in relation to the thickness and the content of their interior hollow region. The theory is modified to include the effect of hysteresis type absorption of compressional and shear waves in the material. The results of numerical calculations are presented for two viscoelastic (lucite and phenolic polymer) materials, with the hollow region filled with water or air indicating how damping and change of the interior fluid inside the shell's hollow region affect the acoustic radiation force. The acoustic radiation force acting on cylindrical lucite shells immersed in a high density fluid (in this case mercury) and filled with water in their hollow region, is also studied.     

A Brillouin study of the temperature-dependence of the acoustic modes across the insulator-metal transitions in V2O3 and Cr-doped V2O3

Brillouin scattering studies have been carried out on high-quality single crystals of undoped and 0.9% Cr-doped V₂O₃. The observed modes in both the samples at ∼12 and ∼60 GHz are associated with the surface Rayleigh wave (SRW) and bulk acoustic wave (BAW), respectively. In the undoped sample, the mode frequencies of the SRW and BAW modes decrease as the temperature is lowered from room temperature to the insulator-metal transition temperature (T_IM=T_N=∼130 K). Below the transition, the modes show hardening. In the doped sample, the SRW mode shows a similar temperature-dependence as the undoped one, but the BAW mode shows hardening from room temperature down to the lowest temperature (50 K). This is the first measurement of the sound velocity below T_IM in the V₂O₃ system. The softening of the SRW frequency from 330 K to T_IM can be qualitatively understood on the basis of the temperature-dependence of C₄₄, which, in turn, is related to the orbital fluctuations in the paramagnetic metallic phase. The hardening of the mode frequencies below T_IM suggests that C₄₄ must increase in the antiferromagnetic insulating phase, possibly due to the orbital ordering.     

Critical behavior of sound attenuation in a metamagnetic Ising system

In this study, the sound attenuation coefficient of a spin- metamagnetic Ising system is calculated by the method of thermodynamics of irreversible processes. The behavior of sound attenuation near the phase transition temperatures is analyzed according to various values of phenomenological rate coefficients (γij). For all γm and γs values it is found that sound attenuation peaks occur below TN(H) and depend on frequency ω and the value of the off-diagonal rate coefficient γ. On the other hand, the critical behavior of the sound attenuation in the hydrodynamic regime is obtained analytically via the critical exponents. Moreover, the behavior of the sound attenuation as a function of frequency is also investigated and ω² dependence is observed for the attenuation coefficient. These results are in a good agreement with ultrasonic investigations of magnetic systems.     

The effect of competition between the frequency of the field and the frequency of the spin flipping on the kinetics of Ising metamagnet

We consider the effect of a variable representing the competition between the frequency of the field and the frequency of the spin flipping (Ω) on the dynamics of the metamagnetic Ising model in a cubic lattice under the presence of a time varying (oscillating) external magnetic field. The system is modelled with a formalism of master equation at a mean-field level. The time averaged staggered magnetization (Ms) acts as the order parameter and divides temperature field plane into three regions: anti-ferromagnetic, paramagnetic and coexistence of anti-ferromagnetic and paramagnetic phases. It is observed that the topology of the dynamical phase diagram depends strongly on Ω as well as the ratio between interlayer and intralayer couplings.     

Aspect sensitivity of sound backscattering in the atmospheric boundary layer

The dependence of backscatter intensity on the inclination of a SODAR beam was measured in the nocturnal boundary layer. The aspect sensitivity was revealed, but the angular dependence was not so strong as that of radiowave backscattering in the upper atmosphere. The mechanism of the phenomenon is under discussion as well as its influence on the accuracy of SODAR measurements of C _T ² values which are very useful for air-pollution meteorology.     

On the use of SODAR data to estimate mixing height

Estimation of the mixing height is one of the most common applications of SODAR data for more than twenty years and SODAR has been shown to provide reliable results under a great variety of meteorological and site conditions. Nevertheless there are still some unsolved questions. The paper gives a brief review of results obtained during the last two decades. Additionally some of the open problems are discussed in more detail. These are the determination of the stable boundary layer height from digitized SODAR data, its relation to other common height scales, its parameterization, and finally the estimation of the height of a deep convective boundary layer, exceeding the SODAR probing range.     

Elastic properties of lanthanum manganites

We give a brief overview of the data on elastic properties of the CMR manganites published during last decade. The main emphasis is put on the results obtained for single crystals.     

The vibration and rotation of piezoelectric particles in high frequency electric fields observed by a thermoacoustic method

We have observed, by a thermoacoustic method, the acoustic resonance vibration of small (diameter 115 m) piezoelectric particles in high frequency (15 MHz) electric fields. The frequency dependence of the thermoacoustic signal gives direct information about the size distribution of the particles.In very strong rf electric fields we also observed a rotation of the vibrating particles by large angles — as originally suggested by Melcher and Shiren [3] — orienting the initially random rf dipoles parallel to the rf electric fields.The observed strong forces acting on vibrating piezoelectric particles in rf fields can probably be used as a new method of selectively extracting particles of a given size from a broad distribution of very small particles.     

Windowless resonant acoustic chamber for laser-photoacoustic applications

A windowless resonant photoacoustic chamber is described for environmental and agricultural applications. The windowless design is a solution to the lack of perfect windows in the infrared region. The design was made insensitive to external noise with the help of acoustic filters. The chamber was designed to have a high quality factor of about 400. Experimental results show good agreement with the theoretical calculations.     

Crystal growth and dielectric, piezoelectric and elastic properties of Ca3TaGa3Si2O14 single crystal

Single crystals of Ca₃TaGa₃Si₂O₁₄ (CTGS) were successfully grown from stoichiometric melts by the conventional Czochralski technique. The relative dielectric constants, the piezoelectric strain constants and the elastic compliance constants of CTGS single crystal have been determined by an electric bridge and resonance-antiresonance method. At room temperature, the two piezoelectric strain constants d₁₁ and d₁₄ are −4.58×10⁻¹² coulombs per newton (C/N) and 10.43×10⁻¹² coulombs per newton (C/N), respectively. The velocities of the bulk acoustic wave are also calculated.     

Hysteresis in response of nonlinear bistable interface to continuously varying acoustic loading

In many experimental situations it is an equation of the forced relaxator and not of the forced oscillator that describes a variation in the acoustic field of the interface width (i.e. of a characteristic distance between the surfaces composing the interface). The developed theory predicts that some types of the nonlinear relaxators (depending on the structure of the nonlinear interaction force between the surfaces) exhibit hysteresis in their response to continuous acoustic loading of first increasing and then decreasing amplitude. Nonlinear (unharmonic) variation of the interface width starts at threshold amplitude of the incident sinusoidal acoustic wave, which is higher than threshold amplitude for returning to sinusoidal motion. This dynamic hysteresis (and accompanying it bistability) are possible, in particular, if the dependence of the effective interaction force on the interface width admits two quasi-equilibrium positions of the interface (bistable interface) or if the force itself is hysteretic (hysteretic interface). These theoretical predictions are relevant to some recent experimental observations on the interaction of powerful ultrasonic fields with cracks.     

Laser ultrasound characterization of chemically prepared nano-structured silver

The ultrasonic properties of nano-structured silver (hereafter nm-Ag) are investigated by the laser ultrasonic technique. The nm-Ag superfine particles with a size of 20 to 27 nanometer (nm) are prepared by using a chemical method. Wafers of nm-Ag are fabricated under different pressing pressures and used as samples. The experimental results show that the ultrasonic velocity and attenuation of nm-Ag prepared by the chemical method depends on the pressing pressures and thus on the relative density. The elastic moduli values of nm-Ag prepared by the chemical method are deduced. The values are lower than those of polycrystalline and single-crystalline Ag. The detailed results, discussions, and comparisons with nm-Ag samples prepared by a physical method are presented. Received: 23 November 1998 / Accepted: 13 September 1999 / Published online: 24 March 2000     

Acoustic characteristics of Ni–Nb–Zr–H glassy alloys by ultrasonic shear waves

The propagation characteristics of shear horizontally polarized (SH) waves passing through (Ni₄₂Nb₂₈Zr₃₀)_100–x H_x (x = 0–15.2) glassy alloys were investigated as a function of hydrogen content. With an increase in hydrogen content, the propagation time and main frequency of the receiving waves show increase and decrease, respectively, indicating expan‐ sion in average atomic distance which comes from solution of hydrogen. In sharp contrast to crystalline alloys, the decrease in damping ratio and the delay in phase with increasing hydrogen suggest a strong settlement of hydrogen into four‐coordination sites surrounded tetrahedrally by four Zr atoms and the resulting increase in dynamic elasticity, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anisotropy of sound velocity for a new PbB4O7 crystal as determined by the laser ultrasonic technique

4 O₇ crystal is a new nonlinear optical crystal. For the first time the anisotropy of the velocities of its longitudinal and surface acoustic waves (SAWs) are determined by laser ultrasonic technique. The velocities of surface waves for X-, Y-, and Z-cut crystals are also calculated. The theoretical calculations of slowness curves are in good agreement with experimental results. The SAW slowness curve is elliptical for Y- or Z-cut crystal wafers, and circular for an X-cut wafer. Received: 27 January 1997/Accepted: 30 July 1997     

Similarity methods to derive turbulence quantities and mixed-layer depth from SODAR measurements in the convective boundary layer: A review

A brief review is made of similarity methods which have been developed to calculate turbulence parameters and mixed-layer depth from SODAR measurements. Emphasis is placed on surface fluxes and mixed-layer depth. A variety of parametric relations valid in the convective boundary layer are surveyed and relevant similarity methods are outlined. The methods are primarily intended for flat and relatively homogeneous terrain. It is shown that, in most cases, horizontal homogeneity provides a less stringent constraint.Finally, some limitations of the existing methods are presented and discussed. It is proposed that the most serious errors are mainly associated with the poor accuracy of some parameters measured by the SODAR and also with the limitations of the theoretical basis employed.     

The Casimir atomic pendulum

We want to introduce an atomic pendulum whose driving force (torque) is due to the quantum vacuum fluctuations. Applying the well-known Casimir-Polder effect to a special configuration (a combined structure of an atomic nanostring and a conducting plate), an atomic pendulum (Casimir atomic pendulum) is designed. Using practically acceptable data corresponding to the already known world of nanotechnology and based on reasonable/reliable numerical estimates, the period of oscillation for the pendulum is computed. This pendulum can be considered as both a new micro(nano)-electromechanical system and a new simple vacuum machine. Its design may be considered as a first step towards realizing the visualized vacuum (Casimir) clock!