Propagation of acoustic beams in a paratellurite crystal

A general solution to the problem of acoustic beam diffraction in an anisotropic medium has been obtained. The effect of acoustic anisotropy of paratellurite crystal on the structure of beams propagating in the XY crystallographic plane has been analyzed. Ray spectra are calculated for different propagation directions of ultrasound. The effects of beam focusing, defocusing, and autocollimation caused by acoustic anisotropy have been analyzed.     

Incident-angle dependence of the phase conjugate reflectivity by nonlinear piezoelectric interaction in PZT ceramics.

The reflectivity of an acoustic phase conjugator by nonlinear piezoelectric interaction in PZT ceramics was measured as a function of the incident angle. The phase conjugator was designed as a solid block with a flat surface for the acoustic incidence from water. The phase conjugate reflectivity showed incident angle dependence, which is mainly determined by the transmissivity of the acoustic waves, together with some effects of anisotropy. The field-reconstructing ability of this phase conjugator was simulated for focused incident beams.     

Acoustic radiation force on a sphere in standing and quasi-standing zero-order Bessel beam tweezers

Starting from the exact acoustic scattering from a sphere immersed in an ideal fluid and centered along the propagation axis of a standing or quasi-standing zero-order Bessel beam, explicit partial-wave representations for the radiation force are derived. A standing or a quasi-standing acoustic field is the result of propagating two equal or unequal amplitude zero-order Bessel beams, respectively, along the same axis but in opposite sense. The Bessel beam is characterized by the half-cone angle β of its plane wave components, such that β = 0 represents a plane wave. It is assumed here that the half-cone angle β for each of the counter-propagating acoustic Bessel beams is equal. Fluid, elastic and viscoelastic spheres immersed in water are treated as examples. Results indicate the capability of manipulating spherical targets based on their mechanical and acoustical properties. This condition provides an impetus for further designing acoustic tweezers operating with standing or quasi-standing Bessel acoustic waves. Potential applications include particle manipulation in micro-fluidic lab-on-chips as well as in reduced gravity environments.     

Acoustooptical and elastic properties of laminated KY(MoO4)2 crystals

Data on acoustic (absorption and velocity of sound), optical (refractive index and optical absorption coefficient), and photoelastic (coefficients of acoustooptical quality and photoelastic constants) properties of KY(MoO₄)₂ crystals are obtained. It is shown that, not only does the anisotropy of binding forces lead to a significant anisotropy of acoustic and photoelastic properties, but it also determines anomalously high elastic nonlinearity in the direction of the Y-axis perpendicular to cleavage planes.     

Observation of light-induced anisotropy in ferric borate by acoustic resonance

A light-induced anisotropy of two-fold symmetry in the magnetically easy c-plane has been observed at 77 K in c-plane platelets of FeBO₃ using the method of acoustic resonance. The anisotropy field is shown to be negative (positive) in the direction of the net magnetization during illumination in those samples for which the susceptibility increases (decreases) with light.     

Doppler spectroscopy on relativistic particle beams in the light of a test theory of special relativity

Modern experimental techniques based on collinear laser spectroscopy have led to greatly improved versions of the classical Ives-Stilwell experiment. The new experiments employ collinear laser beams and measure the Doppler shift as it is seen in the rest frame of a relativistic particle beam. This note analyzes these experiments in the framework of a test theory of special relativity due to Mansouri and Sexl. It is found that the experiments can test the coefficient functiona(υ² (responsible for time dilation), but are insensitive tob,d and?. While the Mansouri-Sexl theory in principle allows for an anisotropy of the velocity of light with respect to a moving reference frame, anisotropy effects cancel out in Ives-Stilwell type experiments. These experiments by themselves are not sufficient for a direct observation of such an anisotropy.     

Partially polarized fields generated by constant-amplitude (non-thermal) beams

The necessity for polarization-tensors of higher order than the 2×2 polarization matrix is clearly demonstrated, by considering two inequivalent situations arising from the superposition of two constant-amplitude beams with a random phase difference between them. The partially- and unpolarized fields from the superposition of two circularly polarized beams show no anisotropy in intensity fluctuations when viewed by a plane-polarized detector. This is not the case for the partially- and even the unpolarized beam formed from two plane-polarized beams. In passing, thesimple form of the polarization curve observed by a rotatable plane-polarized detector subject to an elliptically polarized beam, is derived.     

Collective oscillations of the order parameter and electromagnetic response in niobium

Caroli and Maki predict a strong anisotropy in the transverse electromagnetic response of pure type II superconductors due to the effect of transverse collective fluctuations of the order parameter. The predicted anisotropy in the transverse acoustic wave attenuation near H_c2 was studied experimentally in niobium and found to be absent for q parallel to [100].     

Three-dimensional acoustic Fabry-Perot interferometer of α-quartz as converter of acoustic modes

A cube of α-quartz with polished flat and parallel sides was used as an acoustic three-dimensional Fabry-Perot interferometer. The sides of the cube are perpendicular to the crystallographic axesx, y, z of the quartz. By means of a transducer, transverse elastic waves were injected along thez-axis. Because of anisotropy, this produces a diffuse acoustic field in the cube. From this diffuse field, the interferometer selects pure longitudinal acoustic waves along thex andz axis, which were identified by means of the different velocities of propagation.     

Thermopower anisotropy in the layered compound PbSb2Te4

The possible origin of the considerable anisotropy of the Seebeck coefficient in crystals of the multicomponent layered compound p-PbSb₂Te₄ is analyzed. It is shown that the experimental data on the thermopower anisotropy in p-PbSb₂Te₄, together with the temperature dependences of the electrical conductivity along the trigonal axis and in the cleavage plane, can be explained by in terms of a one-band spectrum model and a mixed mechanism of hole scattering under the assumption that scattering from acoustic phonons and impurity ions is dominant in the cleavage plane and along the C ₃ trigonal axis, respectively.     

计算水下凹面目标散射声场的声束弹跳法

为了解决含有多次散射时水下目标声散射场的计算问题,提出了一种声束弹跳方法。把入射声波划分为若干声束,根据几何声学方法计算每条声束在目标表面的反射方向和能量损失,利用物理声学方法计算最后一次反射的声束所对应的面元的散射场,通过计算所有声束产生的散射场的叠加得到整个目标的散射场。计算了直角凹面圆锥体的散射声场,并对具体模型进行了水池测量实验,理论计算和实验测量结果一致。表明该方法作为一种高频近似的数值计算方法,可以计算存在多次散射时水下目标的散射声场。      

The anisotropy of the basic characteristics of Lamb waves in a (001)-Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> piezoelectric crystal

The orientation dependences of the phase velocity, the effective electromechanical coupling coefficient, and the angle between the wave normal and the energy flux vector are numerically calculated for zeroand first-order Lamb waves propagating in the (001) basal plane of a Bi₁₂SiO₂₀ cubic piezoelectric crystal. It is shown that the anisotropies of these modes are different and depend on the plate thickness h and the wavelength λ. For h/λ < 1, the mode anisotropy can exceed the anisotropy of the corresponding characteristics of surface acoustic waves propagating in the same plane; for h/λ > 1, it approximately coincides with the SAW anisotropy for all the characteristics.     

Acoustic scattering of a high-order Bessel beam by an elastic sphere

The exact analytical solution for the scattering of a generalized (or “hollow”) acoustic Bessel beam in water by an elastic sphere centered on the beam is presented. The far-field acoustic scattering field is expressed as a partial wave series involving the scattering angle relative to the beam axis and the half-conical angle of the wave vector components of the generalized Bessel beam. The sphere is assumed to have isotropic elastic material properties so that the nth partial wave amplitude for plane wave scattering is proportional to a known partial-wave coefficient. The transverse acoustic scattering field is investigated versus the dimensionless parameter ka(k is the wave vector, a radius of the sphere) as well as the polar angle θ for a specific dimensionless frequency and half-cone angle β. For higher-order generalized beams, the acoustic scattering vanishes in the backward (θ = π) and forward (θ = 0) directions along the beam axis. Moreover it is possible to suppress the excitation of certain resonances of an elastic sphere by appropriate selection of the generalized Bessel beam parameters.     

Zweiquantenabsorption bei freien Exzitonen in Zinkoxid

The two photon absorption spectrum at the intrinsic absorption edge of ZnO is measured with various directions and modes of polarization of the incident light beams. Several peaks are observed, corresponding to the 2P _0,±1-exciton states of the 3 seriesA, B, C (which occur on account of the threefold upper valence band). The anisotropy of the product of reduced effective mass times dielectric constant determined from the line positions is compatible with theoretical predictions. The lack ofS-exciton states in the two quantum absorption spectrum of ZnO is discussed in detail.     

Anisotropic potential energy functions for carbon atoms in graphite

Recent experimental studies of the scattering of He atomic beams by graphite have shown that the electronic anisotropy of the C atoms must be taken into account in calculating the interaction between a graphite lattice and an external He atom. The empirical compressibility of graphite is used to determine an appropriate value for the polarizability of graphite C atoms parallel to the graphite c axis, a parameter needed to calculate He-graphite interaction potentials.     

Plasmons in double quantum wells in a parallel magnetic field

Collective intraband charge-density excitations in the quasi-two-dimensional electron system of double GaAs/AlGaAs quantum wells in an external parallel magnetic field B_∥ are studied by inelastic light scattering. It has been found that the energy of the excitations under study (acoustic and optical plasmons) exhibits anisotropy depending on the mutual orientation of B_∥ and the excitation quasi-momentum k. It is shown theoretically that, in a strong parallel magnetic field, the effects associated with the finite width of the quantum wells dominate over the effects associated with interlayer tunneling and determine the anisotropy of plasmons. The experimental data are compared with a theoretical calculation.     

Bosonization of coupled electron-phonon systems

We calculate the single-particle Green’s function of electrons that are coupled to acoustic phonons by means of higher dimensional bosonization. This non-perturbative method is not based on the assumption that the electronic system is a Fermi liquid. For isotropic threedimensional phonons we find that the long-range part of the Coulomb interaction cannot destabilize the Fermi liquid state, although for strong electron-phonon coupling the quasi-particle residue is small. We also show that Luttinger liquid behavior in three dimensions can be due to quasi-one-dimensional anisotropy in the electronic band structure or in the phonon frequencies.     

Modification of the parabolic approximation in the theory of ultrasonic-beam diffraction in a strongly anisotropic crystal

A modification of the parabolic approximation of the diffraction theory is based on the approximation of the wave surface with allowance for the shape of the angular spectrum of the ultrasonic beam. It is demonstrated that such an approach is more accurate in the analysis of the acoustic field at a relatively high diffraction divergence and/or high acoustic anisotropy. An acoustic field is simulated in paratellurite, which is a leading acoustic material with anomalously high acoustic anisotropy.     

The effect of hydrostatic and uniaxial pressure on the elastic constants of cadmium

The third order elastic constants of the hep metal cadmium have been determined from measurements of the hydrostatic and uniaxial pressure dependences of the velocities of ultrasonic waves propagated through single crystals. The hydrostatic pressure dependences of the second order elastic constants have also been obtained. The compression of cadmium has been estimated by extrapolation of the data up to high pressures by using the Murnaghan equation of state. Using the generalised Grüneisen theory in the quasiharmonic approximation, the long wavelength acoustic mode Grüneisen parameters have been obtained; the mean acoustic mode parameter is compared with the thermal Grüneisen parameter. In general the anisotropy of the vibrational anharmonicity of the acoustic modes is found to be consistent with the deviation of the c/a ratio of cadmium from that expected for an ideally close-packed metal.     

Excitation of ion acoustic waves at the difference frequency of two microwave beams in a semiconductor

This letter presents an investigation of the resonant excitation of the electrostatic ion acoustic wave at the difference frequency of two microwave beams in a semiconductor, viz., n-type InSb. The ponderomotive force at the difference frequency on electrons drives the ion acoustic wave at the difference frequency. The resonance conditions are satisfied over a wide range of semiconductor parameters. For typical plasma parameters of n-InSb and microwave beams of power densities 1 MW cm^?2, the power density of the excited ion acoustic wave is ≈ 1.76 kW cm^?2.