Reflection and refraction of bulk acoustic waves in piezoelectrics under uniaxial stress

Basic equations that describe the conditions of reflection and refraction of bulk acoustic waves at the boundary between acentric crystalline media under uniaxial stress are presented. The effect of uniaxial stress on the anisotropy of reflection and refraction of bulk acoustic waves is numerically analyzed for boundaries of the crystal-vacuum, piezoelectric crystal-piezoelectric crystal, and piezoelectric crystal-elastic isotropic medium types.     

Manifestation of basal-plane anisotropy and mechanical boundary conditions in magnetic birefringence of sound in hematite

The angular dependence of the magnetic birefringence of sound in hematite is experimentally investigated as a function of the direction of a magnetic field applied in the basal plane of the hematite crystal. It is found that, at room temperature, the curve of magnetoacoustic oscillations in the magnetic field, i.e., the oscillatory dependence of the amplitude of an acoustic wave transmitted through the crystal on the magnetic field strength, is characterized by hexagonal and uniaxial anisotropy. It is shown that the hexagonal anisotropy is governed by the basal-plane anisotropy of higher orders. The appearance of the uniaxial magnetic anisotropy in the basal plane of the crystal is explained by the mechanical stresses arising in the sample when piezoelectric transducers are glued to the sample ends. This assumption is confirmed by the observed change in the direction of the uniaxial anisotropy axis under variations in the boundary conditions.     

Effect of Anisotropy Fluctuations on the Piezomagnetic Properties of Magnetostrictive Ferromagnets

The effect of the angular and amplitude anisotropy fluctuations in an uniaxial magnetostrictive ferromagnet on its piezomagnetic parameters is examined. It is shown that the increase of the angular anisotropy fluctuations decreases the piezomagnetic parameters of the ferromagnet, while the increase of the amplitude anisotropy fluctuations increases these parameters.     

Rotationally invariant theory of magnetoacoustic wave attenuation in cubic ferromagnets

Rotationally invariant theory is used to study the effect of magnetization relaxation on the spectrum of magnetoacoustic waves propagating in a cubic ferromagnet with an induced uniaxial anisotropy along the [011] direction. It is found that the inclusion of rotationally invariant terms leads to certain contributions to the propagation velocity and rate of attenuation of magnetoelastic waves, thus increasing the degree of anisotropy of these characteristics. Among different types of coupled waves, only quasi-sound modes exhibit a relaxation nature in the region of magnetic phase stability loss. The introduction of dissipation and rotational invariance affects the acoustic birefringence, while the latter gives rise to an additional term in the expression for the phase shift.     

Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory

The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation of the magnetization is determined by the spin components (), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient. Results for the Green's function theory are compared with those obtained with mean-field theory (MFT). We find significant differences between these approaches. Received 6 April 1999 and Received in final form 9 July 1999     

The change in the sound wave energy due to the scattering by a medium rotating with acceleration

Scattering amplitudes and intensities of partial azimuthal harmonics and the total intensity of the scattered field are calculated for the case of transmission of longitudinal acoustic waves through a liquid rotating with acceleration. It is shown that the intensity of transmitted waves depends on the angular acceleration and does not depend on the angular velocity of the vortex rotation.     

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.     

The molecular-statistical calculation of the elastic constants of uniaxial nematics

Using the molecular-statistical theory of the thermodynamic properties of nematics and perturbation theory, the expressions for the elastic constants of uniaxial nematics are derived in terms of the mutual interactions between their constituent molecules. Next the elastic constants of the Maier-Saupe model are calculated and compared with the original result. The appearing difference is discussed briefly. Finally, in view of existing discrepancies, the elastic constants of the Onsager model are recalculated. For that purpose the hard rod interaction is conceived as the limit of a non-singular interaction. The elastic constants of hard core models are shown to increase with increasing temperature at constant density. In view of this behaviour the usefulness of hard core models for an understanding of the nematic state seems rather limited.     

Generation of acoustic solitons by a single-mode electromagnetic field

Mechanisms of acoustic pulse generation by a single-mode electromagnetic field propagating in a photoelastic material are analyzed. The anisotropy induced by acoustic excitations in an isotropic medium leads to nonlinear coupling between the polarization components of a single-mode electromagnetic field. For different conditions, it is shown that the acoustic-electromagnetic wave interaction due to mixing of the polarization components of light and acoustic waves can give rise to soliton-like coherent acoustic excitations in a thin crystal plate. When spatial dispersion is ignored, the governing system of equations for unidirectional acoustic solitons can be reduced to an integrable model. It is shown that qualitatively different scenarios of formation of acoustic solitons are possible, depending on the directions of deformation and field polarization.     

Optical properties of superlattices in the long-wavelength approximation

Specific symmetry features of dielectric and gyrotropic properties of superlattices comprised of arbitrary crystalline layers are considered. It is shown that by changing the orientation of crystallographic axes of the high-symmetry components of the superlattice with respect to their interfaces, one can obtain structures with induced uniaxial or biaxial anisotropy. Expressions are derived that determine the orientation of optical axes in these structures. Propagation of light beams along directions of the induced binormals is analyzed. The feasibility of and conditions for lensless focusing of an optical beam in superlattices comprised of high-symmetry layers are established.     

The energy bands of quantized spin waves and the uniaxial anisotropy in a biferromagnetic system

The eigenproblems of quantized spin waves in a (100) ferromagnetic bilayer system have been investigated theoretically using the interface rescaling approach. The energy-band structure of the system has been obtained and the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands are explored thoroughly. It is found that all kinds of eigenmodes of the spin waves in the biferromagnetic system are determined by the bulk exchange coupling constant, the interface exchange coupling constant, the spin of lattice, the number of atomic layers and the uniaxial bulk anisotropy. Especially, we explore the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands of first Brillouin zone and the existence condition of all kinds of the eigenmodes.     

Calculated correlations between material parameters and electro-optical performance of twisted nematic displays

Computer calculations of the optical transmission of TN-LCDs as a function of the applied voltage are presented. In particular, the wavelength and angular dependence of half transmission voltages and sharpness of transmission curves are examined. Furthermore the lowest relaxation rates of the linearized dynamical equations, neglecting backflow effects are calculated. The dielectrical anisotropy and the elastic parameters are varied systematically in the calculations over ranges that actually occur in nematics. Although the results are too complex to be condensed in simple general statements they are helpfull in designing optimal nematic mixtures for a given display application.     

声表面波在声栅上的反常Bragg衍射

本文讨论了在考虑基片各向异性因素时,声表面波在声栅上的Bragg衍射现象。理论分析时,将有关波式和不同模式之间的差别唯象地以它们对应的相速不同来表征之,而不具体讨论这些波式或模式的偏振特性。在分析时,与讨论声-光反常Bragg衍射情形一样采用二波近似,但不借助于引入对应于极化矢量那样的量,而直接由栅区声速分布给出各波式之间的耦合系数。由此而得出的“反常Bragg衍射”公式,与声-光情形下采用极化矢量概念给出的公式一一对应。由这些公式计算的结果,与实验结果良好地符合。 关键词：     

Wave anisotropy of shear viscosity and elasticity

The paper presents the theory of shear wave propagation in a “soft solid” material possessing anisotropy of elastic and dissipative properties. The theory is developed mainly for understanding the nature of the low-frequency acoustic characteristics of skeletal muscles, which carry important diagnostic information on the functional state of muscles and their pathologies. It is shown that the shear elasticity of muscles is determined by two independent moduli. The dissipative properties are determined by the fourth-rank viscosity tensor, which also has two independent components. The propagation velocity and attenuation of shear waves in muscle depend on the relative orientation of three vectors: the wave vector, the polarization vector, and the direction of muscle fiber. For one of the many experiments where attention was distinctly focused on the vector character of the wave process, it was possible to make a comparison with the theory, estimate the elasticity moduli, and obtain agreement with the angular dependence of the wave propagation velocity predicted by the theory.     

Diffraction of light by an acoustic wave with three frequency components

Collinear light diffraction by three-frequency sound is investigated theoretically. The amplitude distributions of transmitted and diffracted light waves along the cell are calculated for different amplitudes of sound signals. The dependence of the intensity of principal diffraction peaks on the frequency difference between acoustic signal components is studied for different amplitude ratios of these components. It is shown that the character of this dependence for a wave being in synchronism differs substantially from that for two other waves characterized by detuning. The dependence of the amplitudes of principal and parasitic diffraction peaks on the efficiency of acoustooptical interaction is investigated. It is demonstrated that parasitic sideband components in diffracted light can play a considerable role if the diffraction efficiency is sufficiently high and exceeds 80%.     

Angular spectrum approach for the computation of group and phase velocity surfaces of acoustic waves in anisotropic materials

The decomposition of an acoustic wave into its angular spectrum representation creates an effective base for the calculation of wave propagation effects in anisotropic media. In this method, the distribution of acoustic fields is calculated in arbitrary planes from the superposition of the planar components with proper phase shifts. These phase shifts depend on the ratio of the distance between the planes to the normal component of the phase slowness vector. In anisotropic media, the phase shifts depend additionally on the changes of the slowness with respect to the direction of the propagation vector and the polarization. Those relations are obtained from the Christoffel equation. The method employing the fast Fourier transformation algorithm is especially suited for volume imaging in anisotropic media, based on holographic detection in transmission of acoustic waves generated by a point source. This technique is compared with measurements on crystals performed by phase-sensitive scanning acoustic microscopy.     

Sagnac effect in ring interferometers on “slow” waves

We consider the Sagnac effect in ring interferometers on magnetostatic and surface acoustic waves. It is shown that the Sagnac effect for waves of arbitrary type (including both magnetostatic and surface acoustic waves) propagating in an arbitrary medium cannot be calculated using Galilean transformations but is explained within the framework of the special relativity and is related to the difference between the phase velocities rather than group velocities of counter-propagating waves in the rotating reference frame. We also show that the phase difference of counterpropagating waves due to the Sagnac effect depends on neither the phase velocity of the wave in a medium at rest nor the dispersion of the medium; it depends only on the wave frequency and the angular velocity of interferometer rotation. The minimum angular velocity that can be measured in the ring interferometers using magnetostatic and surface acoustic waves is estimated. N. I. Labachevsky State University, Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 4, pp. 373–382, April 1999.     

Exchange bias for ferromagnetic/antiferromagnetic bilayers with the uniaxial anisotropy being misaligned with the exchange anisotropy

Using the principle of minimal energy and S-W model, the exchange bias for ferromagnetic/antiferromagnetic bilayers has been investigated when the uniaxial anisotropy is misaligned with the exchange anisotropy. According to the relation between the energy of the bilayer and the orientation of ferromagnetic magnetization, it is found that the bilayer will be in the monostable state or bistable state when the external field is absent in the initial magnetization state. The monostable state or bistable state of the bilayer, which determines the angular dependence of exchange bias directly, is controlled by the competition between the exchange anisotropy and uniaxial anisotropy. When the applied field is parallel to the intrinsic easy axes and intrinsic hard axes, one of the switching fields of the hysteresis loop shows an abrupt change, while the other keep continuous by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show a jump phenomenon. The numerical calculations indicate that both the magnitude and direction of the exchange anisotropy will significantly affect the angular dependence of exchange bias. The jump phenomenon of exchange bias is an intrinsic property of the bilayer, which is dependent on the interfacial exchange-coupling constant, the orientation of the exchange anisotropy, the thickness and uniaxial anisotropy constant of the ferromagnetic layer.     

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.