On existence of one-partial bulk waves in semi-infinite piezoelectric media

The conditions for the existence of one-partial bulk waves satisfying the boundary conditions on a mechanically free surface of a semi-infinite piezoelectric medium have been analyzed. In purely elastic media, similar wave solutions are known to exist along the propagation directions m which form lines on the sphere m ? m = 1, passing through all the available degeneracy points (acoustic axes). It is shown that in a triclinic piezoelectric half-space with a metallized surface, one-partial bulk waves may exist solely along isolated propagation orientations, whereas for a nonmetallized surface, such waves can exist only if an additional condition for the material constants of the medium is fulfilled. It is also shown that the one-partial bulk solution may not exist along an arbitrary acoustic axis in a piezoelectric. These conclusions are valid in the general case of unrestricted anisotropy, i.e., they do not take into account the material symmetry. In addition, the orientations providing the propagation of one-partial bulk waves because of the existing symmetry are specified for piezoelectric media of various symmetry classes.     

Dynamic and static orientational disorder in mixed K1 − x (NH4)xI crystals

Earlier, we have shown that, at low temperatures, the mixed K_{1 ? x} (NH₄)_xI crystals show two low-energy resonance modes in the dynamically orientationally disordered region of the α-phase and statically orientationally disordered phase of the orientational glass, absent in the orientationally ordered γ-phase. Below, we report the results of the experimental study of the low-energy resonance and local modes in K_{1 ? x} (NH₄)_xI crystals obtained by the method of inelastic incoherent neutron scattering (IINS) over the wide temperature range.     

On the theory of plane inhomogeneous waves in anisotropic elastic media

In the theory of plane inhomogeneous elastic waves, the complex wave vector constituted by two real vectors in a given plane may be described with the aid of two complex scalar parameters. Either of those parameters may be taken as a free one in the characteristic condition assigned to the wave equation. This alternative underlies the two fundamental approaches in the theory, namely, one associated with the Stroh eigenvalue problem and the other with the generalized Christoffel eigenvalue problem. The two approaches are identical insofar as a partial nondegenerate wave solution (partial mode) is concerned, but they differ in the fundamental solution (wave packet) assembling, and their dissimilarity is also revealed in the presence of degeneracies, which may involve either of the two governing parameters or both of them. Therefore, use of both approaches is essential for studying the degeneracy phenomenon in the theory of inhomogeneous waves. The criteria for different types of degeneracy, related to a double eigenvalue of the Stroh matrix or the Christoffel matrix and at the same time to a repeated root of the characteristic condition, are formulated by appeal to the matrix algebra and to the theory of polynomial equations. On this basis, dimensions of the manifolds, associated with degeneracy of different types in the space of variables, are established for elastic media of unrestricted anisotropy. The relation to the boundary-value problems is discussed.     

The matrix sign function for solving surface wave problems in homogeneous and laterally periodic elastic half-spaces

The matrix sign function is shown to provide a simple and direct method to derive some fundamental results in the theory of surface waves in anisotropic materials. It is used to establish a shortcut to the basic formulas of the Barnett–Lothe integral formalism and to obtain an explicit solution of the algebraic matrix Riccati equation for the surface impedance. The matrix sign function allows the Barnett–Lothe formalism to be readily generalized for the problem of finding the surface wave speed in a periodically inhomogeneous half-space with material properties that are independent of depth. No partial wave solutions need to be found; the surface wave dispersion equation is formulated instead in terms of blocks of the matrix sign function of i$i$ times the Stroh matrix.     

The high-frequency dispersion coefficient for the Rayleigh velocity in a vertically inhomogeneous anisotropic half-space

For an arbitrary anisotropic half-space with continuous vertical variation of material properties, an explicit closed-form expression for the coefficient B of high-frequency dispersion of the Rayleigh velocity v(R)(omega) approximately v(R)(0)(1+B/omega) is derived. The result involves two matrices, one consisting of the surface-traction derivatives in velocity and the other of its Wentzel-Kramers-Brillouin coefficients, which are contracted with an amplitude vector of the Rayleigh wave in the reference homogeneous half-space. The "ingredients" are routinely defined through the fundamental elasticity matrix and its first derivative, both taken at v=v(R)(0) and referred to the surface.     

Numerical Simulation of Self-Pumped Phase Conjugate Plane--Curve Loop Mirror Based on Photorefractive Nonlinearity

We deal with computer simulation of a transient process in a self-pumped phase conjugate plane-curve loop mirror based on BaTiO3. In optimal circumstances the nonlinear reflectivity and fidelity of such a mirror respectively achieve 0.80-0.90 and 0.95-0.98. The generation of conjugate wave-front occurs due to scattering from the dynamic hologram which is produced in the region of self-intersection of forward and backward beams. In such a model the scenario of passing to unstable generation regimes is similar to the self-pumped phase conjugate plane-plane loop mirror and substantially differs from a single-crystal double phase conjugate mirror.     

Structure-activity studies of irumamycin type macrolides from Streptomyces sp. INA-Ac-5812

Three natural glycosylated macrolide compounds, known irumamycin 1 and X-14952B 2, as well as new isoirumamycin 3, were isolated from ethyl acetate mycelium extract of Streptomyces sp. INA-Ac-5812. Structures of the compounds were elucidated using 1D and 2D NMR. Isoirumamycin 3 was found to be an isomer of irumamycin with an 18-membered macrolactone ring instead of 20-membered macrolide in irumamycin. A previously unknown stereo configuration of irumamycin epoxide (C23, C24) and hemiketal (C3, C7) fragments was deduced from NMR data (ROESY/NOESY and HSQMBC). Cytotoxic, antifungal and antibacterial activities were studied for all isolated compounds. Comparison of the collected data showed crucial importance of 20-membered macrolactone ring for antimicrobial properties of this antibiotic family.     

Shear horizontal waves in transversely inhomogeneous plates

Shear horizontal (SH) waves in free and clamped monoclinic plates with an arbitrary inhomogeneity across the plate are considered. Firstly, some general properties of dispersion spectra of, specifically, the SH waves are pointed out. Secondly, analytical and numerical modeling of the SH dispersion branches is presented. Closed-form estimations are compared with exact curves computed for a free plate with continuously varying properties.