V.I. Alshits

On the existence problem for localized acoustic waves on the interface between two piezocrystals

Abstract The investigation of the existence problem for inhomogeneous acoustic waves on the interface between two piezocrystals of arbitrary anisotropy is performed on the basis of the surface impedance method. It is shown that not more than two localized waves may exist if two piezoelectric half-infinite spaces are in welded contact and the electrical boundary conditions require the electrical potential to vanish on the interface. However, if the normal projection of electrical induction is assumed to be zero on the interface, then at most one interfacial wave comes into existence. For two piezoelectric crystals in sliding contact at most three or at most two “slip” intrinsic waves may appear depending on the electrical boundary conditions. But if two piezocrystals are separated by an air gap, then not more than four inhomogeneous “gap” waves propagate along the gap. The existence problem for localized acoustic waves on the interface between two piezomagnetic crystals is also discussed.

Magnetoplastic effect in non-magnetic crystals and internal friction☆

Abstract A new physical phenomenon, the magnetoplastic effect, was found and investigated in a series of non-magnetic crystals (NaCl, CsI, LiF, Zn, Al). The phenomenon manifests itself in dislocation displacements as a result of exposing the specimens to a static magnetic field in the absence of mechanical loading. The effect was studied experimentally as a function of the time of magnetic treatment of the samples, the magnitude and orientation of the magnetic field ( B =0.05−;2 T), the temperature ( T =4.2–293 K), the type and concentration of impurities, the density of “forest” dislocations and the frequency ( v =0−200 Hz) of the alternating (over orientation) magnetic field. A strong effect of a weak static electric field ( E =2–30 V cm −1 ) on the dislocation mobility in NaCl specimens simultaneously exposed to a static magnetic field ( B =0.1−0.4 T) is found (with no electric influence at B =0). A physical interpretation of the phenomenon is proposed. Possible manifestations of the magnetoplastic effect in the internal friction are discussed.

Magnetoplastic effect in nonmagnetic crystals

Abstract The main features of the magnetoplastic effect displaying itself in dislocation displacements in nonmagnetic crystals under the action of external magnetic field are briefly discussed. The influences of external electric field and X-ray irradiation on the magnetoplastic effect are demonstrated for NaCl and LiF crystals. A magnetic induction threshold B c below which the magnetoplastic effect does not exist is predicted and experimentally observed in Al, NaCl and LiF crystals. The existence of a threshold is associated with the fact that for B c the spin-lattice relaxation time τ slc in a system is less than the time required for a spin evolution in a magnetic field resulting in the removal of the spin forbiddenness of a process that ‘switches off’ the dislocation-pinning-center interaction. It is shown that the threshold field B c is sensitive to temperature and X-ray irradiation of the samples.

Effects of magnetic fields on the dislocation unlocking from paramagnetic centers in non-magnetic crystals

Abstract Displacements of dislocations as a result of exposing the specimens to a static magnetic field (B=0.1−2 T) in the absence of mechanical loading in the temperature interval T=4.2−293 K were found and investigated for NaCl, CsI, LiF, Zn and Al single crystals. This effect is characterized by the following properties: the mean dislocation path l increases linearly with the time t of the magnetic treatment; the mean dislocation velocity ν is proportional to the square of the magnetic magnetic induction (ν∝B2) and inversely proportional to the square root of the concentration of paramagnetic impurities ( ν∝1 √C ); the effect is temperature independent in the range T = 4.2–77 K and it grows only by 20%–30% with a further increase in T up to room temperature; l(t) and l(B) saturate at large values of t and B at a level corresponding to an average distance between “forest” dislocations; an abrupt decrease in the effect is found when the frequency ν of an alternating magnetic field exceeds some critical value νc ∝ B2 (νc ≈ 15 Hz at B = 0.3 T). The phenomenon is interpreted as a result of unlocking of dislocations from paramagnetic centers under the action of an external magnetic field and motion of dislocations in the long-range internal stress field.

Surface acoustic waves in piezocrystals: an example of surface wave existence with clamped boundary

Abstract It is shown by a model calculation that in a half-infinite piezoelectric-piezomagnetic a subsonic surface wave solution will exist even with a mechanically clamped boundary, for suitable material constants and suitable electric and magnetic boundary conditions. This settles a problem left open in a recent paper by Alshits, Darinskii and Lothe, “On the existence of surface waves in half-infinite anisotropic media with piezoelectric and piezomagnetic properties”, Wave Motion 16 , 265–283 (1992).

Electric amplification of the magnetoplastic effect in nonmagnetic crystals

An experimental study of the electric field influence on the magnetoplasticity is performed for NaCl and LiF crystals. It is shown that even rather weak electric fields of E∼1–10 kV/m strongly amplify the magnetoplastic effects of both “signs.” In crystals, such as NaCl(Ca), where the magnetic transformation of impurities in dislocation cores decreases the dislocation pinning, the mean dislocation path l in the magnetic field B abruptly increases under an electric field applied along with B. Also in crystals such as NaCl(Pb), where, conversely, a magnetically induced growth of the pinning force occurs, the electric field again amplifies the magnetic effect, this time suppressing a dislocation mobility. In both cases, no electric effects at B=0 are observed. Measurements at the temperatures of 77, 165, and 293 K displayed a thermally activated character of the electric actions. An activation analysis was performed for the two types of NaCl crystals, manifesting a magnetic softening. The found activation en...

Surface Waves, Limiting Waves and Exceptional Waves: David Barnett's Role in the Development of the Theory

A short review is presented of progress in the theory of localized eigenwaves in anisotropic solids. The main attention is paid to the problem of existence of surface wave solutions in semi-infinite elastic bodies. Here the contribution of David Barnett is most important. The general theorems of existence and uniqueness are formulated and their modifications for media with piezoelectric and/or piezomagnetic couplings are considered. The conditions for existence of supersonic surface waves in purely elastic half-spaces are displayed. The situations on both sides of the vicinity of exceptional transonic states are discussed. The criterion for the existence of quasi-bulk subsonic surface waves in elastic media of unrestricted anisotropy is obtained in terms of the general Barnett and Lothe theorem. The conditions for the existence of exceptional bulk waves and their main properties are considered. The interface wave theory of David Barnett et al is reviewed for rigid and slip contacts between elastic and piezoelectric sub-media.

Wedge refraction of electromagnetic waves in absorbing crystals

Topological features of the self-intersection of wave surfaces near singular optical axes of an absorbing crystal are investigated. Distributions of complex polarization fields in the neighborhood of singular directions are obtained. It is shown that, when the wave normal m circumvents an optical axis, the corresponding rotation of polarization ellipses is characterized by the PoincarÉ index n=1/4. Using the example of an orthorhombic crystal, a wedge refraction of electromagnetic waves on the intersection line of the sheets of the surface of refractive indices is predicted and theoretically investigated. It is shown that the directions of the mean energy fluxes P̄± are close to the direction of normals n± to the refraction surface only in the central region of a wedge, i.e., only in the domain where the polarization is almost linear and the group velocity of waves is well defined. When m moves to singular axes, the ellipticity of the polarization increases at the ends of the edge of the wedge and the orientations of the vectors P̄± and n± gradually diverge, yet remain in the same plane that is orthogonal to the edge. The angle between P̄+ and P ̄ − monotonically decreases, and P̄+ || P̄− for the propagation along singular axes; in this case, the angle between n+ and n− increases, and they have a plane-fan-type orientational singularity along the optical axes. When m is scanned along the edge of the wedge, the unaveraged vectors P± describe per period the same conical surface that coincides with the refraction cone of a transparent crystal, while the endpoints of the vectors P± run over elliptic orbits whose shape and slope depend on m. The possibilities of observing a wedge refraction are analyzed.

Anomalous dispersion of SH acoustic waves in a piezoelectric sandwich structure

A transformation of the dispersion spectrum of shear horizontal (SH) acoustic eigenwaves in a sandwich structure due to a piezoelectric effect is described. The structure consists of two plates separated by a gap whose thickness is considerably less than the wavelength. Under these conditions, acoustic fields induced in the plates interact through the piezoelectric effect. The piezoelectric effect brings about a distortion and divergence of the initially (in the zeroth approximation) independent dispersion curves; i.e., all points of intersection of the dispersion curves disappear. Each of the new dispersion branches is formed by a set of adjacent portions of initial branches. A change in the wave number (or in the frequency) results in a periodic gradual displacement of the localization zone of the acoustic field from one plate to the other.

Bulk polaritons in a biaxial crystal at the interface with a perfect metal

The orientation of the plane where the tangential electric field component becomes zero is indicated for any plane bulk electromagnetic wave propagating in an infinite transparent medium of arbitrary anisotropy. Thus, the existence of this wave (bulk polariton) in this plane (interface with an ideal conductor) is ensured. The characteristics of such polaritons of two independent branches with coinciding wave normals (isonormal polaritons) or Poynting vector directions (isoray polaritons) are compared.