High-Q oscillations in a layered elastic medium

Conditions of the existence of long-lived acoustic resonances that occur in a layered medium because of its oscillation with low damping factors are considered. A priori theoretical estimates relating the distribution of resonance frequencies in the complex plane to the parameters of inhomogeneity of the layered system are obtained. A scheme of resonance calculations in numerical modeling is described. Examples of geophysical media with long-lived resonances are presented.     

Bloch oscillations of an acoustic field in a layered structure

This study is devoted to experimentally achieving the phenomenon of periodic modulation of the acoustic-wave intensity, which is observed upon passage of an acoustic signal through a quasi-periodic structure and is the acoustic analog of the effect of Bloch oscillations (BO). Ultrasound at a frequency of 1 MHz is used, and a layered structure that consists of alternate plane-parallel glass and water layers serves as a super-lattice. In order to create an analog of the external electric field, the thickness of the water layers was changed inversely with respect to their ordinals. It is shown that the transmission spectrum of such a structure has the form of narrow equidistant peaks (an analog of the Wannier-Stark ladder), and the envelope of a transmitted signal undergoes periodic oscillations (analogous to BO). The experimental results are in good agreement with theoretical calculations performed by the transfer-matrix method.     

Reaction-diffusion fronts in periodically layered media

We compute the effective wavefront speeds of reaction-diffusion equations in periodically layered media with coefficients that have small-amplitude oscillations around a uniform mean state. We compare them with the corresponding wavefront speeds in the uniform state. We analyze a one-dimensional model where wave propagation is along the layering direction of the medium and a two-dimensional shear flow model where wave propagation is othogonal to the layering direction. We find that the effective wave speed is smaller in the one-dimensional model and is larger in the two-dimensional model for both bistable cubic and quadratic nonlinearities of the Kolmogorov-Petrovskii-Piskunov form. We derive approximate expressions for the wave speeds in the bistable case.Dedicated to Jerry Percus on the occasion of his 65th birthday.     

Friedell oscillations in a layered electron gas

The oscillations appearing in the potential of a layered electron gas in the random phase approximation are investigated for arbitrary plane distances. Due to the two-dimensional Fermi sphere, they turn out to be always of two-dimensional character, in contrast to the three-dimensional screening behaviour of the system.     

Photothermoacoustic oscillations in a solid-piezoelectric layered structure

Photothermoacoustic oscillations in a thin plane-layered structure consisting of an isotropic solid and a piezoelectric crystal of class 6mm (or piezoelectric ceramics) are studied theoretically and experimentally. Expressions for the potential difference across an arbitrary layer of the piezoelectric transducer are derived. For the case of a two-layer transducer, the amplitude-frequency and phase-frequency dependences of the signal are analyzed. It is shown that, from these experimental dependences, one can determine certain elastic and thermal parameters of a solid. An experiment is performed with samples of Cu, Zn, and TsTS-19 piezoceramics in the frequency range within 9–1000 Hz. The experimental data are used to determine the values of the reduced Young’s modulus and the thermal diffusivity of the materials under study.     

An inverse algorithm for reconstructing the acoustic parameters of layered medium

I.IntroductionThcinvcrsescattcringprob1cmisbasicinarcassuchasradar,sonar,geophysica1explora-honandnondestructivctesting.Whenthescatteringobjectisaconstitutiveparametersvaryinacontinuousmannerandtheinvcrseproblcmistodctcrmineoneormoreoftheseparametcrsfromtheobserveddata.Muchofthcpreviousworkonthisproblcmforthecaseofp1anewaveatnormalincidencchasconsistcdofderivingaSchr6dingerequationfromthebasicacousticandstrcss-straincquations,andthenreconstruchngthepotentialappearinginthisequationbyusingtheGe…     

Self-localization of excitons in a periodically modulated molecular medium

Electromagnetic field propagation is analyzed in a one-dimensional Bragg grating consisting of periodically arranged linear molecules making up a resonant medium. Dye J-aggregates and conjugated polymers are considered as examples of the medium. Both adiabatic and nonadiabatic dynamics of the acoustic waves generated by electromagnetic field in the system are examined. The effects of exciton-phonon and exciton-phonon-photon interactions on the band structure and formation of self-localized excitations are examined on various time scales. A new mechanism for controlling bandgap parameters in a bistable regime is described. Some effects of electromagnetic-field nonuniformity on generation of phonons in molecules and exciton self-localization are investigated.     

Neutrino oscillations in a homogeneous moving medium

The quasi-classical equation that describes both neutrino flavor oscillations and spin rotation in dense matter is obtained in the framework of quantum field theory. The solution of this equation for homogeneous medium is found. The probabilities of the spin-flavor transitions in unpolarized moving matter are calculated in the two-flavor model. We show that if the medium is at rest the neutrino helicity is conserved and the flavor transition probabilities are equal to those obtained using the phenomenological approach.     

Localization of acoustic phonons in a layered lattice

The possibility of weak localization of acoustic phonons in a nonideal layered-crystal lattice is studied. Analytical expressions are obtained for the thermal conductivity and diffusion coefficient tensors at low temperatures. The role of specific processes of interference scattering of phonons by phonon density fluctuations near defects under conditions of weak interaction between the layers is analyzed. It is shown that because of such processes a substantial renormalization of the diffusion coefficient in the presence of strong diagonal disorder is possible at relatively low frequencies, where the dispersion laws for the acoustic phonons display quasi-two-dimensional properties. A nonstandard low-temperature dependence of the thermal conductivity of the layered systems BSCCO and BSYCO is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 132–141 (January 1998)     

SH-wave propagation and scattering in periodically layered composites with a damaged layer

Plane SH-wave propagation in periodically layered elastic composites with a damaged layer is investigated. Two different models are developed to approximate the damaged layer, namely, a periodic array of cracks and continuously distributed springs in the layer. In the first model, the total wave field in the elastic stack of layers with cracks is described as a sum of incident wave field modeled by the transfer matrix method and the scattered wave field governed by an integral representation in terms of the crack-opening-displacements on the crack-faces. The integral equation derived from the boundary conditions on the crack-faces is solved numerically by a Galerkin method. By using Bloch–Floquet theorem the crack-opening-displacements for a periodic array of cracks are expressed by the crack-opening-displacement on a reference crack. In the spring model, the spring constant is estimated by the material properties and the crack density and the modified transfer matrix method is used to compute the wave reflection and transmission coefficients. Numerical results obtained by both models are presented and discussed. Special attention of the analysis is devoted to wave transmissions and reflections, band gaps, wave localization and resonance phenomena due to damages. The influences of the damage types (periodic cracks and stochastic cracks approximated by distributed springs) on the wave field pattern and the band gaps are analyzed.     

SH-wave propagation and resonance phenomena in a periodically layered composite structure with a crack

This paper presents a dynamic analysis of time-harmonic plane SH-waves propagating in periodically multilayered elastic composites with a strip-like crack. The total wave field in the multilayered elastic structure is described as a sum of incident wave field modeled by the transfer matrix method and the scattered wave field governed by an integral representation containing the crack-opening-displacement. The integral equation derived from the boundary conditions on the crack-faces is solved numerically by a Galerkin method. The paper focuses on resonant and non-resonant regimes of anti-plane wave motion in a stack of elastic layers weakened by a single strip-like crack and wave localization in the vicinity of the crack. The scattered extra displacement induced by the presence of the crack is investigated in detail for both situations of high and low contrast in material properties. Numerical results for the average crack-opening-displacement, the transmission coefficient, the stress intensity factor and the average energy flow are presented and discussed to reveal wave resonance and localization phenomena within the band-gaps and the pass-bands.     

Spectrum of plasma oscillations in structures with a periodically inhomogeneous two-dimensional electron plasma

We develop a rigorous electrodynamic theory of plasma oscillations in a periodically inhomogeneous two-dimensional electron system with a rectangular profile of the spatial modulation of the equilibrium electron concentration. We calculate the frequencies and radiative damping of the main plasma-oscillation types with a zero reduced wave vector. We show that the frequency splitting and the radiative damping of the oscillations are nonmonotonic functions of the modulation percentage and the ratio of the widths of the bands of two-dimensional plasma with low and high electron concentrations. The results of calculations are compared with experimental data and with the results of a perturbation theory developed in earlier work of other researchers. We discuss the physical mechanism for the emergence of radiative damping of plasma oscillations. Zh. éksp. Teor. Fiz. 113, 988–999 (March 1998)     

Parametric resonance in neutrino oscillations in periodically varying electromagnetic fields

It is shown that a parametric resonance may arise in neutrino oscillations in varying electromagnetic fields. For two types of electromagnetic fields—an amplitude-modulated electromagnetic wave and a transverse magnetic field that is constant in time, but which has an amplitude periodically varying in space—the probabilities of the ν _i ? ν _j neutrino transitions are found, and it is shown that the probability amplitudes increase with time for a specific choice of the parameters of external electromagnetic fields.