Electrooptical modulation in the Stark-ladder regime

A direct measurement of the in-plane birefringence below the absorption edge of a GaAs/AlAs superlattice (SL) under electric fields shows a unique type of electrooptical modulation. The SL is sandwiched between two doped AlGaAs alloy layers, which play the simultaneous role of positive (p ⁺) and negative (n ^?) contacts, as well as clad layers, to achieve optical waveguiding. The p-(SL)-n structure is chosen so that, as a function of the externally applied bias, it displays Stark-ladder localization and the quantum confined Stark effect at low and high fields, respectively. We show that this results in an electrooptical modulation, in which the built-in birefringence of the SL initially decreases and shows a crossover to a quadratic increase for larger fields.     

Experimental solution of the local-field problem in discotic liquid crystals

Polarized light-absorption spectra are obtained for single-domain planar-oriented samples of the discotic D _ho . Previously predicted spectral effects induced by resonant dipole-dipole interactions of the molecules are observed. New methods for determining the parameters of the local field in the D ho phase are developed which take into account the mixing of the molecular excitations. It is shown that the two-dimensional crystalline ordering of the molecular columns decreases the anisotropy of the local field for this phase. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 1, 30–35 (10 July 1999)     

Analysis of the photoelastic effect in ionic crystals

An analysis of the photoelastic effect in ionic crystals has been presented within the framework of Clausius-Mossotti theory of the dielectric constant. The values of the strain derivative of the electronic dielectric constant have been calculated in alkali halides and MgO crystals by taking into account the variation of electronic polarizabilities with compressive stress. The results obtained are found closer to the experimental values. The photoelastic behaviour of MgO crystal is predicted to be of opposite nature to that of alkali halides, in conformity with the experimental observations.     

Enhancement of the magnetorefractive effect in magnetophotonic crystals

The magnetorefractive effect in a one-dimensional magnetophotonic crystal, more specifically, a photonic crystal (SiO₂/Ta₂O₅) containing a built-in defect in the form of a thin layer of the Co-(Al-O) magnetic nanocomposite, is studied in a computer simulation experiment. The structure of the unit cell of the photonic crystal in which most of the field energy is concentrated in cells nearest to the defect is determined. This makes it possible to increase the defect-mode Q factor and, owing to the multiple passage of light along the defect, to enhance the magnetorefractive effect by more than one order of magnitude as compared to a thin film on a substrate and by two orders of magnitude as compared to thick films. The reflectance of these structures in an applied magnetic field can be as high as 60%.     

Polarization effect in langasite crystals

The variation of the resonance frequency of piezoelectric resonators subjected to a constant electric field is demonstrated with longitudinal vibrations in La₃Ga₅SiO₁₄ (langasite) crystal rods at temperatures between −60 and +80°C. For the rods whose longtudinal axis makes angle α with the crystal axis in the plane of the axes and [0001], the change in the frequency is proportional to the field strength and depends on α. For a field of ±1.5 × 10⁶ V/m and α = 15°, the relative change in the frequency is ±120 × 10⁻⁶. It is shown that La₃Ga₅SiO₁₄ crystals offer a set of physical parameters that makes them promising for electrically controlled acoustoelectronic devices and for devices based on nonlinear interaction between acoustic waves.     

Piezoelectric effect in liquid crystals

A piezoelectric effect has been observed in a cholesteric liquid crystal layer (an electric field effect type, a mixture of cholesteryl oleyl carbonate, cholesteryl chloride and cholesteryl nonanoate) [1]. The layer, subjected to shear vibration, generates an alternating electric potential of the same frequency as the exciting vibration. In the present paper an extension of these investigations is reported to cases with various mixing ratios and also to one of smectic liquid crystals (lecithin with water). For a cholesteric liquid crystal of a particular mixing ratio, the strength of the electric generation is found to reach as high as 450 mV(p-p) for a vibratory shear displacement of 1 μm(p-p) under a certain molecular orientation preparation. Small electric generation is also observed for the above smectic liquid crystal but not for the nematic liquid crystals (MBBA and EBBA). A simple continuum elasticity model can qualitatively explain the behaviour of this electric potential generation.     

The Raman effect in crystals

A review is given of progress in the theoretical and experimental study of the Raman effect in crystals during the past ten years. Attention is given to the theory of those properties of long-wavelength lattice vibrations in both cubic and uniaxial crystals which can be studied by Raman scattering. In particular the phenomena observed in the Raman scattering from crystals which lack a centre of inversion are related to the theory. The angular variations of the scattering by any type of lattice vibration in a crystal having any symmetry can be easily calculated using a complete tabulation of the Raman tensor. Recent measurements of first-order lattice vibration spectra are listed. A discussion of Brillouin scattering is included. The relation of second-order Raman spectra to critical points in the lattice vibration density of states is discussed, and measurements of the second-order spectra of diamond and the alkali halides are reviewed. The theory and experimental results for Raman scattering by electronic levels of ions in crystals are examined, and proposals for Raman scattering by spin waves, electronic excitations across the superconductive gap and by plasmons are collected together. Finally, the prospects for applying lasers as sources for Raman spectroscopy are discussed, and progress in the new technique of stimulated Raman scattering is reviewed.     

The Raman effect in crystals

A review is given of progress in the theoretical and experimental study of the Raman effect in crystals during the past ten years. Attention is given to the theory of those properties of long-wavelength lattice vibrations in both cubic and uniaxial crystals which can be studied by Raman scattering. In particular the phenomena observed in the Raman scattering from crystals which lack a centre of inversion are related to the theory. The angular variations of the scattering by any type of lattice vibration in a crystal having any symmetry can be easily calculated using a complete tabulation of the Raman tensor. Recent measurements of first-order lattice vibration spectra are listed. A discussion of Brillouin scattering is included. The relation of second-order Raman spectra to critical points in the lattice vibration density of states is discussed, and measurements of the second-order spectra of diamond and the alkali halides are reviewed.

The theory and experimental results for Raman scattering by electronic levels of ions in crystals are examined, and proposals for Raman scattering by spin waves, electronic excitations across the superconductive gap and by plasmons are collected together.

Finally, the prospects for applying lasers as sources for Raman spectroscopy are discussed, and progress in the new technique of stimulated Raman scattering is reviewed.     

Magnetoelectric effect in antiferromagnetic crystals

Magnetoelectric experiments provide information on spin configurations, critical indices and microscopic ionic interactions. The basic theory of the magnetoelectric effect in antiferromagnets is rederived in this article and extended to situations in which there is an external magnetic field including the spin-flop phase. It is found that measurements of the magnetoelectric tensor α as a function of field in the antiferromagnetic and spin-flop phases of an antiferromagnet can provide more complete information in all three categories above. Certain higher-order terms are examined and it is found that it is possible to measure the staggered susceptibility directly in magnetoelectric materials. It is shown that a non-zero value of α causes a tetragonal antiferromagnet to be optically biaxial. A pedagogical discussion of the origins of magnetoelectric effects is given in the Appendix.     

Iterative solution of the inverse problem of dynamic diffraction in inhomogeneous crystals

An iterative method for solving the inverse problem of dynamic x-ray diffraction in crystalline layers that are inhomogeneous across their thickness is developed. The structural characteristics of an InGaAsSb/AlGaAsSb/GaSb heteroepitaxial system are calculated using the method. Zh. Tekh. Fiz. 69, 39–42 (March 1999)     

The effect of ion-ion correlations on the ionic hall effect in crystals

Abstract

Because an external magnetic field does no work on moving charges, it is argued that independently migrating point defects in an ionic crystal cannot exhibit any Hall effect, to first order. This is in sharp contrast to the cooperative motions displayed by superionic conductors. Experimental results on interstitial migration in AgBr and α—AgI agree with these expectations.     

Hypergeometric solutions of Knizhnik-Zamolodchikov equations

Explicit integral formulas are presented for the solutions of Knizhnik-Zamolodchikov equations associated with an arbitrary Kac-Moody Lie algebra.     

Zeeman effect in the absorption spectrum of EuMg-nitrate crystals

Large single crystals of EuMg-nitrate were grown of optical quality. The polarized absorption spectra were investigated with high optical resolution at helium temperatures and in external magnetic fields up to 6.6T parallel as well as perpendicular to the trigonal axis. The energy, symmetry quantum number,g-factor, and Zeeman shift of the various crystal field levels were determined, as well as first order matrix elements of the crystal field energy. The second order Zeeman effect and the temperature independent van Vleck paramagnetic susceptibility of the ground level⁷ F ₀ were calculated from these constants, in excellent agreement with the spectroscopic and magnetic measurements.Project of the Sonderforschungsbereich Festkörperspektroskopie Darmstadt-Frankfurt/Main     

Magnetic isotopic effect in the plasticity of diamagnetic crystals

A magnetic isotopic effect in the plasticity of diamagnetic crystals is predicted. This effect consists in that the replacement of nonmagnetic isotopic nuclei by magnetic nuclei in ionic crystals (e.g., the introduction of ²⁵Mg²⁺ or ⁴³Ca²⁺ in place of ²⁴Mg²⁺ or ⁴⁰Ca²⁺ in NaCl) increases the plasticity of the crystals even in the absence of an external magnetic field. On the contrary, such a replacement in covalent crystals (e.g., the introduction of ²⁹Si into silicon or ¹³C into diamond) is expected to reduce the plasticity and to produce dislocation strengthening.     

Experimental evidence of the Halperin-Lubensky-Ma effect in liquid crystals

The orientational order parameter decrease, due to nematic director macroscopic fluctuations, is obtained by birefringence measurements around the nematic <--> smectic-A transition temperature of the liquid crystal 4'-n-octyl-4-cyanobiphenyl. The measured nematic order reduction shows the same amplitude as the secondary order parameter discontinuity at the transition providing direct evidence of the Halperin-Lubensky-Ma effect. The importance of nematic director thermal fluctuations on the character of the transition is revealed as their quenching by an electric field of approximately 20 V/micrometer restores the second order character of the transition.     

Optical quenching of the magnetoplastic effect in NaCl crystals

Observations indicate that illuminating NaCl crystals by ultraviolet light (λ=350 nm) suppresses the magnetoplastic effect. The processes induced by illumination take place in a subsystem of point defects and are related to a change in the state of magnetically sensitive dislocation pinning sites. Fiz. Tverd. Tela (St. Petersburg) 39, 1389–1391 (1996)