Excitonic line shapes of disordered solids

The optical absorption for a disordered one-dimensional excitonic solid has been studied by numerical methods. We have considered both Gaussian and binary energy fluctuations. Our results indicate that the physical properties associated with the spectra are selfaveraging. Results obtained on a single realization of random variables are consistent with recent ensemble averaged results obtained numerically on smaller samples. We also show that for binary disordered solids in the band splitting limit, spectra are dominated by cluster effects and can not be described through analytical treatments.     

Critical broadening of EPR line in molecular crystals of free radicals

Using the EPR method, the members of free radicals of the hydrazyl group have been studied within a wide temperature range. In all the samples at temperatures 0.05?3K the existence of a phase transition to an antiferromagnetic state is found out. Dynamics of critical fluctuations near the phase transition have been investigated. Two regimes of critical spin relaxation are discovered.     

Mechanoluminescence of ferroelectric crystals

Mechanoluminescence spectra of triglycine sulfate, guanidine aluminium sulfate, rochelle salt, diglycine manganous chloride dihydrate, lithium ammonium tartate monohydrate and potassium dihydrogen phosphate crystals resemble the discharge spectra of nitrogen gases. Glycine silver nitrate, thiourea, ammonium sulfate and azobenzene crystals, the structure of which is centrosymmetric at room temperature, do not show mechanoluminescence. Following the role of crystal structure, the mechanoluminescence excitation is discussed on the basis of the piezo-electrification of the new surfaces created during fracture of the crystals.     

NMR dynamics in disordered magnets

The excitation dynamics of site diluted magnets can be described at low energies (long length scales) by magnons, and above a crossover frequency, ω_c, (short length scales) by fractons. The density of fracton states is given by , where is the fracton dimensionality. Dilution gives rise to a characteristic length ξ∝(p−p _c)^ν, wherep _c is the critical concentration for (magnetic) percolation. The crossover frequency ω_c is proportional to ξ^-1[1+(θ/2)], where θ is the rate at which the diffusion constant decays with distance for diffusion on an equivalent network. A fractal dimensionD describes the density of magnetic sites on the infinite network, and . For percolating networks, for all dimensions ≥2. Neutron scattering structure factor measurements by Uemura and Birgeneau compare well with calculations using fracton concepts. Magnons are extended at low energies, while the fracton states are geometrically localized, with a wave function envelope proportional to exp . Here, is the fracton length scale at frequency ω. The exponentd _ϕ lies between 1 andd _min, the chemical length index (of the order of 1.6 in three dimensions). The localization of the magnetic excitations causes a spread in the NMR relaxation rates. A given nuclear moment will experience only a limited set of fracton excitations, resulting in an overall non-exponential decay of the NMR relaxation signal. When strong cross-relaxation is present, the relaxation will be exponential, but the temperature dependence will be strongly altered from the concentrated result.     

Domain switching kinetics in disordered ferroelectric thin films

We investigated domain kinetics by measuring the polarization switching behaviors of (111)-preferred polycrystalline Pb(Zr,Ti)O3 films, which are widely used in ferroelectric memories. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of logarithmic domain-switching times. We suggested that the local field variation due to dipole defects at domain pinning sites could explain the Lorentzian distribution.     

EPR of the ferroelectric phase transition in Li2Ge7O15:Cr3+ crystals

A Cr³⁺ EPR study of lithium heptagermanate crystals, Li₂Ge₇O₁₅ (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the vicinity of the transition point. Doublet splitting of the EPR lines was observed to occur below T _C as a result of emergence of two structurally nonequivalent positions for Cr³⁺ ions. Fiz. Tverd. Tela (St. Petersburg) 40, 1102–1105 (June 1998)     

Phase transitions in ferroelectric liquid crystals

Molecular chirality plays an important role in the science of liquid crystals, leading to cholesteric liquid crystal, blue phases, ferroelectric and antiferroelectric smectic phases and twist grain boundary phases. In all of these mesogens, chirality is an intrinsic property built into the chemical structure of mesogenic molecules. The study of ferroelectric liquid crystals has seen substantial experimental strides. In theoretical aspects, there has been relatively little basic work on this fascinating class of material. This review will try to present a comprehensive overview of the current status of the phase transitions in ferroelectric smectic liquid crystals. The article begins with a brief introduction about the symmetry and structure of ferroelectric mesophases. An attempt is made to identify a range of problems and related questions associated with the study of phase transitions. In the remaining parts of the article the important experimental and theoretical developments are summarized. Finally, some of the future directions have been identified.     

Optical autowaves in photorefractive ferroelectric crystals

The effect of appearance of traveling structures, referred to as scattering autowaves, under illumination of photorefractive media by coherent light under certain conditions (power, wavelength, and focusing of pump radiation) is theoretically described. A model is proposed that takes into account both longitudinal and transverse photovoltaic currents in crystals with a spatially isotropic distribution of seed permittivity fluctuations.     

Photorefractive effect in ferroelectric liquid crystals

In this paper, we review recent progress of research on the photorefractive effect of ferroelectric liquid crystals. The photorefractive effect is a phenomenon that forms a dynamic hologram in a material. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon is applicable to a wide range of devices related to diffraction optics including 3D displays, optical amplification, optical tomography, novelty filters, and phase conjugate wave generators. Ferroelectric liquid crystals are considered as a candidate for practical photorefractive materials. A refractive index grating formation time of 8–10 ms and a large gain coefficient are easily obtained in photorefractive ferroelectric liquid crystals.     

Echo response in ferroelectric liquid crystals

It is proposed that the kinetics and dynamics of ferroelectric liquid crystals may be studied by using the echo responses to the pulsed action of a microwave electric field. For ferroelectric liquid crystals, with homogeneously oriented molecules, calculations are made of the echo signals for two pulses and it is shown that these have values which are quite easily measured. Fiz. Tverd. Tela (St. Petersburg) 40, 1370–1372 (July 1998)     

Aging of the field-induced asymmetry in a disordered ferroelectric

The isothermal aging of the asymmetry induced in the disordered dielectric crystal (x=0.027) submitted to the biasing electric field E, is investigated. To this end, the response of the complex dielectric constant to infinitesimal field changes , applied to the sample after a variable aging delay, has been measured for different magnitudes of E and after different aging delays. Two different experimental procedures have been used: in both cases the response strongly depends on the time spent under field. For short aging delays, the response has a strong contribution proportional to δE and a weak quadratic contribution proportional to . As time elapses, the linear and the quadratic contributions age in opposite ways: the former decreases whereas the latter increases. This paradoxical behaviour is analyzed in the framework of a model which attributes aging and the related effects (rejuvenation, memory) to the evolution of polarization domain walls: the decrease of the linear contribution is related to the decrease of the total wall area, while the increase of the quadratic term is attributed to wall reconformations.     

Long-range interlayer interactions in ferroelectric liquid crystals

Some smectic liquid crystals exhibit a series of phases, including ferroelectric, antiferroelectric, and ferrielectric commensurate structures as well as an incommensurate phase. A long-standing problem has been to understand the origin of the long-range interaction responsible for this rich variety of phases. We study a model that incorporates thermal fluctuations in the flexing of layers and find that it supports commensurate and incommensurate structures. The vibrational entropy competes with an assumed helical interaction between nearest-neighbor layers. An increase in temperature then leads to an unwinding of the helix that proceeds at first through commensurate phases and then into an incommensurate phase. This result is consistent with the experimentally observed "distorted clock model."     

Magnetoplastic effect in NaNO2 ferroelectric crystals

This paper reports on a laser interferometric study of the effect of a dc magnetic field (MF) on the rate of plastic deformation (creep) $\dot \varepsilon$ of NaNO₂ ferroelectric crystals under compression. It is established that the application of a dc MF to a loaded specimen results in an increase in the creep rate and that removal of the MF brings about a decrease in $\dot \varepsilon$ . Subjecting an unloaded specimen to a dc MF beforehand also affects its strain rate under the subsequent loading. The observed magnetoplastic effect is most clearly pronounced within a certain $\dot \varepsilon$ interval, and the magnitude of this effect for the NaNO₂ ferroelectric is several times larger than that for LiF crystals.     

Microsphere manipulation using ferroelectric liquid crystals

We propose a strategy for a micromanipulation method using SSFLC (surface stabilized ferroelectric liquid crystals). By adjusting the frequency of the applied ac electric field, the surface layers that cannot follow an applied ac electric field are constructed in SSFLC. In addition, by applying a sawtooth wave voltage, net flow along the smectic layer is generated. The flow direction is reversed by changing the polarity of the sawtooth wave. Consequently, the particles dispersed in SSFLC can be driven bidirectionally along the smectic layer. The particle velocity depends on the temperature, amplitude, and frequency of the applied voltage.     

Surface effects and defect structures in ferroelectric liquid crystals

Summary The influence of boundaries on the smectic structure dielectric properties and polarization reversal processes in thin films of ferroelectric liquid crystals are discussed. Work presented at the First USSR-Italy Bilateral Meeting on Liquid Crystals held in Portonovo, Ancona (Italy), September 30-October 2, 1987.     

Secondary ferroic properties of ferroelectric crystals

Twenty five types of structural phase transitions that are accompanied by the appearance of a spontaneous optical activity in the low-temperature phase are analyzed. A technique is proposed for describing the enantiomorphic domain structure (orientation states) in these crystals.     

Dielectric spectra of disordered ferroelectric systems: Polycrystals and composites

The dielectric spectra of heterogeneous systems that have very large dielectric constants and exhibit the Maxwell-Wagner relaxation are analyzed. The spectra of the disordered systems are found to be of a non-Debye type, and attempts to fit them using either the Cole-Cole or Davidson-Cole equations sometimes fail to produce satisfactory results. Experimental spectra of disordered systems are proved to be close to the spectra obtained by computer simulation of statistical mixtures and ferroelectric ceramics. A new formula is suggested to describe the dielectric relaxation in polycrystalline and composite materials, and reasons for the non-Debye nature of the dielectric spectra are examined.     

Correlation of acoustic and thermal anomalies in ferroelectric crystals

The Pippard-Janovec relations are derived for correlating the anomalous elastic coefficient and the anomalous specific heat near the phase transitions of ferroelectric crystals. These relations are verified in the case of ferroelectric triglycine selenate crystal.