Uniaxial Ferroelectrics Research Articles

Slow relaxations of the even-field current in some uniaxial ferroelectrics

Processes of the formation of even-field currents in Pb5Ge3O11 and LiNbO3 single crystals have been studied. It has been shown that the even-field currents in these crystals are present under conditions of a weak injection of carriers or when it is absent altogether. Possible causes leading to the even-field currents are discussed.

A Phenomenological Theory of Nonlinear Dielectric Response

The nonlinear after-effect functions and susceptibilities for uniaxial ferroelectrics and ferroelectric liquid crystals are derived on the basis of equations of motion. The ferroelectrics and the ferroelectric liquid crystals are quite different in the functional forms of nonlinear after-effect functions and the nonlinear susceptibilities, though they are the same in those of the linear ones. On the basis of the results the validity of so-called Nakada's formula, which has been often used in analyzing experimental data, is also discussed.

Periodic diamagnetic domain structures in metals under a quantizing magnetic field.

The spatial distributions of magnetization (i.e., domain structures due to magnetic interaction) in metals under conditions of strong de Haas--van Alphen (dHvA ) oscillations at any magnetic field within the period of magnetic oscillations (PMO) are investigated. At fields away from the center of the PMO the distribution of magnetization is markedly different from the well-known strictly symmetrical domains, which exist only at the center of the PMO. For temperatures near the diamagnetic phase transition distributions of magnetization without sharp domain walls are found. Similar results are predicted for domain structures in uniaxial ferroelectrics or ferromagnetics. The discovered structures are strongly asymmetrical with narrow spikelike domains separating wide domains of opposite magnetization. Spikelike domains of more complex structures, which may be important in kinetic processes, are obtained at sufficiently low temperatures. In contrast to the existing theory, which yields a temperature-independent magnetic susceptibility below the phase transition, we find a minimum in the spatially averaged differential susceptibility just below the phase transition, which reflects strong interactions between diamagnetic domain walls. The form and magnitude of the calculated dHvA oscillations is in good agreement with the experimentally observed magnetic oscillations in gold, which exhibit significant deviations from the Lifshitz-Kosevich theory.

Effect of the uniaxial compression on the phase transitions in Sn2P2S6‐type ferroelectrics

AbstractThe effect of uniaxial compression on the temperature dependences of birefringence and dielectric constant is studied for the proper uniaxial ferroelectrics Sn2P2S6 and Sn2P2Se6. The influences of compression on the ferroelectric phase transition (PT) in Sn2P2S6 and on the PT which restricts the incommensurate (IC) phase in Sn2P2Se6 are compared. The observed variations of PT temperature and IC‐phase range with pressure agree with those calculated in an approximation which takes into account the presence of a tricritical point on the phase diagram of the crystals under study along with the Lifshits point. The variation of the ratio of the rates of PT temperature change for different uniaxial mechanical stresses on the replacement of sulphur by selenium as well as the weak sensitivity of the IC‐phase to compression along the structure modulation axis are found. It is suggested that these manifestations of the anisotropy are related to the special features of the linear interaction of soft optical and acoustic phonon branches.

Weak ferroelectrics: Specific acoustic anomalies

A review of acoustic anomalies at the transition to a weak ferroelectric phase is given. Different approaches have been analyzed in order to reveal the specific modification due to a crossover in the order parameter dynamics from the dipole regime, characteristic of ordinary uniaxial ferroelectrics, to Ising-type behavior. Some comments are made on extremely large electrostriction coefficients in weak ferroelectrics. Additional modifications of the acoustic anomalies due to fluctuation contributions and frequency dispersion are considered.

Specific heat and quadrupole interactions in uniaxial ferroelectrics

Abstract The inclusion of quadrupolar and higher order terms into the effective field in second order transition in uniaxial ferroelectrics is shown to describe well the jump in specific heat at T⋍Tc for crystals of the TGS family. It is shown that the numerical values for the coefficient g≡(γ/β)(Nμ)2, corresponding to the quadrupolar interaction term, obtained (a) from specific heat data and (b) from spontaneous polarization data (both at T⋍Tc are in good agreement with each other for crystals of the TGS family.

Crystal growth and domain structure evolution

Abstract The evolution of ferroelectric domain structure is considered as an example of kinetics of phase transformation process. Some analogies are revealed between crystal growth and domain kinetics in model uniaxial ferroelectrics. Within this approach the mechanisms of arising regular shaped, chaotic and fractal-type domain structures are explained. It is shown that observed peculiarities of arising domain patterns are due to the complicated origin of local electric field which is the driving force of the switching process.

Directed polymer melts and quantum critical phenomena

The statistical mechanics of directed linelike objects, such as directed polymers in an external field, strands of dipoles in both ferro- and electrorheological fluids, and flux lines in high-Tc superconductors, bears a close resemblance to the quantum mechanics of bosons in 2+1 dimensions. We show that single-component and binary mixture critical phenomena in these systems are in the universality class of three-dimensional uniaxial dipolar ferromagnets and ferroelectrics. Our results also apply to films of two superfluid species undergoing phase separation well below their λ-points near T=0. In the case of directed polymers and electrorheological fluids we analyze the effects of free ends occurring in the sample as well as a novel directionally-dependent compressibility.

Field induced motion of domain walls in uniaxial ferromagnets and ferroelectrics

The phenomenological description of the field-induced motion of linear domain walls in uniaxial ferromagnets and ferroelectrics is presented on the basis of the time-dependent Ginzburg-Landau equation. Expressions for the velocity width and shape of domain walls are obtained both for low and for arbitrary fields. The comparison with experimental data is carried out.

Lifshitz point on the state diagram of ferroelectrics

An analysis is made of the shape of the phase diagrams of substances in the neighborhood of the Lifshitz point, which separates the second-order phase transitions going from the initial phase to a commensurate and to an incommensurate phase. The conclusions of the thermodynamic theory in the mean field approximation and in a treatment including the effects of the interaction of well-developed fluctuations are compared. The possible complication of the phase diagram by a crossing of the line of Lifshitz points with a line of tricritical points and the possible existence of a tricritical Lifshitz point are discussed. The changes in the character of the critical behavior in the neighborhood of the Lifshitz point are discussed in reference to the results of experimental studies of the transformation of anomalies of the thermodynamic characteristics, principally for the proper uniaxial ferroelectrics Sn2P2(SexS1?x)6.

Nonlinear dielectric properties of CsH2PO4

Abstract The temperature dependence of the fourth order dielectric coefficient e2222 of CsH2PO4 was determined above the ferroelectric transition temperature Tc. The critical behaviour of this coefficient differs from those of proper uniaxial ferroelectrics and can be described using a pseudo- one-dimensional Ising model. Below Tc the sample capacitance is in the range of small driving voltages strong voltage dependent.

Time-domain soft mode spectroscopy

Abstract The application of impulsive stimulated light scattering, an entirely time-domain form of light scattering spectroscopy, to the study of lattice dynamics in crystals near structural phase transitions is reviewed. Advantages of the time-domain approach are demonstrated for study of low-frequency, heavily damped, and/or dispersive modes such as soft acoustic phonons. Impulsive stimulated Brillouin and Raman scattering data showing time-dependent oscillations and decay of acoustic and optic phonons, respectively, are presented and explained. Results of experiments on acoustic phonons in uniaxial and incommensurate ferroelectrics are summarized. Time-domain spectrosocopy of soft optic phonons and other overdamped modes is discussed.

Influence of defects on the properties of uniaxial ferroelectrics near the lifshitz point

Abstract We calculated and compared the contributions of point defects and of the order parameter fluctuations to the anomalies of physical properties of ferroelectrics with one axis of spontaneous polarization near the Lifshitz point. The example of such a system is Sn2P2(SexS1-x)6. It was demonstrated that at realistic parameters the temperature behaviour of thermodynamic quantities is governed by the defects.

Brillouin scattering at the phase transitions in some ferroelectrics

This paper reviews the experimental studies of acoustic anomalies at the phase transition in some ferroelectrics. Primary emphasis is upon possibilities of Brillouin light scattering in revealing of some new and unusual features occurring at “usual” phase transitions. We shall consider in detail a modification of typical anomalies in the behavior of sound velocity and attenuation due to existence of specific phenomena in the crystals under investigation. The specialities are determined by belonging of the TGS crystals to a group of uniaxial ferroelectrics. In the case of TSCC and Li2Ge7O15 the situation is more complicated because we have to attribute these two formally uniaxial ferroelectrics to a new class of weak ferroelectrics. At last, in Gd2(MoO4)3 there is an unusual broad central peak correlating with a pronounced acoustic anomaly. Taking into account individual features of these crystals one can explain their very unusual behavior in the framework of the Landau phase transition theory.

Dynamics of domain structure in uniaxial ferroelectrics

The switching of polarization in ferroelectrics is considered as the first-order phase transition. By the analogy with the theory of crystal growth it is assumed that the switching process is determined by the oversaturation degree which correspond in ferroelectrics to the magnitude of electric field on the domain boundary. It is supposed that the mechanism of domain wall motion in strong field is due to the twodimensional nucleation and in weak field is due to the one-dimensional nucleation. The criterion of strong field IS defined. From this point of view it have been explained the peculiarities of domain structure dynamics in two uniaxial ferroelectrics: lead germanate and gadolinium molybdate.

Switching pulse shape in single crystals of the TGS family

Abstract It is shown that the rate equations for polarization reversal by domain wall motion provide a satisfactory picture for the switching pulse shape in good overall agreement with the observed behaviour at subsonic switching for uniaxial ferroelectrics of the TGS family.

Ferroelectric switching at low, intermediate and high fields

Abstract The observed switching current associated with polarization reversal in uniaxial ferroelectrics shows three distinct regimes which may be labelled the low, intermediate and high field regimes. Most previous theoretical work attempting a description of the switching in the low and intermediate field regimes has been done using semiempirical approaches. In this work the rate equations for polarization reversal are solved for the three cases of low, intermediate and high fields, and solutions are shown to provide a unified physical picture in good overall agreement with the observed switching behavior.

Stabilization of Polarization Vector in Uniaxial Ferroelectrics by Means of Chiral Dopants. Macroscopic Symmetry Conditions

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Temperature anomalies in the dispersion and absorption of sound near phase transition points at low temperatures

We examined the attenuation and dispersion of sound during phase transitions with an overdamped soft mode at low temperatures. The obtained temperature and frequency dependences differ from results known at the high-temperature asymptote. The temperature anomalies are sharper. At low frequencies, the attenuation behaves like (T − Tc)−5/2 for a critical phonon spectrum isotropic in k, and like (T−Tc)−2 for uniaxial ferroelectrics. The presence of a temperature anomaly is characteristic also for high-frequency attenuation, behaving like (T−Tc)−1/2 and ln(T−Tc)., respectively. We discuss briefly the results obtained from an experimental test.

The investigation of critical behaviour of uniaxial cluster ferroelectrics

Abstract The description of uniaxial cluster ferroelectrics in a wide temperature range, including a phase transition point, by means of the collective variables method is suggested. The peculiarities of critical behaviour of ferroelectrics cluster system with dipole-dipole interaction are investigated.