S. A. Pikin

Ferroelectric liquid crystals

Abstract The idea either to reveal or to construct a liquid ferroelectric was never abandoned by researchers since the discovery of ferroelectricity in solid crystals at the beginning of this century. In principle, nature does not forbid the existence of ferroelectric liquids. For instance, a centro-symmetric liquid can change its point symmetry from the K h , to C∝v , group with the formation of a uniform ferroelectric phase. A liquid without the inversion center can have a phase transition K → C∝, into the inhomogeneous, helical structure. In both these hypothetical cases the spontaneous polarization, P s , would play the role of the true order parameter. However, such liquid ferroelectric phases (proper ferroelectrics) have not been discovered as yet.

Molecular Models for the Ferroelectric Smectic C* Phase

Abstract The molecular-statistical theory of the ferroelectric smectic C* phase is summarized and generalued to the cases of the ferroelectric liquid crystals with large dipole in the chiral part of the molecule and to the mixtures of liquid crystals. The molecular model is proposed which is used to explain both ferroelectric and flexoelectric properties of the C* phase using a small number of molecular model parameters. In this model we consider the chiral intermolecular interaction, modulated by the banana-like shape of the molecules. The analysis of the model yields a simple chiral and polar interaction potential which is used in the statistical theory. The phenomenological constants of the smectic C* liquid crystals are expressed via the molecular model parameters. In this report we consider also the microscopic interpretations of flexoelectricity in the smectic C phase. It is shown, that steric interaction of banana-like molecules results in the strong temperature dependence of flexoelectric coeffici...

Experimental verification of the theory of electrohydrodynamic instability in nematic liquid crystals

Abstract Williams domains threshold voltage are given as functions of dielectric and conductivity anisotropy. Experimental data confirm the theory of electrohydrodynamic (EHD) instability in nematic liquid crystals (NLC) based on Helfrichs model and taking into account the boundary conditions.

Crystalline and spin chiralities in multiferroics with langasite-type structure and Fe1–xCoxSi crystals

It is shown that, when magnetic ordering occurs in layered iron-containing langasites (sp. gr. P321), one of the reasons for spin chiralities of different signs is the presence of structural chirality (the existence of inversion twins), which, in turn, is due to the nonsymmetricity of these crystals. Spin helicoids arise in these multiferroics at split sites of Fe3+ ions below the Néel point. The direction of electric polarization vectors coincides with the direction of the magnetic helicoid axes because of the piezoelectric properties of these materials. Due to the magnetostriction effects, structural chirality wave vector kz exceeds the magnetic helicoid wave vector by a factor of 2: kz = 2qz. The temperatures of transitions to the chiral structural and chiral magnetic states may differ. In particular, if the structural transition initial temperature exceeds the magnetic transition temperature (ТU> ТМ), structural displacements may arise in the absence of magnetism at ТМ < Т < ТU. In noncentrosymmetric Fe1–xCoxSi crystals (sp. gr. P213), which are not multiferroics, magnetic chirality is due to the Dzyaloshinski–Moriya interaction. The dependence of the moduli of incommensurate wave number of the corresponding helicoid on the atomic composition of the crystals under consideration is nonmonotonic.

Bistable and monostable polarized states of a liquid-crystalline ferroelectric in an electric field

Abstract A condition for obtaining bistable states in a liquid-crystalline ferroelectric (S*C) has been found by computer simulation and analytical estimates; it depends on the value of the applied electric field, the magnitude of the polar contribution to anchoring energy and the material parameters of S*C

New Types of Instabilities in Liquid Crystals with Tilted Orientation

Abstract We have considered theoretically a new kind of instability formed of rolls perpendicular to Williams domains when the molecules are initially tilted with respect to the substrate plane. The threshold voltage versus the tilt angle, dielectric constants, electroconductivity, viscosity coefficients and the frequency of an external field is calculated within the framework of the two-dimensional model. The contribution of electrokinetic processes observed in an isotropic liquid to the effect under consideration is discussed and the respective estimates for the threshold are given.

Molecular Aspects of the Main Phase Transition in Lipid Systems as a Weak First-Order Phase Transition: 1. Model of Thermodynamic Behavior of Lipid Membranes

The possibility to describe the principal phase transition in lipid systems as a weak orientational first-order phase transition with characteristics close to those of a second-order phase transition characterized by pretransition phenomena and rather strong fluctuations has been considered. A first order transition is explained by interactions between fluctuations of molecule orientations and density fluctuations at a low shear modulus. The jumpwise behavior of enthalpy, volume, heat capacity, compressibility, sound velocity and absorption, and the order parameter are analyzed. It is shown that because of molecular van der Waals interactions the shear modulus should give rise to more pronounced jumps of the above quantities with an increase in molecule lengths.

Ferroelectricity and antiferroelectricity in CuCrO2-type multiferroic semiconductors

Multiferroic semiconductors of the CuCrO2 type are considered. These materials, in the presence of free charge carriers, allow for the existence of opposite domains (in the ferroelectric FEd phase), along with ordinary 180° domains (in the FE phase). The magnetization phase transition in a chiral multiferroic, allowing for piezoelectric effects in an antiferromagnet with a layered triangular structure which result in an incommensurate (helicoidal) spin structure, is phenomenologically described. The behavior of the ferroelectric polarization in the considered phases is characterized. The antiferroelectric is considered a system of parallel layers alternating in chirality sign with oppositely directed polarization vectors. The possibility of antiferroelectric phase transition due to the specific features of the dipole-dipole interaction is discussed.

Domain structures in ferroelectric liquid crystals

Abstract Domain structures called steric (or striped) domains are investigated in ferroelectric liquid crystals (FLCs). The domain lines are aligned perpendicular to the smectic layers with a period proportional to the FLC cell thickness L. Neither FLC polarization nor helix pitch affects the domain pattern. Striped domains disappear in thick cells (L > 8–10 μm) and transform to a new type of surface domain without any preferred orientation in thin samples (L < 1·0 μm). The existence and structure of the steric domains define properties such as contrast and bistability of the FLC cell. We propose a qualitative explanation of this domain pattern and compare its characteristics with ferroelectric domains discovered earlier by Beresnev et al. in FLCs.

On the problem of the “Colossal” magnetoelectric effect

In connection with the recent experiments on manganites, it is shown in the framework of the Landau theory that the magnetoelectric effect appears under a critical magnetic field. The signs of the spinorbit interaction constant and the helix wave number are interrelated. A helical structure and weak ferromagnetism occur simultaneously due to the first-order phase transition at a critical temperature, which depends on the magnetic field. The temperature range of the effect increases with the field. In this range, the magnetic moment increases and the correction to the electric polarization, being negative, also increases in magnitude with the magnetic field above its threshold. These changes are directly related to the increase in the helix wave number with a decrease in temperature below the phase transition point.