Elastic properties of liquid crystals

The structural and elastic properties of 4-n′-pentyl-4′-cyanobiphenyl (5CB) in the nematic liquid-crystal phase are investigated in the framework of the statistical-mechanical theory and the molecular dynamics method.     

Some properties of liquid crystals

The present paper contains the results of measuring the velocity and absorption of ultrasonic waves in materials of the homologous series of dialkoxyazoxybenzene. The existence of pretransition states in liquid-mesophase and mesophase-mesophase temperature transitions was detected for the first time. Regular fluctuations of the velocity and absorption were noted. The ferroelectric properties of the nematic and smectic phases of para-normal octyloxybenzoic acid (domains, hysteresis effects) were discovered. Experimental data on the scattering of light by cholesteric-type liquid crystals and their mixtures were obtained. It is shown that the sequence and rate of change of specimen color depended strictly on temperature.     

Structure investigation of liquid crystals composed of terminally polar molecules

Summary Recent experimental studies of X-ray scattering in mesogenic compounds with polar endgroups are discussed. The evidence of different types of layering in the nematic and smecticA phases was found. The lamellar packing in smeticA phases corresponds to either the monolayersA ₁ with an interlayer distance equal to the molecular lengthd≈L, or a bilayerA ₂ withd≈2L, or partially bilayerA _d phase whose period is incommensurate with the molecular lengthL<d<2L. The influence of the molecular structure of terminally polar molecules on the stability of layered structures of different types is discussed. The smectic ? phase with bilayer density wave modulated along the direction perpendicular to the directorn is described. The properties of liquid crystals consisting of polar molecules are discussed in terms of frustrated smectics with competing length scales. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Macroscopic properties of smectic liquid crystals

We discuss the macroscopic behavior of smectic C_G liquid crystals. Smectic C_G is the most general tilted smectic phase that is fluid in the layers. It is characterized by global C₁ symmetry. Consequently, it is ferroelectric, pyroelectric and piezoelectric, opening up a number of possible applications for such a phase. As smectic C_G-phase has a macroscopic hand due to its structure, it is a natural candidate to explain the recent experimental observations of left and right-handed helices in a system composed of achiral molecules. We also discuss critically to what extent smectic C_G could be important for liquid crystalline phases formed by banana-shaped molecules. Phase transitions involving a smectic C_G phase and defects of its in-plane director are briefly discussed. Received: 25 March 1998 / Revised: 15 June 1998 / Accepted: 15 July 1998     

Structure and physical properties of liquid crystals

In writing this review article on the “state of the art” in liquid crystals we assume that the reader has an acquaintance with the fundamentals of the subject. For those who may wish to start with more elementary and general material we cite two articles.     

Reflection properties of distorted cholesteric liquid crystals

The optical reflection properties of distorted cholesteric liquid crystals are theoretically investigated. It is shown that higher-order reflection bands occur with distorted helical structures even for light propagation parallel to the twist axis. For aperiodic structures, the positions of the main reflection bands differ from that of periodic ones. Ferroelectric helical structures could be detected by reflection spectroscopy when the liquid crystal is deformed by an electric field.     

极性晶体膜中强耦合激子的性质

在强耦合极化子模型基础上,采用Lee-Low-Pines(LLP)变分法研究了极性晶体膜中激子与表面光学(SO)声子强耦合、与体纵光学(LO)声子弱耦合体系的性质.讨论了极性晶体膜中激子的诱生势与膜厚度和温度的变化关系.结果表明:激子的诱生势不仅与电子-空穴间距离有关,而且与极性晶体膜厚度有关,同时温度对激子诱生势的影响十分显著.     

Dielectric properties of liquid crystals in polycapillary matrices

This paper reports on the results of investigations into the dielectric properties of liquid crystals embedded in polycapillary matrices and describes a technique for their measurement. It has been revealed that the chemical structure of the rigid core and the length of mobile alkyl groups of liquid-crystal molecules of the alkylcyanobiphenyl group substantially affect the equilibrium configuration of the liquid-crystal director in capillaries. The reorientation of liquid-crystal molecules embedded in capillaries in the nematic phase under the influence of an external magnetic field has been investigated.     

Some properties of liquid crystals as infrared modulators

Liquid crystals can be used to modulate low power i.r. radiation. At 10.6 μm, modulation depths of 80%–90% and absorption losses of 15%–30% are typical for devices made with presently available material.     

Optical and electro-optical properties of photonic crystals based on polymer-dispersed liquid crystals

Two-dimensional (2-D) square lattice (SL) photonic crystals (PCs) are fabricated, and their optical/electro-optical properties are studied. The PCs are based on polymer-dispersed liquid crystals (PDLC) that are formed using two-beam interference with double exposures. The PC structure that is observed using a scanning electron microscope matches with the calculated interference pattern. The results of optical/electro-optical studies demonstrate that superprism and negative refraction effects occur at certain incident angles over a range of frequencies and are consistent with the simulated ones. Moreover, the negative refraction efficiency is electrically controllable.     

General properties of lasing effect in chiral liquid crystals

Numerical simulations and experimental studies of the lasing effect in chiral liquid crystals are presented. It is shown that ring-like light emission, which is often observed experimentally, is a true lasing effect. Lasing condition and different modes in thin liquid crystal layers are discussed.     

Optical properties of acoustically induced domains in liquid crystals

Optical properties of acoustically induced domains in planar liquid crystal layers are considered: the diffraction of light and the focusing of light by domains.     

Electron-TO-phonon interaction in polar crystals

In the present work, the interaction between electrons and long-wavelength transverse optical (TO) vibrations in a polar insulator is considered. The crystal model employed for the theoretical analysis includes classical potentials and electronic polarizabilities. A significant enhancement of the strength of the electron-TO-phonon interaction in ferroelectrics has been found. A microscopic justification of this effect is given. A bridge that relates the interaction of electrons with the polar long-wavelength TO modes of lattice vibrations to the long-range dipole-dipole interaction is established. As an application of our analysis, a novel method for quantitative predictions of the electron-TO-phonon interaction constants in polar crystals via the data of experimental studies is suggested.     

Anchoring transitions with polar and non-polar nematic liquid crystals on incompletely oxidized silane substrates

The active oxygen gas arising from a plasma reactor is used to realize progressive chemical modifications onto silane coatings that could be particularly interesting as alignment layers for liquid crystal display applications. Depending on the oxygen density grafted onto the substrate, these alignment layers provide different zenithal anchoring angles, or pretilt angles, with anchoring transitions, for polar and non-polar nematic liquid crystals as 5CB and MBBA, respectively. The anchoring transitions are found to be smoother with the polar nematics. Such a behavior is discussed in terms of the differential wetting model by adding a cosine term to the interaction energy between the nematic and the substrate. A local justification is proposed for this symmetry breaking term. Received: 18 May 1998     

Dielectric properties of antiferroelectric liquid crystals composed of banana-like molecules

The dielectric properties of an antiferroelectric B₂ phase representing a homolog (n=14) from a series of 4-chloro-1,3-phenylene-bis[4-(4-n-alkylphenyl)aminobenzoates] with banana-like molecules were studied. The temperature dependence of the dielectric relaxation time was measured, and the corresponding activation energy was determined. The kinetics of the dielectric permittivity was studied in the course of the polarization vector variation under the action of a low-slope triangular bias voltage in the entire temperature range of existence of the B₂ phase. Dependence of the dielectric permittivity on the electric field strength (bias voltage) is determined by the presence of two polar subsystems forming the antiferroelectric B₂ phase.     

Optical properties of frustrated cholesteric liquid crystals in planar cell

ABSTRACT

We investigated the alignment director of a frustrated cholesteric liquid crystal (CLC) confined in a planar cell. Three cells with different confinement ratio (c?=?d/p) (where p is the pitch and d is the cell thickness) are prepared. Under an electric field, the CLC planar texture is transformed into a cholesteric fingerprint (CF). The results showed that CF contrast depends on c. When c?≈?2, CLC stripes are formed by a periodic CF, with a period equal to the CLC pitch. The CF is developed and slowly extended to the whole cell surface along the rubbing direction and the contrast of the grating stripes keeps unchangeable. Yet, the CLC finger borders have a different light intensity. However, when c?≈?1, the CF contrast increases with time. When c?θ between the polarizer and the CFs.     

Magnetic properties of a multi-domain model for smectic-C liquid crystals

The magnetic properties of a multi-domain smectic-C phase liquid crystal are calculated as a function of tilt angle and magnetic field orientation. Numerical results are presented for the overall diamagnetic susceptibility and magnetic torque density. It is predicted that a first-order reorientation phase transition can occur for the case of a free smectic-C whose tilt angle exceeds 35° at some temperature. These calculations can be used for critically evaluating the multi-domain model and for studies of the temperature dependence of the smectic-C tilt angle.     

Exciton interaction potential in polar crystals

The problem whether polaron effects can lead to a repulsive barrier in the exciton interaction potential is reinvestigated, because this effect is crucial for the possibility of a Bose-Einstein condensation of excitons (rather than of excitonic molecules). The energy for two excitons is calculated accurately as a function of their mean distance in the case of a heavy hole mass by using an integral expansion method of molecular physics. The resulting interaction potential shows for certain polaron radii a very weak repulsive barrier. Taking into account corrections to the Haken potential which have been introduced by Pollmann and Büttner, it is shown that the interaction potential between two excitons in the relative singlet state is always attractive, so that the exciton system is also in polar materials unstable against exciton molecule formation.This is a project of the Sonderforschungsbereich Frankfurt/Darmstadt, financed by special funds of the Deutsche Forschungsgemeinschaft