Chirality has become arguably the most important and complex topic of research in liquid crystals today. The reduced symmetry in these organized phases leads to a variety of novel phase structures, properties, and applications. Molecular asymmetry imparts form chirality to liquid-crystalline phases, which is manifested in the formation of helical ordering of the constituent molecules of the phase. Similarly, molecular asymmetry imposes a reduction in the space symmetry, which leads to some phases having unusual non-linear properties, such as ferroelectricity and pyroelectricity. In the following article the various effects of chirality in liquid-crystalline systems are described in general terms.

Chirality in liquid crystals

The discovery of the first liquid-crystalline material in 18881,2 also heralded an age of fascination with chirality and optical activity in ordered fluids. The cholesteric mesophase, which was the first liquid crystal to be found, exhibits form optical activity by virtue of a helical arrangement of its constituent molecules. One hundred years after the discovery of this first liquid crystal, we report the discovery of a new helical smectic liquid crystal, the smectic-A* phase. In this phase the lath-like molecules are arranged in layers with their long axes on average normal to the layer planes. Parallel to the layers there is a helical ordering of the molecules (Fig. 1). We suggest that this phase may be described by a model in which grain boundaries of screw dislocations rotate blocks of layers with respect to each other.

Characterization of a new helical smectic liquid crystal

A new electrooptic mode of operation of ferroelectric chiral smectic C liquid crystal displays (LCDs) is proposed and demonstrated. The effect, which is called the deformed helical ferroelectric (DHF) effect, is based on the deformation of the helical structure by weak electric fields. In the unbiased device the smectic layers are arranged in the bookshelf geometry with the helix axis parallel to the electrodes [1]. Systems with a very small pitch (<1 μm) and a large tilt angle are especially well suited for this mode. The key characteristics of DHF-LCDs are: (a) low driving fields (1 Vp-pμm−1 for maximum contrast); (b) grey scale which is approximately linear with the applied electric field; (c) easy alignment even for thick cells using standard wall-aligning methods; and (d) response times at room temperature of 300 μs.

Deformed helix ferroelectric liquid crystal display: A new electrooptic mode in ferroelectric chiral smectic C liquid crystals

Different contributions to the dielectric permittivity in ferroelectric liquid crystals are discussed, with emphasis on the soft mode and the Goldstone mode and their location in the dielectric spectrum. Experimentally, the complex dielectric permittivity has been studied as a function of temperature and frequency in the range 5 Hz - 13 MHz for three different ferroelectric liquid crystal materials. The main problems encountered in dielectric measurements at low and high frequency are discussed in some detail. The soft mode dielectric behaviour has been studied as a function of temperature, frequency and bias electric field. The applicability of the Curie-Weiss law for the soft mode dielectric contribution in the A* phase was analyzed. In the C* phase the temperature dependence of the dielectric contribution of the Goldstone mode has been measured. By applying a bias electric field, we have been able to study the soft mode dielectric behaviour also deep into the C* phase. In the A* and C* phases ...

Dielectric studies of the soft mode and Goldstone mode in ferroelectric liquid crystals

Defects are local breakings of symmetry in an ordered medium. The physics of defects has long been reduced to the study of dislocations in solids, and to the main physical phenomena they are responsible for, like plastic deformation. Dislocations break translational symmetries. Disclinations break rotational symmetries and are the basic defects of media with continuous symmetries, like liquid crystals. In this review, it is stressed how their study has contributed to a renewal of the physics of defects. Static and dynamic properties in the nematic, cholesteric, blue, smectic and columnar phases of liquid crystals are described in detail, up to the most recent results, for small-molecule thermotropic liquid crystals as well as for lyotropic and polymeric liquid crystals. The authors also discuss the homotopy classification of defects, including point defects, and compare it with the Volterra classification; finally they present the curved-crystal description of frustration in the light of the relevant situation in liquid crystals. The usefulness of the concepts introduced for liquid crystals for the study of other systems (structural problems in biology, dissipative structures, magnetic domains, etc.) is also emphasised.

Defects in liquid crystals

The mechanism of a field induced transition from helical Sm C∗ to Sm C structure in a DOBAMBC liquid crystal situated between two glass plates has been studied by optical methods. The important role of a regular system of twist disclination lines (dechiralization lines) in unwinding the helical structure by electric field has been revealed. A simplified model of ±2π twist disclination pairs is proposed which describes the unwinding of the Sm C∗ structure. Under the external electric field the pairs of ±2π twist disclinations move and annihilate each other and thus effectively unwind the helical structure of the sample. Critical fields for switching the helical Sm C∗ structure, as well as the unwound structure, are estimated.

The Influence of an External Electric Field on the Structure of Chiral Sm C∗ Liquid Crystal

The review describes possible structures of the chiral smectic C (Sm C*) liquid crystals (DOBAMBC and a particular Sm C + cholesteric mixture) in samples with planar surface orientation of molecules. In two of them (planar helicoidal PH1 and PH2 configurations) the helicoidal structure is realized in the centre of the plate-like samples. In the other two (planar twisted PT and planar uniform PU configurations) the helix is unwound. Behaviour of these structures in an electric field is discussed. In the field induced transition PH → PT.motion and mutual annihilation of ±2π twist disclinations plays an important role. On the other hand the transition PT → PU or the switching processes between two PU states (PU+ and PU-) with opposite dipole-moments are governed by ferroelectric domain-like mechanisms (nucleation and growth of domains). Hysteresis loop P(E) is observed at frequencies higher than 1 Hz, when the transition PU+ → PU− takes place. The small signal dielectric permittivity of PH or PT con...

The structure of ferroelectric liquid crystals in planar geometry and its response to electric fields

On a ferroelectric liquid crystal DOBAMBC transitions between helical and twisted structures are observed if changing the temperature. The twisted structure appears when the helical pitch becomes c...

The Behaviour of Thin Samples of Ferroelectric Liquid Crystals

Dielectric permittivity of ferroelectric liquid crystals has been measured in dependence on temperature, bias electric field and frequency. It is shown that the permittivity increase occurs only below TC in ferroelectric SmC* phase and it is a consequence of real defect structure in planar samples.

Jiří Pavel

Papers

The Influence of an External Electric Field on the Structure of Chiral Sm C∗ Liquid Crystal

The structure of ferroelectric liquid crystals in planar geometry and its response to electric fields

The Behaviour of Thin Samples of Ferroelectric Liquid Crystals

Permittivity of ferroelectric liquid crystals