ANTIFERROELECTRIC CHIRAL SMECTIC LIQUID-CRYSTALS

In a brief history of the discovery of antiferroelectricity in liquid crystals, the important role played by tristable switching, i.e. an el ectric field induced phase transition from antiferroelectric SC(A) to ferroelectric SC, has been emphasised and the antiferroelectric herr ingbone structure of SC(A) has been presented. Then we have explained how to identify the subphases in the SC region, eg. SC(gamma)*, SC(a lpha); the clarification of the subphase structures is essential for understanding antiferroelectricity in liquid crystals. After summarizi ng the evidence for the SC(A) structure presented, we have suggested the pair formation of transverse dipole moments in adjacent smectic la yers as the cause of its antiferroelectricity, showing that the smecti c layer is much closer to the usual picture of molecules lying on equi distant planes; the packing entropy due to the sinusoidal density wave character stabilizes ferroelectric SC. The competition between the i nteractions stabilizing SC(A) and SC* is responsible for the occurren ce of several varieties of ferrielectric and antiferroelectric subphas es, which constitutes the Devil's staircase. We have further suggested that the essentials of the SC(alpha) phase are its considerably redu ced ability to form SC(A) and SC*. At least when the spontaneous pola rization is zero, SC(alpha) is a smectic C-like phase with molecular tilting that is non-correlated on the visible wavelength scale When th e spontaneous polarization is not zero, as suggested by Prost and Brui nsma recently, a novel type of Coulomb interaction between smectic lay ers due to the collective polarization fluctuations causes the antifer roelectricity in the high-temperature region of SC(alpha); the compet ition between this antiferroelectricity and the SC ferroelectricity m ay form another staircase, causing the complexity in SC(alpha). Appli cations and some future problems have been described in the final sect ion.