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

Abstract

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 the relaxation frequency and the inverse of the dielectric strength of the soft mode vary linearly with |T-T_C|, which can be used to determine the A^*-C^* transition temperature to a good accuracy. The ratio of the slopes of the soft mode relaxation frequency vs temperature line and the inverse of dielectric strength vs temperature line in the C^* and A^* phases has been calculated for the three compounds and compared with solid ferroelectric systems. The soft mode rotational viscosity in the A^* phase has been calculated for one of the compounds.