Abstract

An analysis of the dielectric response of ferro-electric liquid crystals at the $S_A-S_C^{\cdot }$ transition temperature is presented. The dielectric permittivity $\left({\epsilon }^{\ast }={\epsilon }^{\prime }-j{\epsilon }^{\prime \prime }\right)$ was measured as a function of frequency in the planar (bookshelf) and homeotropic geometries, respectively. The frequency range was 5-10⁹Hz. To obtain these measurements, a new dielectric cell was designed allowing a direct optical check of the sample alignment during the experiments. The data are consistent with previously published theories which show that in addition to the usual dielectric relaxation processes of conventional liquid crystals appearing in the high-frequency range of the dielectric spectrum, two modes dominate the dielectric response in the low-frequency range connected with the existence of long-range polarizationP.The first mode is often called the Goldstone mode. It is only apparent in the $S_c^{\cdot }$ phase and is connected with the inphase fluctuations of the phases of the tilt angleθand the polarizationP.The second one is the soft mode which is only observed close to the transition in the two phases. The inphase fluctuations ofθandPamplitudes are responsible for this mode existence. The frequency and temperature dependences of the amplitudes and relaxation frequencies of these two modes are investigated and compared to the data. The analysis yields values for molecular parameters which are consistent with previously reported values.

© 1988 Optical Society of America