Soft mode behaviour in the smectic A* and smectic C* phases as studied by dielectric spectroscopy

Abstract The soft mode behaviour up to about 8 K below the smectic A*-smectic C* transition temperature has been studied by applying a D.C. bias field of ca. 10 V/25 μm to a planarly aligned sample of the ZLI-3654 ferroelectric liquid crystal mixture. The ϵ*⊥ complex electric permittivity component has been measured in the frequency range from 0.66 to 13000 kHz in the smectic C* phase and in the smectic A* phase up to about 3 K above the transition temperature. From the dielectric spectra obtained the dielectric increments and the critical frequencies have been computed for the soft mode. On both sides of the smectic C*-smectic A* transition the critical frequency of the soft mode obeys a linear temperature dependence with much different slopes and about the same intercepts. The smectic C*-smectic A* transition temperature agrees well with the value found in the absence of the bias field.