Study on the Dynamics of an Antiferroelectric Liquid Crystal by Fluctuation-Induced Kerr Effect

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Several novel intermediate phases, so-called SmC_a^* phase and some femelectnc phases, have been found in chiral smectic liquid crystals in addition to the antiferroelectric SmC_A^* phase. Among them, SmC_a^* phase has been attracting much attention because of its unique physical properties. We have performed Kerr effect and nonlinear dielectric spectroscopy to study the dynamic properties near the SmA-SmC_a^* phase transition point of an antiferroelectric liquid crystal 4-(l-methylheptyloxycarbonyl)phenyl 4-octylcarbonyloxybiphenil-4-carboxylate (MHPOCBC). A Landau-type theory was developed in order to analyze the experimentally obtained frequency dispersions. From the experimental and theoretical results we have successfully obtained the temperature dependence of the relaxation frequency of the soft mode both in the SmA and SmC_a^* phases, which gave the direct evidence that the transition is brought'about by the soft mode condensation. Furthermore, from the slopes of the relaxation frequencies of the soft mode (the amplitude mode) and the ferroelectric mode we could conclude directly that the phase transition is close to a tricritical point, and the discrete model, in which the transitions to tilted phases from the SmA phase take place in each layer even without interactions between neighboring layers, is valid.