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

在手性近晶液晶中，除了反铁电SmC_A^* 相外，还发现了几种新的中间相，即SmC_A ^* 相和一些铁电相。其中SmC_a^* 相由于其独特的物理性质而备受关注。利用克尔效应和非线性介电谱研究了反铁电液晶4-（1-甲基庚氧羰基）苯基-4-辛基羰基氧基联苯-4-羧酸酯（MHPOCBC）在SmA-SmC_a^* 相变点附近的动力学性质.一个朗道型理论，以分析实验获得的频散。从实验和理论结果中，我们成功地得到了SmA和SmC_a^* 相的软模弛豫频率随温度的变化关系，这直接证明了相变是由软模凝聚引起的。此外，从软模式（振幅模式）和铁电模式的弛豫频率的斜率，我们可以直接得出结论，相变是接近三临界点，和离散模型，其中从SmA相的过渡到倾斜相发生在每一层中，即使没有相邻层之间的相互作用，是有效的。

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.