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

除了反铁电的SmC_a^*相外，还在手性近晶液晶中发现了几种新的中间相，即所谓的SmC_a^*相和一些微电相。其中，SmC_a^*相因其独特的物理性质一直备受关注。利用Kerr效应和非线性介电光谱研究了反铁电液晶4-（l-甲基庚氧羰基）苯基4-辛基羰基氧联苯-4-羧酸盐（MHPOCBC）在SmA-SmC_a^*相变点附近的动力学性质。为了分析实验得到的频散，建立了朗道型理论。从实验和理论结果中，我们成功地得到了SmA和SmC_a^*相中软模弛豫频率的温度依赖关系，从而直接证明了软模的转变是由软模凝聚引起的。此外，从软模（振幅模）和铁电模的弛豫频率的斜率可以直接得出相变接近三临界点的结论，并且即使相邻层之间没有相互作用，也可以在每层中从SmA相过渡到倾斜相的离散模型是有效的。

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.