The two main classes of liquid-crystal (LC) phases of rodlike molecules are nematics, where the rods align in the same direction (the nematic director $\mathbf{n}$), and smectics, where the rods not only are aligned but also form layers. The electro-optic effects in LC devices that are a backbone in today’s display industry mainly use the Fréedericksz transition, which is the bulk reorientation of a surface-anchored nematic by an electric field. Conventional (uniaxial) smectics do not present a Fréedericksz transition, because, due to their layered structure, the director reorientation would distort the layers, which would cost too much energy. In a worldwide ongoing effort to extend the variety of LC compounds suitable for applications in the display industry, bent-shaped molecules have recently raised much attention, since they present multiple new LC phases with unusual properties. In this paper, we report on a structural and electro-optic study of the LC phases of a bent-shaped dimer. On cooling from the isotropic liquid, this compound shows a usual nematic ($N$), a twist-bend nematic ($N_{\mathrm{TB}}$), and a biaxial smectic-$A$ phase ($\mathrm{Sm}{A}_b$). Quite surprisingly, contrary to usual smectics, $\mathrm{Sm}{A}_b$ presents a remarkable electro-optic response, with low ($<4\mathrm{V}$) voltage threshold, no reorganization of the smectic layers, and low ($<1\mathrm{ms}$) response time (i.e., 30 times faster than the $N$ phase at higher temperature). We interpret this unexpected electro-optic effect as a Fréedericksz transition affecting the secondary director $\mathbf{m}$ of the $\mathrm{Sm}{A}_b$, and we model it by analogy with the usual Fréedericksz transition of the $\mathbf{n}$ director of the uniaxial $N$ phase. Indeed, a Fréedericksz transition affecting only $\mathbf{m}$ in this biaxial fluid smectic does not alter its layered structure and costs little energy. From the point of view of applications, thanks to its low relaxation time, this “biaxial” Fréedericksz transition could be exploited in electro-optic devices that require fast switching.

中文翻译：

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双轴近晶-A相中的类Fréedericksz转变

棒状分子的两大类液晶 (LC) 相是向列相，其中棒以相同方向排列（向列指向 $\mathbf{n}$）和近晶，其中杆不仅对齐而且还形成层。作为当今显示行业支柱的 LC 器件中的电光效应主要使用 Fréedericksz 跃迁，即通过电场对表面锚定向列相进行体重新定向。传统的（单轴）近晶不会出现 Fréedericksz 转变，因为由于它们的分层结构，导向器重新定向会扭曲层，这会消耗太多能量。在全球范围内不断努力扩展适用于显示器行业应用的各种 LC 化合物中，弯曲形状的分子最近引起了很多关注，因为它们呈现出多种具有不同寻常特性的新 LC 相。在本文中，我们报告了弯曲形二聚体 LC 相的结构和电光研究。$N$), 一个扭弯向列 ($N_{\mathrm{结核病}}$)，和双轴近晶-$\mathrm{一种}$ 阶段 （$\mathrm{SM}{\mathrm{一种}}_{乙}$）。相当令人惊讶的是，与通常的近端法相反，$\mathrm{SM}{\mathrm{一种}}_{乙}$ 呈现出显着的电光响应，具有低 ($<4\mathrm{伏}$) 电压阈值，近晶层没有重组，并且低 ($<1\mathrm{小姐}$) 响应时间（即比响应速度快 30 倍） $N$相在较高温度）。我们将这种意想不到的电光效应解释为影响二级导演的 Fréedericksz 转变$\mathbf{米}$ 的 $\mathrm{SM}{\mathrm{一种}}_{乙}$，我们通过与通常的 Fréedericksz 过渡的类比对其进行建模 $\mathbf{n}$ 单轴导演 $N$阶段。事实上，Fréedericksz 过渡仅影响$\mathbf{米}$在这种双轴流体中，近晶不会改变其层状结构并且消耗很少的能量。从应用的角度来看，由于其低弛豫时间，这种“双轴”Fréedericksz 跃迁可用于需要快速开关的电光器件。