First -principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics
First -principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics
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DOI：
10.1073/pnas.2002290117
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发表时间：
2020-06-23
影响因子：
11.1
通讯作者：
Clark, Noel A.
中科院分区：
文献类型：
--
作者：
Chen, Xi;Korblova, Eva;Clark, Noel A.
我们报道了先前报道的钙质化合物4-[(4-nitrophenoxy)carbonyl]phenyl2，4-dimethoxybenzoate(RM734)低温向列相的结构和对外加电场的响应的实验测定。我们利用它的电光来显示在没有外加电场的情况下，永久极化密度的外观，表现为在不同的相反极向取向的不同区域中自发破坏的对称性。极化反转是通过场致磁化壁移来实现的，这使得这一相成为铁电相，这是一种3D单轴向列相，具有自发的、可重定向的极化，局部平行于指向矢。这种偏振密度在接近6µC/cm2的低温下饱和，这是迄今测量到的流体或玻璃材料的最大温度值。这种极化与固态铁电体的极化相当，接近于假设分子偶极子在向列相中完美、极向排列而获得的平均值。我们发现了一系列壮观的光学和流体动力学效应，这些效应是由大偏振与向列相双折射和流动耦合而产生的，由超低外场(E类似于1V/cm)驱动。极化电荷的静电自相互作用使向列相的平均类场弱一级跃迁，并控制着铁电相的指向矢场结构。原子分子动力学模拟揭示了有利于铁电有序的短程极性分子相互作用，包括头尾结合成具有极性横向关联的极性链状组装的趋势。这些结果表明，基于对分子静电相互作用的更好的理解、开发和利用，在变革性的、新的向列相物理、化学和应用方面具有巨大的潜力。
We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of similar to 6 mu C/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E similar to 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic selfinteraction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.