Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers.
Second harmonic light scattering induced by defects in the twist-bend nematic phase of liquid crystal dimers.
复制标题
由液晶二聚体扭转弯曲向列相中的缺陷引起的二次谐波光散射。
DOI：
10.1039/c6sm00585c
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发表时间：
2016
期刊：
影响因子：
3.4
通讯作者：
S. Sprunt
中科院分区：
文献类型：
--
作者：
S. Pardaev;S. Shamid;M. Tamba;C. Welch;G. Mehl;J. Gleeson;D. Allender;Jonathan V Selinger;B. Ellman;A. Jákli;S. Sprunt
某些热致液晶（LC）二聚体表现出的向列扭转弯曲（NTB）相代表了一种新的定向有序中间相——这是多年来发现的第一个独特的向列变体。 NTB相的特点是平均分子长轴（导向器）的螺旋缠绕具有非常短的（纳米级）节距，并且在非手性二聚体系统中，以相同的概率形成右手和左手域。因此，NTB 结构提供了自然界中自发手性对称性破缺的另一个有趣的例子。理论上，驱动螺旋态形成的有序参数被推测为偏振场，源自二聚体的弯曲构象，其以与指向矢场相同的纳米级螺距螺旋旋转。因此，它对实验检测提出了重大挑战。在这里，我们报告了对两种非手性 NTB 形成液晶的二次谐波光散射 (SHLS) 研究，由于与自然发生的纹理缺陷相关的螺旋结构的微米级变形，该液晶对偏振场敏感。这些缺陷是类近晶“赝层”的抛物线焦锥，由 NTB 态粗粒度描述中的等效相位平面定义。我们的 SHLS 数据通过粗粒度自由能密度来解释，该密度结合了偏振场的朗道-德热内展开式、向列相的弹性能以及两者之间的线性耦合。
The nematic twist-bend (NTB) phase, exhibited by certain thermotropic liquid crystalline (LC) dimers, represents a new orientationally ordered mesophase - the first distinct nematic variant discovered in many years. The NTB phase is distinguished by a heliconical winding of the average molecular long axis (director) with a remarkably short (nanoscale) pitch and, in systems of achiral dimers, with an equal probability to form right- and left-handed domains. The NTB structure thus provides another fascinating example of spontaneous chiral symmetry breaking in nature. The order parameter driving the formation of the heliconical state has been theoretically conjectured to be a polarization field, deriving from the bent conformation of the dimers, that rotates helically with the same nanoscale pitch as the director field. It therefore presents a significant challenge for experimental detection. Here we report a second harmonic light scattering (SHLS) study on two achiral, NTB-forming LCs, which is sensitive to the polarization field due to micron-scale distortion of the helical structure associated with naturally-occurring textural defects. These defects are parabolic focal conics of smectic-like "pseudo-layers", defined by planes of equivalent phase in a coarse-grained description of the NTB state. Our SHLS data are explained by a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a linear coupling between the two.