[论文解读] Dielectric spectroscopy of ferroelectric nematic liquid crystals: Measuring the capacitance of insulating interfacial layers

Noel A. Clark, Xi Chen+2 more|arXiv (Cornell University)|Aug 21, 2022

Liquid Crystal Research Advancements参考文献 53被引用 6

一句话总结

本文表明，铁电向列相液晶（N_F）中报道的极高介电常数（高达~30,000）并非材料的本征性质，而是源于纳米尺度绝缘界面层的高电容。作者提出一种极化-电容耦合（PCG）模型，表明界面层主导了测得的阻抗，当未考虑自屏蔽效应和界面效应时，导致对ε′的严重高估。

ABSTRACT

Numerous measurements of the dielectric constant $ε$ of the recently discovered ferroelectric nematic ($N_F$) liquid crystal (LC) phase report extraordinarily large values of $ε^\prime$ (up to ~30,000). We show that what is in fact being measured in such experiments is the high capacitance of the non-ferroelectric, interfacial, insulating layers of nanoscale thickness that bound the $N_F$ material in typical cells. We analyze a parallel-plate cell filled with $N_F$ material of high-polarization $\mathbf{P}$, oriented parallel to the plates at zero applied voltage. Minimization of the dominant electrostatic energy renders $\mathbf{P}$ spatially uniform and orients it to make the electric field in the $N_F$ as small as possible, a condition under which the voltage applied to the cell appears almost entirely across the high-capacity interfacial layers. This coupling of orientation and charge creates a combined polarization-external capacitance (PCG) Goldstone reorientation mode requiring applied voltages orders of magnitude smaller than that of the $N_F$ layer alone to effectively transport charge across the $N_F$ layer. The $N_F$ layer acts as a low-value resistor and the interfacial capacitors as reversible energy storage reservoirs, lowering the restoring force (mass) of the PCG mode and producing strong reactive dielectric behavior. Analysis of data from several experiments on ferroelectric liquid crystals (chiral smectics C, bent-core smectics, and the $N_F$ phase supports the PCG model, showing that deriving dielectric constants from electrical impedance measurements of high-polarization ferroelectric LCs, without properly accounting for the self-screening effects of polarization charge and the capacitive contributions of interfacial layers, can result in overestimation of the $ε^\prime$ values of the LC by many orders of magnitude.

研究动机与目标

• 解决报道的铁电向列相液晶中介电常数（ε′）高达~30,000的悖论。
• 确定实验测量中这些异常高值的物理起源。
• 建立一个理论框架，以解释高极化铁电液晶介电谱中界面电容与极化自屏蔽效应的影响。
• 纠正因未考虑界面效应而导致的铁电液晶本征介电常数的高估。

提出的方法

• 建立一个平行板电容器模型，其中铁电向列相（N_F）层被纳米尺度厚度的绝缘界面层夹在中间。
• 应用静电能最小化方法，确定极化（P）和电场分布的平衡构型。
• 推导出极化-外接电容（PCG）模式，该模式是极化与界面电容耦合所产生的类似戈尔德斯通模式的效应。
• 利用PCG模型解释实验中观察到的强反应性介电响应和低有效驱动电压。
• 分析胆甾相SmC、弯曲分子向列相以及N_F相的实验阻抗数据，以验证PCG模型。
• 量化当忽略界面电容时ε′估计值的误差，表明其被高估了多个数量级。

实验结果

研究问题

• RQ1 为何介电谱测量在铁电向列相液晶中报告的介电常数（ε′）高达~30,000？
• RQ2 导致铁电向列相液晶器件中测得高电容的物理机制是什么？
• RQ3 绝缘界面层如何贡献于高极化铁电液晶中的表观介电响应？
• RQ4 当忽略界面效应时，N_F相的本征介电常数被高估了多少？
• RQ5 能否建立一个统一模型，解释不同铁电液晶相中的异常介电行为？

主要发现

• 铁电向列相液晶中高达~30,000的表观介电常数ε′并非本征性质，而是源于纳米尺度绝缘界面层的高电容。
• 界面层主导了测得的阻抗，导致外加电压几乎完全落在界面层上，而非N_F层内部。
• 由于极化与界面电容的耦合，出现一种极化-电容耦合（PCG）戈尔德斯通模式，使得电荷传输所需的电压比预期低多个数量级。
• N_F层充当低电阻通路，而界面层则作为可逆的能量储存库，降低了PCG模式的有效质量，增强了反应性介电响应。
• 若未考虑界面电容和极化自屏蔽效应，将导致铁电液晶本征ε′被高估多个数量级。
• PCG模型成功解释了包括胆甾相SmC、弯曲分子向列相及N_F相在内的多种铁电相的介电数据，证实了其广泛适用性。