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arXiv 2208.09784cond-mat.soft
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铁电向列相液晶的介电谱:测量绝缘界面层的电容

Dielectric spectroscopy of ferroelectric nematic liquid crystals: Measuring the capacitance of insulating interfacial layers

Noel A. Clark, Xi Chen, Joseph E. Maclennan, Matthew A. Glaser

AI总结:

本文提出极化-外电容(PCG)模型,指出铁电向列相液晶介电测量中异常大的介电常数实际来自纳米级绝缘界面层的高电容,而非液晶本体,并通过对多种铁电液晶实验数据的分析验证了该模型,揭示了忽略自屏蔽效应和界面电容会导致介电常数被高估多个数量级。

摘要:

针对近期发现的铁电向列相($N_F$)液晶(LC)相的大量介电常数 $ε$ 测量报告了异常大的 $ε^\prime$ 值(高达约 30,000)。我们表明,这类实验中实际测得的是典型液晶盒中包围 $N_F$ 材料的非铁电、绝缘、纳米厚度界面层的高电容。我们分析了一个平行板液晶盒,其中填充了高极化强度 $\mathbf{P}$ 的 $N_F$ 材料,在零外加电压下 $\mathbf{P}$ 平行于平板取向。主导静电能的极小化使 $\mathbf{P}$ 在空间上保持均匀,并使其取向以尽可能减小 $N_F$ 中的电场,在这种条件下,施加到液晶盒上的电压几乎全部落在高电容界面层上。这种取向与电荷的耦合产生了一种极化-外电容(PCG)Goldstone 重取向模式,其有效跨 $N_F$ 层输运电荷所需的外加电压比单独 $N_F$ 层所需电压小几个数量级。$N_F$ 层充当低值电阻,界面电容器充当可逆储能库,降低了 PCG 模式的恢复力(质量),并产生强无功介电行为。对铁电液晶(手性近晶 C 相、弯曲核近晶相和 $N_F$ 相)若干实验数据的分析支持 PCG 模型,表明在不适当考虑极化电荷的自屏蔽效应和界面层电容贡献的情况下,从高极化铁电液晶的电阻抗测量推导介电常数,可能导致对液晶 $ε^\prime$ 值的高估达多个数量级。

英文摘要:

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.

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