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

Abstract

Numerous measurements of the dielectric constant $\epsilon$ of the recently discovered ferroelectric nematic ($N_F$) liquid crystal (LC) phase report extraordinarily large values of ${\epsilon }^{\prime }$ (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 ${\epsilon }^{\prime }$ values of the LC by many orders of magnitude.

30 pages of text, 11 figures, 59 references (47 pages total)

References in corpus (11)

• First-Principles Experimental Demonstration of Ferroelectricity in a Thermotropic Nematic Liquid Crystal: Spontaneous Polar Domains and Striking Electro-Optics
• Ferroelectric-ferroelastic phase transition in a nematic liquid crystal
• Development of polar nematic fluids with giant-\k{appa} dielectric properties
• Polar In-Plane Surface Orientation of a Ferroelectric Nematic Liquid Crystal: Polar Monodomains and Twisted State Electro-Optics
• Observation of Spontaneous Helielectric Nematic Fluids: Electric Analogy to Helimagnets
• Antiferroelectric Smectic Ordering as a Prelude to the Ferroelectric Nematic:Introducing the Smectic $Z_A$ Phase
• Ideal Mixing of Paraelectric and Ferroelectric Nematic Phases in Liquid Crystals of Distinct Molecular Species
• Dielectric spectroscopy of a ferroelectric nematic liquid crystal and the effect of the sample thickness
• Role of electrostatics in the texture of islands in free standing ferroelectric liquid crystal films
• Collective and non-collective molecular dynamics in a ferroelectric nematic liquid crystal studied by broadband dielectric spectroscopy
• Dynamics of the molecular orientation field coupled to ions in two-dimensional ferroelectric liquid crystals

Cited by in corpus (6)

• Polar order in a fluid like ferroelectric with a tilted lamellar structure -- observation of a polar smectic C (SmC${}_{\text{P}}$) phase
• Twist, splay, and uniform domains in ferroelectric nematic liquid crystals
• A molecular perspective on the emergence of long-range polar order from an isotropic fluid
• Colossal Dielectric Permittivity and Superparaelectricity in phenyl pyrimidine based liquid crystals
• Half-integer Topological Defects Paired via String Micelles in Polar Liquids
• Deformed states in paraelectric and ferroelectric nematic liquid crystals