Ferroelectric nematics: Materials with high permittivity or low resistivity?

Abstract

Two models have recently been proposed for a description of dielectric spectroscopy measurements of ferroelectric nematics (NF) in thin planar capacitors. The polarization-external capacitance Goldstone reorientation mode (PCG model) considers the NF layer between the electrodes as an effective low resistivity material, the resistivity being inversely proportional to the square of polarisation magnitude. The high-ε model considers the NF material as having a huge permittivity due to the ease of polarisation rotation. In this paper we study implications of both models and show, why both models describe majority of the observed dielectric spectroscopy results equally well. We point out differences among the models predictions and explain why some observations can be explained only by the high-ε model. The major difference between the models is that the high-ε model predicts that the increase in the cell thickness can lead to an increase in the frequency range within which capacitors filled with NF material can be used for energy storage while within the PCG model this frequency range reduces with increasing capacitor thickness. Within both models a crucial parameter which determines the behaviour of the capacitors filled with a NF material is parasitic resistance, primarily due to the electrode resistance. We present measurements of electrode resistance and find that in ITO cells it is of the order of few hundred ohms.