Periodic splay Fr\'eedericksz transitions in a ferroelectric nematic

TL;DR

This paper investigates how an ac electric field induces various polarization patterns in ferroelectric nematic liquid crystals, revealing a novel splay cancellation mechanism that reduces bound charge without free ions.

Contribution

It demonstrates the emergence of periodic splay and twist patterns in ferroelectric nematics under ac fields and introduces a geometrical splay cancellation mechanism.

Findings

Low voltage oscillations around field-free orientation

Formation of stripe and square lattice polarization patterns

Reduction of bound charge via splay cancellation

Abstract

Electric field-induced splay of molecular orientation, called the Fr\'eedericksz transition, is a fundamental electro-optic phenomenon in nonpolar nematic liquid crystals. In a ferroelectric nematic NF with a spontaneous electric polarization P, the splay is suppressed since it produces bound electric charges. Here, we demonstrate that an alternating current (ac) electric field causes three patterns of NF polarization. At low voltages, P oscillates around the field-free orientation with no stationary deformations. As the voltage increases, the polarization acquires stationary distortions, first splay and twist in a stripe pattern and then splay and bend in a square lattice of +1 and -1 defects. In all patterns, P oscillates around the stationary orientations. The stationary bound charge is reduced by a geometrical splay cancellation mechanism that does not require free ions: the charge created by splay in one plane is reduced by splay of an opposite sign in the orthogonal plane.