Landau theory applied to antiferroelectric ordering in ferroelectric nematic liquid crystals

TL;DR

This study investigates antiferroelectric ordering in ferroelectric nematic liquid crystals, analyzing polarization and density modulations via X-ray diffraction and Landau theory, revealing different modulation profiles in two compounds.

Contribution

It applies Landau theory to experimental data on antiferroelectric ordering in nematic liquid crystals, highlighting differences in modulation profiles between a pure compound and a mixture.

Findings

qS and polarization analyzed with Landau theory.

Sinusoidal modulation in DIO; soliton-like in FNLC919.

Modulation profiles vary near phase transition.

Abstract

The polarization and density modulation associated with antiferroelectric ordering is studied experimentally as a function of temperature in two ferroelectric nematic liquid crystals, the prototypical single compound (DIO) and a commercial mixture (FNLC919). The modulation wavenumber qA is determined by small angle X-ray diffraction from the weak smectic-like density wave (wavenumber qS = 2qA) that accompanies the polarization modulation. Results for qS and the saturated value of the polarization are analyzed in terms of Landau theory previously developed to describe the para-/antiferro-/feroelectric sequence of phase transitions in solid ferroelectrics. The analysis indicates that the polarization modulation is reasonably well approximated by a simple sinusoid in the antiferroelectric phase of DIO, whereas in FNLC919 the modulation develops a strongly soliton-like profile (with sharply decreasing wavenumber) close to the antiferro- to ferrolectric transition.