A molecular perspective on the emergence of long-range polar order from an isotropic fluid

TL;DR

This study explores how polar order emerges from an isotropic fluid in ferroelectric nematic liquid crystals, combining experiments and theory to reveal the role of molecular interactions and correlations.

Contribution

It provides new experimental evidence of polar correlations in the isotropic phase and employs a theoretical model to explain the stability of the ferroelectric nematic phase.

Findings

Polar correlations exist in the high-temperature isotropic phase.

Electric fields can induce ferroelectric order in the isotropic phase.

Theoretical calculations suggest dipole alignment is driven by electrostatic and volume interactions.

Abstract

The ferroelectric nematic phase (N$_{\text{F}}$) has quickly become the most studied system in liquid crystal research. In this work, we investigate the origin of such polar structure by studying a compound for which the N$_{\text{F}}$ phase directly follows the isotropic liquid phase on cooling, making it a particularly interesting system. Our experimental results evidence the presence of polar correlations already in the high-temperature phase, in which ferroelectric order can be induced under a sufficiently strong electric field. In the N$_{\text{F}}$ phase, molecular dynamics and polar correlations are investigated through detailed dynamic dielectric measurements, while second harmonic generation experiments evidence a large value of the main coefficient of the second order dielectric susceptibility tensor. Lastly, experimentally determined parameters are employed for calculations based on a recently proposed theoretical model for the stability of the N$_{\text{F}}$ phase. The obtained results suggest that the parallel alignment of dipoles is driven by a subtle interplay between electrostatic and excluded volume interactions.