Fluctuation modes of a twist-bend nematic liquid crystal

TL;DR

This study uses dynamic light scattering to analyze fluctuation modes in a twist-bend nematic liquid crystal, revealing distinct overdamped fluctuation spectra in different phases and providing a theoretical model that matches experimental data.

Contribution

It introduces a comprehensive theoretical framework for fluctuation modes in twist-bend nematic phases, validated by experimental measurements and a coarse-grained Landau-deGennes model.

Findings

Identification of overdamped fluctuation modes in N_TB phase

Agreement between theoretical predictions and experimental relaxation rates

Distinct fluctuation spectra in nematic and twist-bend phases

Abstract

We report a dynamic light scattering study of the fluctuation modes in a thermotropic liquid crystalline mixture of monomer and dimer compounds that exhibits the twist-bend nematic ($\mathrm{N_{TB}}$) phase. The results reveal a spectrum of overdamped fluctuations that includes two nonhydrodynamic and one hydrodynamic mode in the $\mathrm{N_{TB}}$ phase, and a single nonhydrodynamic plus two hydrodynamic modes (the usual nematic optic axis or director fluctuations) in the higher temperature, uniaxial nematic phase. The properties of these fluctuations and the conditions for their observation are comprehensively explained by a Landau-deGennes expansion of the free energy density in terms of heliconical director and helical polarization fields that characterize the $\mathrm{N_{TB}}$ structure, with the latter serving as the primary order parameter. A "coarse-graining" approximation simplifies the theoretical analysis, and enables us to demonstrate quantitative agreement between the calculated and experimentally determined temperature dependence of the mode relaxation rates.