Nature of nematic to twist bend nematic phase transition

TL;DR

This paper investigates the phase transition from nematic to twist-bend nematic phases, revealing mean field critical behavior and the formation of helices influencing birefringence over a broad temperature range.

Contribution

It provides experimental evidence and analysis showing the mean field nature of the nematic to twist-bend transition and the role of helical fluctuations.

Findings

Critical exponents consistent with mean field theory

Observation of helices above transition temperature

Large correlation length of helical fluctuations

Abstract

The critical behavior at the transition from uniform nematic to twist-bend modulated nematic phase is revealed and shown to be well explained by the mean field approximation. The study was performed on a group of materials that exhibit an unusually broad temperature range of the nematic phase above the twist-bend modulated nematic phase, so the critical range in which the order parameter fluctuations are strong and can be experimentally observed is wide. The formation of instantaneous helices is observed already several degrees above the transition temperature, strongly influencing the birefringence of the system. The analysis of a critical part of the birefringence changes provides a set of critical exponents that are consistent with the mean field approximation, indicating a large correlation length of helical fluctuations in the nematic phase.