Landau theory for helical nematic phases

TL;DR

This paper develops a Landau theory to describe the phase transition from nematic to helical nematic phases in liquid crystals, aligning well with experimental data and predicting a fluctuation-driven first-order transition.

Contribution

It introduces a Landau phenomenological model for helical nematic phases and analyzes the impact of fluctuations on the nature of the phase transition.

Findings

Mean field theory predicts a continuous transition.

Fluctuations induce a first-order transition.

A new Goldstone mode is proposed for the low-temperature phase.

Abstract

We propose Landau phenomenology for describing the phase transition from the conventional nematic into the conical helical orientationally non-uniform structure recently identified in liquid crystals formed by "banana"-shaped molecules. The mean field predictions are mostly in agreement with experimental data. Based on the analogy with de Gennes model, we argue that fluctuations of the order parameter turn the transition to the first order phase transition rather than continuous one predicted by the mean-field theory. This conclusion is in agreement with experimental observations. We discuss the new Goldstone mode to be observed in the low-temperature phase.