Q-tensor model of twist-bend and splay nematic phases

TL;DR

This paper develops a unified Q-tensor model to describe twist-bend and splay nematic phases, explaining their distinct elastic behaviors and phase transitions through elastic and flexoelectric effects.

Contribution

It introduces a single free energy framework capturing both phases by breaking degeneracy between elastic constants and incorporating flexoelectric coupling.

Findings

N-NTB transition driven by increased nematic order

NS transition driven by flexoelectric coupling

Elastic constants measurements support the model's mechanisms

Abstract

The twist-bend nematic phase (NTB) is characterized by a conically twisting director and by a dramatic softening of the bend elastic constant before the transition to NTB phase. In the recently found splay nematic phase (NS) the splay elastic constant tends to zero, resulting in a splay modulation perpendicular to the director. We model both phases with a single Q-tensor free energy including a term that breaks the degeneracy between the splay and bend elastic constant, and a flexoelectric term coupling the divergence of the Q-tensor with polarization. NTB or NS phase is obtained by a change of sign of one elastic parameter. Measured elastic constants show that N-NTB transition is mainly driven by the increase of the nematic order, while the NS transition is due to flexoelectric coupling.