Biaxial nematics: symmetries, order domains and field-induced phase transitions

TL;DR

This paper investigates the symmetry and order of biaxial nematic phases, proposing monoclinic symmetry supported by NMR evidence, and explores how microdomain structures influence field-induced phase transitions and measurements.

Contribution

It provides new evidence for monoclinic symmetry in biaxial nematics and discusses the role of microdomains and NMR techniques in understanding their order.

Findings

Monoclinic symmetry is supported over orthorhombic.

Deuterium NMR can differentiate symmetries.

Field-induced alignment involves biaxial and polar domains.

Abstract

We study the symmetry and the spatial uniformity of orientational order of the biaxial nematic phase in the light of recent experimental observations of phase biaxiality in thermotropic bent-core and calamitic-tetramer nematics. We present evidence supporting monoclinic symmetry, instead of the usually assumed orthorhombic. We describe the use of deuterium NMR to differentiate between the possible symmetries. We present the spatial aspects of biaxial order in the context of the cluster model, wherein macroscopic biaxiality can result from the field-induced alignment of biaxial and possibly polar domains. We discuss the implications of different symmetries, in conjunction with the microdomain structure of the biaxial phase, on the alignment of biaxial nematics and on the measurements of biaxial order.