The Biaxial Smectic-A* Phase -- A New Phase, Already But Unknowingly Discovered?

TL;DR

This paper analyzes the biaxial smectic-A* phase using Landau theory, revealing its helical structure and transition behaviors, and discusses how it might be mistaken for similar phases in experiments.

Contribution

It provides a theoretical analysis of the biaxial smectic-A* phase, highlighting its unique helical structure and transition characteristics, and suggests methods to distinguish it from similar phases.

Findings

Biaxial smectic-A* phase has a shorter helical pitch than Sm-C*.

The Sm-A_B*--Sm-C* transition can be first or second order.

Birefringence and electroclinic effects are similar to Sm-C*_alpha phase.

Abstract

The biaxial smectic-A* (Sm-A_B*) phase, appearing in the phase sequence Sm-A*--Sm-A*_B--Sm-C*, is analyzed using Landau theory. It is found to possess a helical superstructure with a pitch that is significantly shorter than the pitch of the Sm-C* helical superstructure. The Sm-A_B*--Sm-C* transition can be either 1st or 2nd order, and correspondingly there will be either a jump or continuous variation in the pitch. The behaviors of the birefringence and electroclinic effect are analyzed and found to be similar to those of a Sm-C*_alpha phase. As such, it is possible that the Sm-A_B* phase could be misidentified as a Sm-C*alpha phase. Ways to distinguish the two phases are discussed.