De Vries Behavior in Smectics near a Biaxiality Induced Smectic A - Smectic C Tricritical Point

TL;DR

This paper presents a generalized Landau theory explaining how biaxiality influences the nature of the smectic A to C transition, revealing de Vries behavior and birefringence growth near a tricritical point.

Contribution

It introduces a theoretical framework linking biaxiality, orientational order, and transition types, explaining de Vries behavior and birefringence changes in smectics.

Findings

De Vries behavior occurs with small orientational order near tricriticality.

Birefringence increases rapidly near the tricritical point.

Layer spacing contraction is unusually small in de Vries transitions.

Abstract

We show that a generalized Landau theory for the smectic A and C phases exhibits a biaxiality induced AC tricritical point. Proximity to this tricritical point depends on the degree of orientational order in the system; for sufficiently large orientational order the AC transition is 3D XY-like, while for sufficiently small orientational order, it is either tricritical or 1st order. We investigate each of the three types of AC transitions near tricriticality and show that for each type of transition, small orientational order implies de Vries behavior in the layer spacing, an unusually small layer contraction. This result is consistent with, and can be understood in terms of, the "diffuse cone" model of de Vries. Additionally, we show that birefringence grows upon entry to the C phase. For a continuous transition, this growth is more rapid the closer the transition is to tricriticality. Our model also predicts the possibility of a nonmontonic temperature dependence of birefringence.