# A New Twist on the Electroclinic Critical Point: Type I and Type II Smectic $C^*$ Systems

## TL;DR

This paper classifies and analyzes the electroclinic effect in ferroelectric liquid crystals with a first order Sm-$A^*$--Sm-$C^*$ transition, revealing Type I and Type II behaviors with distinct phase diagrams and critical phenomena.

## Contribution

It introduces a classification of the electroclinic effect into Type I and Type II systems, detailing their phase behavior and mapping phase boundaries in a comprehensive parameter space.

## Key findings

- Type I systems have an achiral low-tilt and high-tilt Sm-$C$ critical point.
- Type II systems exhibit a reentrant Sm-$C$--Sm-$C^*$ phase sequence.
- Phase diagrams include multicritical, tricritical points, and reentrant sequences.

## Abstract

This analysis of the electroclinic effect in ferroelectric liquid crystals with a first order Smectic-$A^*$--Smectic-$C^*$ (Sm-$A^*$--Sm-$C^*$) transition, shows they can be either Type I or Type II. In temperature--field parameter space Type I systems exhibit a macroscopically achiral (in which the Sm-$C^*$ helical superstructure is expelled) low-tilt (LT) Sm-$C$--high-tilt (HT) Sm-$C$ critical point, which terminates a LT Sm-$C$--HT Sm-$C$ first order boundary. This boundary extends to an achiral-chiral triple point where the achiral LT Sm-$C$ and HT Sm-$C$ phases coexist with the chiral Sm-$C^*$ phase. In Type II systems the critical point, triple point, and first order boundary are replaced by a Sm-$C^*$ region, between LT and HT achiral Sm-$C$ phases, at low and high fields respectively. When the field is ramped up, the Type II system displays a reentrant Sm-$C$--Sm-$C^*$-Sm-$C$ phase sequence. A discontinuity in the tilt of the optical axis at each of the two phase transitions means the Type II system is tristable. The system is Type I or Type II depending on the ratio of two length scales, one is the zero-field Sm-$C^*$ helical pitch, the other depends on the latent heat at the zero-field first order Sm-$A^*$--Sm-$C^*$ transition. A system could be experimentally tuned by varying enantiomeric excess, between Type I and Type II behavior. We also show this Type I vs Type II behavior is the Ising universality class analog of Type I vs Type II behavior in XY universality class systems. Lastly, we make a complete mapping of the phase boundaries in temperature--field--enantiomeric excess parameter space (not just near the critical point) which shows a variety of interesting features, including a multicritical point, tricritical points and a doubly reentrant Sm-$C$--Sm-$C^*$-Sm-$C$--Sm-$C^*$ phase sequence.