Chiral π Domain Walls Composed of Twin Half-Integer Surface Disclinations in Ferroelectric Nematic Liquid Crystals

Academic Background

π domain walls in ferroelectric materials are interfaces that separate regions of different polarizations. Their structures are not only of fundamental interest but also hold practical importance in many applications. Ferroelectric nematic liquid crystals are polar fluids characterized by microscopic orientational ordering and macroscopic spontaneous polarizations. Unlike traditional ferroelectric crystals, ferroelectric nematic liquid crystals possess continuous translational symmetry and exhibit unique properties such as low driving fields, high optical nonlinear responses, and polar topological structures. These characteristics are not only scientifically significant but also hold potential value in nonlinear optics and optoelectronic applications.

Although extensive research has been conducted on π domain walls in solid ferroelectric materials, their internal structures in fluid systems remain incompletely understood. In particular, the internal structure of π domain walls in ferroelectric nematic liquid crystals and their dynamic behaviors under electric fields have not been fully explained. This study aims to reveal the topological structure of π domain walls in ferroelectric nematic liquid crystals and their polarization switching behavior under electric fields, providing new insights into understanding domain wall structures in polar fluids and their applications.

Source of the Paper

This paper was co-authored by Shengzhu Yi, Zening Hong, Zhongjie Ma, Chao Zhou, Miao Jiang, Xiang Huang, Mingjun Huang, Satoshi Aya, Rui Zhang, and Qi-Huo Wei, and was published in the Proceedings of the National Academy of Sciences (PNAS) on December 19, 2024. The authors are affiliated with the Southern University of Science and Technology, the Hong Kong University of Science and Technology, South China University of Technology, and other institutions.

Research Process and Results

1. Experimental Design and Sample Preparation

The research team first spin-coated polyimide films on glass substrates and induced uniaxial alignment through mechanical rubbing. The treated substrates were then assembled into liquid crystal cells with thicknesses ranging from 1 to 10 micrometers. The cells were filled with two ferroelectric nematic liquid crystal materials: RM734 and DIO. These materials transitioned from the isotropic phase to the ferroelectric nematic phase during the cooling process.

2. Observation and Characterization of π Domain Walls

Using polarized optical microscopy, the researchers observed the formation of stripe structures with alternating polarizations in the ferroelectric nematic liquid crystals, with the boundaries between the stripes being π domain walls. The π domain walls consisted of two parallel lines, one bright and one dark. By applying slight pressure, the researchers found that the distance between these two lines changed, indicating that the π domain walls were composed of twin lines located at the two surfaces.

Further studies revealed that the subdomains within the π domain walls exhibited a π twist in polarization, meaning that the polarization underwent a left-handed or right-handed twist across the thickness of the liquid crystal cell. This twisted topological structure significantly influenced the polarization switching behavior under electric fields.

3. Topological Structure and Dynamic Behavior

Through numerical simulations, the researchers proposed that the π domain walls were composed of two surface disclination lines separated horizontally, forming a subdomain with a π twist in polarization. This topological structure led to a two-step polarization switching process under electric fields: first, the disclination lines on one surface approached each other and annihilated, forming a twisted polarization region; then, the disclination lines on the other surface also annihilated, ultimately aligning the polarization of the entire region with the electric field.

Additionally, the researchers observed the formation of kinks and antikinks along the π domain walls. These topological excitations formed on the π domain walls, separating subdomains with opposite chirality. The dynamic behavior of kinks and antikinks was analogous to the spin-flip process in the one-dimensional Ising model.

4. Comparison of Experimental Results and Theoretical Models

Experimental data showed that the width of the π domain walls and the separation between the twin lines were linearly dependent on the thickness of the liquid crystal cell, which differed from Kittel’s law observed in solid ferroelectric materials. This deviation could be explained by the splay deformation in ferroelectric nematic liquid crystals, which drives the ferroelectric-ferroelastic phase transition through flexoelectric coupling.

Conclusions and Significance

This study revealed the topological structure of π domain walls in ferroelectric nematic liquid crystals and their dynamic behavior under electric fields. The π domain walls are composed of twin half-integer surface disclination lines, forming a subdomain with a π twist in polarization. This topological structure significantly influences the polarization switching behavior. The research provides new insights into understanding domain wall structures in polar fluids and offers theoretical support for domain engineering in nonlinear optics and optoelectronic applications of ferroelectric nematic liquid crystals.

Research Highlights

1. Revealing the Topological Structure: For the first time, the study revealed that π domain walls in ferroelectric nematic liquid crystals are composed of twin half-integer surface disclination lines.
2. Explaining Dynamic Behavior: The study elucidated the two-step polarization switching mechanism of π domain walls under electric fields.
3. Combining Experiment and Theory: The topological structure and dynamic behavior of π domain walls were validated through a combination of experimental observations and numerical simulations.
4. Application Potential: The research provides new ideas for domain engineering in nonlinear optics and optoelectronic applications of ferroelectric nematic liquid crystals.

Other Valuable Information

The study also proposed future research directions, including the exploration of other types of domain walls (such as half π domain walls and 2π domain walls) and the investigation of the electrical conductivity of domain walls under electric fields. These studies will further deepen the understanding of domain wall structures in ferroelectric nematic liquid crystals and provide theoretical support for their applications in novel optoelectronic devices.