arXiv subjects

Shengzhu Yi

Publications and source records attributed to Shengzhu Yi.

5 recordsLinked to original sources

arXiv2026

Hybrid Topological Defects in Ferroelectric Nematic Fluids

We investigate the evolution of topological defects in polar fluids driven by discrete inversion symmetry-breaking acrossthe nematic-to-ferroelectric nematic phase transition. Using photopatterned surface alignment to prescribe well-defined initial defect configurations in the nematic phase, we track their metamorphosis upon entering the ferroelectric phase. Through systematic comparisons between experiments and numerical simulations, we demonstrate that the resulting polar structures are generically hybrid, intertwining topological configurations of different dimensionalities. Specifically, we identify three basic hybrid defects evolving from precursor disclinations of net charge 0 and +1 (via pairs), and +1 (via single defects): domain walls terminated by surface disclinations, monopole-decorated domain walls, and meron-mediated boojum and monopole complexes. We further demonstrate that these stable hybrid polar defects are ubiquitous in the ferroelectric nematic phase, providing the basic motifs needed to understand, predict, and engineer complex topological textures in polar fluids.

cond-mat.soft↗

arXiv2025

Double Splay Nematic Order in Confined Polar Fluids

In this study, we demonstrate that when a ferroelectric nematic is confined between two glass plates coated with ionic polymers, a modulated phase emerges in a narrow temperature range between the nematic and ferroelectric nematic phases. This modulated phase emerges from the nematic phase in a continuous manner and then transforms into the ferroelectric nematic phase via a first-order transition upon cooling. Using optical microscopy, we provide compelling evidence that this modulated phase corresponds to the theoretically predicted double splay nematic phase. In this phase, splay deformations alternate in two orthogonal directions oriented at 45° to the substrate surfaces, creating a modulation wavelength that is twice the thickness of the cell. Our experiments with different ionic coatings reveal that only polymeric cationic coatings effectively promote the formation of this phase, highlighting the critical role of electrical screening. These findings not only confirm the existence of the double splay nematic phase but also provide insights into the distinctive topological defects of this phase in confined geometries.

cond-mat.soft↗

arXiv2024

Half-integer Topological Defects Paired via String Micelles in Polar Liquids

Ferroelectric nematic (NF) liquid crystals present a compelling platform for exploring topological defects in polar fields, while their structural properties can be significantly altered by ionic doping. In this study, we demonstrate that doping the ferroelectric nematic material RM734 with cationic polymers enable the formation of polymeric micelles that connect pairs of half-integer topological defects. Polarizing optical microscopy reveals that these string defects exhibit butterfly textures, featured with a two-dimensional polarization field divided by Néel-type kink-walls into domains exhibiting either uniform polarization or negative splay and bend deformations. Through analysis of electrophoretic motion and direct measurements of polarization divergences, we show that the string micelles are positively charged and their side regions exhibit positive bound charges. To elucidate these observations, we propose a charge double layer model for the string defects: the positive charged cationic polymer chains and densely packed RM734 molecules form a Stern charge layer, while small anionic ions and positive bound charges constitute the charge diffusion layer. Notably, our experiments indicate that only cationic polymer doping effectively induces the formation of these unique string defects. These findings enhance our understanding of ionic doping effects and provide valuable insights for engineering polar topologies in liquid crystal systems.

cond-mat.soft↗

arXiv2024

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

Ferroelectric nematic liquid crystals are polar fluids characterized by microscopic orientational ordering and macroscopic spontaneous polarizations. Within these fluids, walls that separate domains of different polarizations are ubiquitous. We demonstrate that the π walls in films of polar fluids consist of twin half-integer surface disclinations spaced horizontally, enclosing a subdomain where the polarization exhibits left- or right-handed π twists across the film. The degenerate geometric configurations of these twin disclinations give rise to kinks and antikinks, effectively partitioning subdomains of opposite chirality like Ising chains. The hierarchical topological structures dictate that field-driven polar switching entails a two-step annihilation process of the disclinations. These findings serve as a cornerstone for comprehending other walls in ferroelectric and ferromagnetic materials, thereby laying the base for domain engineering crucial for advancing their nonlinear and optoelectronic applications.

cond-mat.soft↗

arXiv2023

Line defects in nematic liquid crystals as charged superelastic rods with negative twist--stretch coupling

Topological defects are a ubiquitous phenomenon in diverse physical systems. In nematic liquid crystals (LCs), they are dynamic, physicochemically distinct, sensitive to stimuli, and are thereby promising for a range of applications. However, our current understanding of the mechanics and dynamics of defects in nematic LCs remain limited and are often overwhelmed by the intricate details of the specific systems. Here, we unify singular and nonsingular line defects as superelastic rods and combine theory, simulation, and experiment to quantitatively measure their effective elastic moduli, including line tension, torsional rigidity, and twist--stretch coefficient. Interestingly, we found that line defects exhibit a negative twist--stretch coupling, meaning that twisted line defects tend to unwind under stretching, which is reminiscent of DNA molecules. A patterned nematic cell experiment further confirmed the above findings. Taken together, we have established an effective elasticity theory for nematic defects, paving the way towards understanding and engineering their deformation and transformation in driven and active nematic materials.

cond-mat.soft↗