Morse Theory and Meron-Mediated Interactions Between Disclination Lines in Nematic Materials

• Published in: Physical Review X
• Main Authors: Joseph Pollard, Richard G. Morris
• Format: Article
• Language: English
• Published: American Physical Society 2025-06-01
• Online Access: http://doi.org/10.1103/lbbj-txnx

The topological understanding of nematic liquid crystals has traditionally centered on defects and their classification via homotopy invariants. Here, we draw upon several aspects of Morse theory to describe a finer approach to topology that characterizes the isotopy invariants within a given homotopy class. This approach unifies various disparate ideas in liquid crystal physics, including the homotopy and Volterra classifications of defects, the winding and self-linking of disclinations, and the Pontryagin-Thom construction. Specifically, we use singularity theory and dividing curves to characterize nematic textures via tomography, as well as to provide a full topological classification of the rewiring and crossing of disclination lines. This process reveals the central role that nonsingular but nontrivial topological solitons—so-called merons—play in mediating interactions between disclination lines. We also clarify the notion of the charge of a disclination line, give a simple method for computing it, and apply this to examples drawn from experiment. In particular, this method shows that restricting a disclination line’s charge modulo 2 is incorrect; we construct examples of disclinations with point defect charges 0 and -2, which have the same local structure but very different global structures, underscoring the need for a more nuanced topological understanding of nematic systems.