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, domain walls that separate regions of different polarizations are ubiquitous. We demonstrate that the π walls in films of the 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 arrangements of the twin disclinations generate kinks and antikinks, parting subdomains of opposite chirality, like the spin-up and spin-down in Ising chains. The hierarchical structures dictate that field-driven polar switching undergo two-step annihilations of the surface disclinations. These findings provide an insight for both comprehending other walls in the polar fluids and domain engineering crucial for advancing their nonlinear and optoelectronic applications.