Universality of Topological Vortex Domains in Layered Perovskites*

In numerous functional materials, such as steels, ferroelectrics and magnets, new functionalities can be achieved through the engineering of the domain structures, which are associated with the ordering of certain parameters within the material. The recent progress in technologies that enable imaging at atomic-scale spatial resolution has transformed our understanding of domain topology: along with simple stripe-like or irregularly shaped domains, intriguing vortex-type topological domain configurations often appear in complex materials. It turns out that vortex domains are rather common in layered perovskites such as Ca_3-xSr_xTi₂O_7. In Ca_3-xSr_xTi₂O_7, the vortex domain formation, associated with the existence of abundant Néel-type ferroelectric walls, results in the presence of numerous conducting charged walls. The relationship between two distinct vortex domains and the emergence of polar-antipolar order at domain walls will be also discussed.