Topological polar structures in ferroelectric oxide films

The continuous rotation of electric dipoles, which is inspired by unusual spin textures in magnetic materials, has been envisioned by theoretical modelings in last two decades. Although in electrically polar systems it was thought to be difficult to introduce continuous rotation of electric dipoles since similar Dzyaloshinskii–Moriya interaction like that of ferromagnets is still under study, external strains and interface depolarization fields have been then identified to be critical for rotating electric dipoles in nano-scale oxide films/superlattices. In this Perspective, we will briefly summarize the experimental finding of the newly identified topological polar structures and corresponding properties, such as polar flux-closure, vortex, skyrmion lattice, and meron. The critical importance of microscopy technologies, especially the advanced aberration-corrected transmission electron microscopy with ultra-high spatial resolutions, will be emphasized. Moreover, physical aspects to be addressed in the future, such as the strain maintenance and relaxation mechanisms of polar systems/superlattices, atomic maps of three-dimensional topological polar structures, and flexoelectricity-related properties, will be highlighted and envisioned.