Phase-field Simulations of Polar Topologies in K0.5Na0.5NbO3 Ferroelectric Thin Films

Polar topologies in ferroelectric materials have attracted significant attention in condensed matter physics and materials science due to their novel physical properties and immense potential for advanced ferroelectric devices. Current polar topologies are predominantly found in tetragonal or rhombohedral ferroelectrics. However, the polar topologies in orthorhombic ferroelectrics remain inadequately understood. Here, taking K0.5Na0.5NbO3 thin film as a model system, we systematically explore the possible polar topologies through manipulating misfit strain, film thickness, electric field, and electrical screening effect via phase-field simulations. We further establish phase diagrams of polar topologies, including vortices, spirals, periodic polarization waves, and skyrmions. Our results reveal that misfit strains can modulate the orientations of vortices, spirals, and periodic polarization waves, as well as the amplitudes of periodic polarization waves. Notably, a periodic in-plane electric field can reversibly switch the chirality of spirals. Moreover, tunable skyrmion density is attained via the coupling of the electrical screening effect with an external electric field. Our simulations also demonstrate the universality of Kittel's law in polar topologies. These results provide systematic theoretical guidance for discovering and manipulating polar topologies in orthorhombic ferroelectric materials, paving the way for rational design of topology-engineered ferroelectric microelectronic devices.