Microscopic formation process of polar skyrmions and strain-dependent topological phase transitions in PbTiO3/SrTiO3 superlattices

Recent discoveries of polar skyrmions have garnered significant attention due to their unique properties beyond conventional polarization orders, requiring a comprehensive understanding of the stabilization mechanisms and topological transitions between other phases. However, the fundamental and critical question “How do polar skyrmions form?” still remains elusive. We provide the first detailed demonstration of the formation process of polar skyrmions and strain-dependent topological phase transitions in PbTiO₃/SrTiO₃ superlattices under various in-plane strain conditions. Our phase-field simulation revealed that in-plane strain stabilizes out-of-plane polarization, influencing the velocity of domain growth and the resulting topological phases. Under low in-plane strain, numerous polarization domains grow rapidly and connect to each other, forming a labyrinth phase. Conversely, under high in-plane strain, domains grow slowly and individually, forming a skyrmion bubble phase. Our results provide profound insights into the formation mechanism of skyrmions and the design principles of next-generation functional devices.