Discovery of Ultrasmall Polar Merons and Rich Topological Phase Transitions: Defects Make 2D Lead Chalcogenides Flexible Topological Materials

Abstract Atomic‐scale polar topological configurations, such as skyrmions and merons, have garnered enormous interest due to their rich emergent physical phenomena and promising applications in next‐generation electronics. Despite recent progress in the exploration of 2D ferroelectrics, isolated polar topological structures in 2D lattices have not yet been explored. Here, an original design principle is proposed to remove the point group limit for polar structures while achieving atomic‐scale polar topological structures in non‐ferroelectric monolayers caused by defects in 2D materials. The first‐principles calculations show that an isolated polar meron with a diameter < 3.0 nm is generated in the deficient lead chalcogenide monolayer, and its formation is attributed to the synergic effects of vacancy‐induced radial atomic displacements and symmetry reduction in 2D materials. The emergent polar meron can transform to rich topological configurations under external stimuli or by manipulation of the defect concentrations. Furthermore, this strategy of atomic‐scale symmetry breaking via point defect engineering can be applied to a wide variety of 2D materials to induce polar topological structures. This work generalizes the polar topology from perovskite oxides to 2D materials, facilitating exciting opportunities to create high‐density topological configurations that enable the exploration of meron/skyrmion‐based functional nanodevices.