Observation of switchable polar skyrmion bubbles down to the atomic layers in van der Waals ferroelectric CuInP2S6

flake of over -100 nm, skyrmion bubbles feature an elliptical hedgehog-like state with center-divergent or center-convergent configurations. Progressively thinning the flake thickness to -8 nm allows a topological transition from elliptical to circular skyrmionic patterns. Interestingly, the skyrmions can be switched with the change in helicity by probe-applied electrical and mechanical stimuli, which is distinct from the creation and annihilation of other reported skyrmions. Both theoretical and experimental data proves that the formation and thickness-dependence of skyrmion textures primarily stem from charge-related energy penalty. This work opens up a new material system (i.e., two-dimensional layered ferroionic materials) for exploring uncharted polar-topology physics and prospective neuromorphic devices.