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

Polar skyrmions are topologically nontrivial polarization textures that demonstrate exotic physical phenomena and novel memory applications. Thus far, these textures have primarily been reported in oxide-ferroelectric-based epitaxial heterostructures because their stabilization requires an elastic energy penalty from the epitaxial strains. Here, without the epitaxial-strain engineering, we discover polar skyrmion bubbles in stand-alone van der Waals ferroelectric CuInPS crystal through the combination of piezoelectric force microscopy, high-resolution transmission electron microscopy, and phase-field simulations. In a thick CuInPS 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.