Creating Ferroelectricity and Ultrahigh-Density Polar Skyrmion in Paraelectric Perovskite Oxide Monolayers by Moiré Engineering.

Atomic-scale polar topologies such as skyrmions offer important potential as technological paradigms for future electronic devices. Despite recent advances in the exploration of topological domains in complicated perovskite oxide superlattices, these exotic ferroic orders are unavoidably disrupted at the atomic scale due to intrinsic size effects. Here, based on first-principles calculations, we propose a new strategy to design robust ferroelectricity in atomically thin films by properly twisting 2 monolayers of centrosymmetric SrTiO. Surprisingly, the emerged polarization vectors curl in the plane, forming a polar skyrmion lattice with each skyrmion as small as 1 nm, representing the highest polar skyrmion density to date. The emergent ferroelectricity originates from strong interlayer coupling effects and the resulting unique strain fields with obvious ion displacements, contributing to electric polarization comparable to that of PbTiO. Moreover, we observe ultraflat bands (band width of less than 5 meV) at the valence band edge across a wide range of twist angles, which show widths that are smaller than those of common twisted bilayers of 2-dimensional materials. The present study not only overcomes the critical size limitation for ferroelectricity but also reveals a novel approach for achieving atomic-scale polar topologies, with important potential for applications in skyrmion-based ultrahigh-density memory technologies.