"Tear-And-Stack" Twisted SrTiO₃ Moiré Superlattices for Precise Interfacial Reconstruction and Polar Topology

Oxide-based moiré superlattice is an emerging field for its exotic properties and abundant design freedom. However, due to the dangling bond and much stronger interlayer adhesion with the supporting substrate, fabricating twisted complex oxide is challenging, and an oxide moiré lattice with a clean interface remains elusive. Here, square moiré superlattices in twisted SrTiO₃ (STO) bilayer are constructed using a 2D like "tear-and-stack" method, achieving unprecedent control resolution and superior interface quality. Through depth-dependent atomic-scale analysis and electronic reconstruction, the upper and lower STO layers are found near the twisted interface exhibit opposite shear strain, evidencing a strong coupling confined within 2 unit cells (±0.8 nm) from the interface. The strain gradient of twisted bilayer STO gives rise to alternating clockwise and counter-clockwise polarization originating from the flexoelectric effect, leading to a large-scale array of polar vortex. This motivates to fabricate twisted STO bilayers with a freestanding parent layer as thin as 0.8 nm, in which a polar vortex is also confirmed. The "tear-and-stack" method is generic to create twisted moiré superlattices in a wide range of oxide material systems, and it demonstrates the feasibility of "2D-like" oxide twistronics.