Twist-angle-engineered ferroelectric vortex lattice in freestanding BaTiO3 bilayers

Abstract

Realizing isolated ferroelectric topological states is challenging, as polar vortices typically form continuous, coupled networks. Here, we demonstrate that tuning the twist angle (θ) in freestanding BaTiO3 bilayers resolves this issue. Leveraging flexoelectric coupling to moiré strain gradients, we map a twist-angle-dependent topological phase diagram using atomic-resolution scanning transmission electron microscopy, with piezoresponse force microscopy confirming the preserved macroscopic ferroelectric response. By modulating θ, the system evolves from diffuse half-vortices to ordered homochiral vortex lattices and ultimately to bound vortex–antivortex pairs. At an optimal angle of θ = 7°, the polar textures are structurally pinned at moiré saddle points (S-sites), stabilizing a long-range ordered array of homochiral vortices. Geometrically compartmentalized by surrounding AA/AB domains, these discrete vortices exhibit suppressed inter-vortex crosstalk. Ultimately, structural engineering via the twist angle provides a scalable platform for generating spatially isolated, weakly coupled topological units for individually addressable oxide nanoelectronics.