Three-Dimensional Chiral Relaxation in Atomically Bonded Oxide Moire Superlattices

Twisted oxides have emerged as a promising platform for strongly correlated states as well as multiferroic and magnetic orders[1,2]. The development of freestanding oxide films now makes it possible to realize moiré-driven phenomena such as polarization vortices, charge-density modulations, and memristive switching. Because oxide interfaces are covalently bonded, their lattice relaxation should be significantly larger than in widely studied van der Waals moire superlattices, profoundly reshaping electronic structures and polar textures. However, this relaxation landscape and its property implications remain underexplored in prior work.

 

Here, we reveal three-dimensional chiral relaxation in a strongly bonded, twisted SrTiO₃ (STO) moiré superlattice using correlative in-plane and cross-sectional electron ptychography. The twisted interface, fabricated via a bicrystal technique, shows a large-area superlattice of alternating AA and AB domains, whose domain boundaries host rotational dislocation cores of opposite handedness. We further identify a preferential relaxation toward AA stacking over AB, reconstructs and alters the conventional periodic vortex-antivortex displacement field. The broken symmetry in the structural relaxation is corroborated by enhanced second harmonic generation compared with unbonded surfaces. The chiral relaxation extends ~6 nm into the bulk from the interface, following a smooth rotational gradient and forming a depth-dependent chiral texture. These results provide new insight into twist engineering in strongly coupled oxide systems.