Chiral Moiré Polar States and Vortex Handedness in Twisted Oxide Membranes

The recent creation of freestanding perovskite-oxide membranes has enabled their deterministic assembly into twisted homobilayers. Such twisted oxide stacks circumvent the constraints of epitaxial growth, which fix the crystalline axes to those of the substrate. The resulting non-coherent atomic registry, coupled to the long-range ionic bonding of oxides, generates entirely new strain textures at the interface. In twisted ferroelectric membranes, this geometry unlocks a chiral degree of freedom, providing an unprecedented route to engineer topological polar textures through twist-induced lateral strain modulation mediated by flexoelectric coupling.

 

Flexoelectricity is a universal electromechanical response known to induce polar textures even in centrosymmetric insulators and semiconductors. Yet, its typically weak coupling has made the direct observation of flexoelectrically driven polar landscapes challenging. In this talk, we will present flexoelectric moiré polar topologies emerging in twisted SrTiO₃ bilayers— a quantum paraelectric that develops a polar response only at very low temperatures. First-principles simulations, informed by machine-learned force fields, reveal polar vortex arrays in striking agreement with experiment and demonstrate the stability of the vortex state. These results point to a flexoelectrically induced polar topology arising from a negative flexoelectric coefficient. We will further show that the chirality is set by the twist angle, and that vortices in the two layers acquire opposite handedness, dictated by the continuous torsional strain across the interface.