Flexoelectrically Induced Polar Topology in Twisted SrTiO₃ Membranes.

Abstract

The recent realization of freestanding membranes of perovskite oxides has enabled their deterministic mechanical assembly into twisted homo bilayers. Twisted oxide interfaces feature a non-homogeneous pattern of shear strains determined by the atomic registry through the strong ionic-covalent bonding of this family of compounds. Twisted ferroelectrics open an unprecedented opportunity to tailor topological polar landscapes in a way determined by the flexoelectric coupling of polarization to the large strain gradients developing with the modulation of the moiré coincidence pattern. Yet, flexoelectricity is a universal phenomenon which may render polar landscapes in non-ferroelectric materials. Here we report a polar topology in twisted membranes of SrTiO₃ (STO), a paraelectric centrosymmetric compound. The polar landscape is generated by the flexoelectrically induced polarization driven by the strain modulation triggered by twisting, as also supported by machine-learned force fields based on first-principles calculations. We further show that the strain and polarization patterns in top and bottom layers are correlated in a way which breaks inversion and mirror symmetries as dictated by the emergent chirality imposed by twisting. This finding opens exciting opportunities for the exploration of novel physical effects and functionalities.