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.
Tenreiro et al. (Thu,) studied this question.