Polarization Vortices, Charge Modulation, Flat Bands, and Moiré Magnetism in Twisted SrTiO₃Bilayers

The discovery of flat bands and unconventional superconductivity in twisted bilayer graphene has led to the emerging field of “twistronics.” While most studies have focused on van der Waals materials, moiré superlattices in oxide heterostructures may be more appealing due to stronger interlayer coupling and variable local atomic registries at twisted oxide interfaces. Motivated by recent advances in the synthesis of freestanding oxide membranes, we study the emergent properties of twisted SrTiO₃ bilayers using density functional theory. Our calculations predict vortex-antivortex displacement patterns of Ti ions, forming a chiral polar phase due to flexoelectric coupling. Significant atomic reconstruction at the interface is induced by twist, which leads to interfacial charge modulation. At large twist angles, we predict the emergence of flat electronic bands with moiré-periodic charge localization, facilitated by Ti-O dangling bonds. Finally, we find that hole doping induces unconventional d⁰ magnetism, arising from spin splitting of high-density O-p bands and resulting in moiré-periodic spin density. These findings suggest that twisted oxide bilayers can host novel electronic, topological, and magnetic phases, expanding the scope of twistronics beyond van der Waals materials.