Prediction of polarization vortices, charge modulation, flat bands, and moiré magnetism in twisted oxide bilayers

The recent surge of interest in moir\'e superlattices of twisted van der Waals compounds has spotlighted the emergence of unconventional superconductivity and novel electronic phases. However, the range of moir\'e phenomena can be dramatically expanded by incorporating complex oxide materials into twisted heterostructures. In this study, motivated by the recent breakthroughs in the synthesis of freestanding oxide membranes, we explore the emergent structural and electronic properties of twisted oxide bilayers. We focus on the classic perovskite oxide, $\mathrm{SrTi}{\mathrm{O}}_{3}$, and design $\mathrm{SrTi}{\mathrm{O}}_{3}$ bilayers with a relative twist between the individual layers. Using density functional theory calculations, we predict the appearance of vortex-antivortex polarization patterns at the interface of the $\mathrm{SrTi}{\mathrm{O}}_{3}$ bilayers driven by twist. We also predict charge modulation of the interfacial Ti ions induced by varying local coordination, which follow the moir\'e pattern. Furthermore, we forecast the emergence of flat bands at large twist angles and the associated localized electronic states with moir\'e-periodic charge density, originating from the interlayer bonding effects resulting in the formation of dangling bonds. Finally, we predict that hole doping induces unconventional ${d}^{0}$ magnetism in otherwise nonmagnetic $\mathrm{SrTi}{\mathrm{O}}_{3}$, driven by the exchange splitting of the high-density O-$p$ bands and producing the spin density with moir\'e periodicity. These results demonstrate a broad landscape of emergent phenomena that may occur in moir\'e-engineered oxide heterostructures showing far-reaching perspectives of these material systems for further fundamental studies and potential applications.