Interface-induced ferroelectric domains and charged domain walls in BiFeO3/SrTiO3 superlattices

Heterostructures comprising of polar and nonpolar perovskites, such as $\mathrm{LaAl}{\mathrm{O}}₃/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}₃$, have been extensively studied because of the discovery of a two-dimensional electron (or hole) gas [2DEG (or 2DHG)] at the interface. $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}₃/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}₃$ is chemically similar to $\mathrm{LaAl}{\mathrm{O}}₃/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}₃$ with polar/nonpolar interfaces. However, $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}₃$ exhibits spontaneous ferroelectric polarization, which leads to interesting effects in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}₃/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}₃$. Our theoretical simulations exhibit some different physical phenomena in this system, including interface-induced ferroelectric domains, monodirectional polarization, and 2DEGs (or 2DHGs) at the domain walls. Furthermore, it was identified that the observed 2DEGs (or 2DHGs) were spin polarized and exhibited ferromagnetism. The origin of the ferromagnetic 2DEGs (or 2DHGs) is discussed. These spin-polarized 2DEGs (or 2DHGs) observed in our $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}₃/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}₃$ model would be important for fundamental research and applications.