Two-dimensional polar metals in KNbO3/BaTiO3 superlattices: first-principle calculations

Polar metals, commonly defined by the coexistence of polar structure and metallicity, are thought to be scarce because free carriers eliminate internal dipoles that may arise owing to asymmetric charge distributions. By using first-principle electronic structure calculations, we explored the possibility of producing metallic states in the polar/nonpolar KNbO3/BaTiO3 superlattice (SL) composed of two prototypical ferroelectric materials: BaTiO3 (BTO) and KNbO3 (KNO). Two types of polar/nonpolar interfaces, p-type (KO)−/(TiO2)0 and n-type (NbO2)+/(BaO)0, which can be constituted into two symmetric NbO2/BaO–NbO2/BaO (NN-type) and KO/TiO2–KO/TiO2 (PP-type) SL, as well as one asymmetric KO/TiO2–NbO2/BaO (PN-type) SL. The spatial distribution of ferroelectric distortions and their conductive properties are found to be extraordinarily sensitive to the interfacial configurations. An insulator-to-metal transition is found in each unit cell of the symmetric interfacial SL models: one exhibiting quasi-two-dimensional n-type conductivity for NN-type SL, while the other being quasi-two-dimensional p-type conductivity for PP-type SL. The anisotropic coexistence of in-plane orientation of free carriers and out-of-plane orientation of ferroelectric polarization in KNO/BTO SL indicates that in-plane free carriers can not eliminate the out-of-plane dipoles. Our results provide a road map to create two-dimensional polar metals in insulating perovskite oxide SL, which is expected to promote applications of new quantum devices.