Strain-tunable giant anisotropic conductivity in the polar metal of KNbO3/BaTiO3 ferroelectric superlattice

Anisotropy in crystals originates from different periodicity atomic arrangements along with different crystallographic directions, leading to different physical properties of the crystal in different directions, which is particularly pronounced in two-dimensional (2D) materials. In this work, by using first-principles calculations, we predicted the tunable giant electrical anisotropy of the KNbO3/BaTiO3 (KNO/BTO) ferroelectric superlattice (SL) between in-plane and out-of-plane directions through the in-plane strain-engineering. The anisotropy ratio of electrical conductivity and electronic thermal conductivity shows an exponentially increase when the KNO/BTO SL undergoes a strain from tensile to compressive, and their anisotropy ratio can be tuned from several times to over 105 at room temperature. In addition, the compressively strained KNO/BTO SL has a large out-of-plane Seebeck coefficient and a high anisotropic thermopower. This anisotropy of the KNO/BTO SL primarily derives from the proportion of electrons occupied by dxy and dxz/dyz orbitals and that proportion is highly dependent on the polarization intensity of the KNO/BTO SL. Moreover, the ferroelectric and ferromagnetic properties of the KNO/BTO SL can be tuned by strain due to the coexistence of polar structure and metallicity. Our results provide a road map for creating multifunctional materials at perovskite oxides interfaces, which may be favorable for designing new volatile memory devices with anisotropy-based ultrafast and low-power properties.