  Interface engineering in perovskite oxide superlattices has developed into a
flourishing field, enabling not only further tuning of the exceptional
properties, but also giving access to emergent physical phenomena. Here, we
reveal a new mechanism for enhancing the electric polarization by the
interface-induced oxygen octahedral tilts in BaTiO3/CaTiO3 superlattices. By
combining a novel genetic algorithm with density functional theory (DFT), we
predict that the true ground states in 1:1 and 2:2 BaTiO3/CaTiO3 superlattices
grown on SrTiO3 adopt Pc symmetry with a large proper electric polarization
(32.8{\mu}C/cm2 for 1:1 and 35.8 {\mu}C/cm2 for 2:2 superlattices), which is
even larger than that of bulk BaTiO3. The tilt of oxygen octahedron is found to
play a key role for the enhancement of out-of-plane polarization in 1:1
superlattices because it reduces greatly the rotation of oxygen octahedron
(out-of-phase) which significantly suppresses the out-of-plane polarization.
