Theoretical predictions—motivated by recent advances in epitaxial engineering—indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component super-lattices. Here we fabricate superlattices consisting of barium titanate (BaTiO_3), strontium titanate (SrTiO_3) and calcium titanate (CaTiO_3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO_3) layers. The strain in BaTiO_3 layers is fully maintained as long as the BaTiO_3 thickness does not exceed the combined thicknesses of the CaTiO_3 and SrTiO_3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO_3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO_3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.
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