Fig. 1 Schematic diagram of composition classifications of ferroelectric superlattices

Fig. 2 Schematic diagrams of topological polar structures found in ferroelectric/paraelectric superlattices[28] (a, b) Maps of the polar atomic displacement vectors showing the flux-closure array in the PbTiO3 layer; (c) Schematics of four newly identified topological polar structures in ferroelectric films

Fig. 3 Position dependence of average net polarization and oxygen vacancy formation energy calculated from the first principles when an oxygen vacancy is introduced into the (SrTiO3)9/(SrRuO3)1 superlattice[56] (a) Average net polarization; (b) Oxygen vacancy formation energy

Fig. 4 STEM images and ferroelectric properties of the (SrTiO3)25/(SrRuO3)2 superlattice[59] (a, b) Low-magnification and high-magnification HAADF-STEM images; (c-g) Atomically resolved EDS mappings; (h-j) Polar vector distribution; (k-m) Ferroelectric properties

Fig. 5 BaTiO3/SrTiO3/CaTiO3 tricolor superlattices[10] (a) Cross-sectional atomic number (Z)-contrast scanning transmission electron microscopy (Z-STEM) image and atomic structure diagram; (b, c) P-E hysteresis loops, lattice constants and polarizations with different period thicknesses Colorful figures are available on website

Fig. 6 Polarization properties of the (BaTiO3)n/(SrTiO3)n superlattices under different period thicknesses[90] (a) BaTiO3 c-axis parameter and remnant polarization as a function of the number (n) of unit cells in (BaTiO3)n/(SrTiO3)n superlattice; (b) Remnant polarization as a function of the c-axis parameter of BaTiO3; (c) Sketch of possible ferroelectric domains with different superlattice period thicknesses 1 Å = 0.1 nm

Fig. 7 180° domain structures in the (PbTiO3)n/(SrTiO3)n superlattices[95] (a) Schematic diagram of the 180° domain structure; (b) Reciprocal space map; (c) Domain periodicities along the [100] direction as a function of PbTiO3 layer thickness 1 Å = 0.1 nm

Fig. 8 Electrical properties of the BaTiO3/Pb(Zr0.52Ti0.48)O3 ferroelectric superlattice[9] (a) Schematic diagram of space charges accumulation; (b) Dielectric properties; (c) Leakage current curves; (d) P-E hysteresis loops

Fig. 9 BaTiO3/LaNiO3[61] and PbZr0.52Ti0.48O3/SrRuO3[62] superlattices with different metallic layer thicknesses (a, b) P-E hysteresis loops and leakage current curves of the BaTiO3/LaNiO3 superlattices with different LaNiO3 thicknesses; (c) TEM, (d) HRTEM images and (e) P-E hysteresis loops of the PbZr0.52Ti0.48O3/SrRuO3 superlattices with different SrRuO3 thicknesses Colorful figures are available on website