Ferroelectric and anti-ferroelectric coupling in superlattices of paraelectric perovskites at room temperature

Results from dielectric and structural measurements on epitaxial SrTiO$_3$/BaZrO$_3$ superlattices reveal properties that cannot be explained simply in terms of those measured on single films of the constituent materials. For large superlattice periodicities (20/20 and 38/38 structures, i.e. samples in which each SrTiO3 and BaZrO3 layer are 20 or 38 unit cells thick, respectively), the capacitance-voltage curves indicate room-temperature ferroelectricity. For smaller periodicities (7/7 and 15/15), anti-ferroelectric-type behavior is observed, suggesting strong coupling between individual polar layers. This is consistent with recent second-harmonic generation results [A.Q. Jiang et al., J. Appl. Phys. 93, 1180 (2003)] of ordering in SrTiO$_3$/BaTiO$_3$ superlattices. However, both constituents of the structures investigated here are paraelectric. Strain-induced room-temperature ferroelectricity in SrTiO$_3$ and distance-dependent coupling between these layers are proposed as mechanisms leading to the observed behavior.