Polarization enhancement and heterointerfacial coupling in artificial perovskite superlattices

Bi- and tri-color superlattices comprised of BaTiO$_3$, SrTiO$_3$, and CaTiO$_3$ with compositionally-abrupt interfaces have been grown on atomically-flat SrRuO$_3$ bottom electrodes on (001) SrTiO$_3$ single crystals by pulsed laser deposition. These superlattices provide additional freedom in tuning the average lattice parameter and the structure’s physical properties. We found that locally asymmetric heterointerfaces (TiO$_6$- octahedra bound by different A-site cations) play a crucial role in the polarization enhancement due to elastic and electrostatic couplings at the interfaces. Such subtle effects, especially in tri-color superlattices, can be attributed to the broken inversion symmetry, although the effects are sometimes weak. A strong polarization enhancement is achieved by the proper balancing between two competing requirements: the ferroelectric layers must thick enough to contain a sufficient amount of non-interfacial TiO6 octahedra, but thin enough to remain fully strained. For a superlattice, this produces a maximum polarization as much as 50\% higher than that of a BaTiO$_3$ single film. Research sponsored by the U.S. Department of Energy under contract with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, as part of a BES NSET initiative.