Asymmetric ferroelectricity by design in atomic-layer superlattices with broken inversion symmetry

In atomic-layer superlattices constructed using three constituent phases, CaTiO3, SrTiO3, and BaTiO3, the stacking sequence of the atomic layers is found to control the symmetry of the high-temperature dielectric response. In such a superlattice when a nanostructured asymmetric strain is programmed into the lattice via the stacking order, the natural symmetry at high temperatures is removed and a polarized sample is obtained in which the polarization increases as the temperature is lowered. In contrast to a ferroelectric characterized by a bistable ground state with two equal and opposite electronic polarizations, our experiments show evidence of asymmetric ferroelectric correlations that set in when such a sample becomes hysteretic below a temperature Tx, with two unequal polarization states. We further show that both the magnitude and direction of this ferroelectric asymmetry can be controlled by the engineered atomic-layer stacking order and periodicity of the superlattice.