Unconventional Polar Phenomena in Epitaxial Multiferroic Heterostructures

Crystal symmetry and boundary conditions in condensed-matter materials largely dictates their micro- and macroscopic properties and, in turn, their functionalities. A key challenge is to stabilize and engineer symmetry breaking and boundary conditions in polar materials. To this end, the intimate coupling between spin, charge, and lattice degrees of freedom in complex oxides provides a platform to manifest correlations between broken inversion symmetry, boundary condtions, and materials properties. Polar materials, such as ferroelectric and magnetoelectric materials, are a particularly pervasive example of broken inversion symmetry, were boundary conditions can drive materials response. Here, we show pathways by which symmetry in unconventional phenomena in polar materials can be engineered via epitaxial constraints. First, by engineering octahedral tilt distortions in epitaxial multiferroic superlattices, we demonstrate that electric (polar) fields can be used to both erase and introduce centrosymmetry, effectively erasing and writing ferroelectric order, and resulting in orders-of-magnitude changes in the nonlinear optical response, resistivity, and piezoresponse. Second, we show that epitaxial strain and heterostructure boundary conditions in multiferroics can be used to establish coupled multiferroic order and topological defects in ultrathin films.