Perspective: Emergent topologies in oxide superlattices

The ability to synthesize high-quality, complex-oxide heterostructures has created a veritable playground in which to explore emergent phenomena and exotic phases which arise from the interplay of spin, charge, orbital, and lattice degrees of freedom. Of particular interest is the creation of artificial heterostructures and superlattices built from two or more materials. Through such approaches, it is possible to observe new phases and phenomena that are not present in the parent materials alone. This is especially true in ferroelectric materials where the appropriate choice of superlattice constituents can lead to structures with complex phase diagrams and rich physics. In this article, we review and explore future directions in such ferroic superlattices wherein recent studies have revealed complex emergent polarization topologies, novel states of matter, and intriguing properties that arise from our ability to manipulate materials with epitaxial strain, interfacial coupling and interactions, size effects, and more. We focus our attention on recent work in (PbTiO3)n/(SrTiO3)n superlattices wherein exotic polar-vortex structures have been observed. We review the history of these observations and highlights of recent studies and conclude with an overview and prospectus of how the field may evolve in the coming years.