Abstract
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.
Highlights
INTERFACES ABOUNDComplex-oxide materials and their heterostructures have garnered enormous interest in the last decade because of the potential for and realization of exotic phenomena, including emergent interfacial conduction, magnetic order, superconductivity, new ferroelectric order, etc
One of the most prominent examples in this regard is the conductivity that emerges at the interfaces between two large bandgap insulating oxides, LaAlO3 and SrTiO3.4,5 Regardless of the system, it has been shown that the interfaces themselves and the subsequent coupling between adjacent layers are of utmost importance and govern the properties of the heterostructures
In zero-dimensional PbZr0.5Ti0.5O3 nanostructures, ab initio simulations predicted a phase transition that leads to the formation of a spontaneous toroidal moment in each (001); a structure that is characterized by a vortex-like configuration of electric dipoles.[39]
Summary
Theoretical studies predicted that at intermediate length scales (n and m = 10-20 unit cells), such superlattices could potentially support the formation of exotic topological structures including vortices and skyrmions as a result of the competition of different energies (i.e., elastic, electrostatic, and gradient).[39,40,41,42] For example, in zero-dimensional PbZr0.5Ti0.5O3 nanostructures, ab initio simulations predicted a phase transition that leads to the formation of a spontaneous toroidal moment in each (001); a structure that is characterized by a vortex-like configuration of electric dipoles.[39] This transition is driven by the finite size of the nanostructures and the strong depolarization fields that occur at their surfaces If such structures could be controlled and utilized in non-volatile ferroelectric memory devices, storage capacities could be orders of magnitude higher than devices using bulk ferroelectrics. The presence of the polar PbTiO3 layers, sandwiched between the dielectric SrTiO3 layers, leads to an effective frustration or competition that drives the system to a new instability and the formation of a novel polarization topology
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