Phase control and hidden order in oxide superlattices: dimensional and interfacial effects*

In electronically correlated systems, including many complex oxides, atomic layer heterostructuring can expose new phenomena – transition temperatures may be modified, order parameters may be strengthened or weakened, and new phases may emerge. Understanding the origin of these effects is vital for designing tailored materials as well as for improving theories of correlated physics. However, many overlapping and competing processes influence the emergent phases. In this talk, I discuss the distinct roles of dimensional confinement and interfacial coupling in correlated oxide heterostructures in the context of experiments on atomically layered NdNiO₃ superlattices. In our work, we systematically track the magnetic, charge, and orbital order parameters of NdNiO₃ and uncover “hidden” phases that arise as the thickness is reduced down to the single atomic layer limit. The layer-dependent phase diagram shows distinct behavior for single- and multilayer structures. In particular, although magnetic order persists down to two atomic layers of NdNiO₃, it becomes 2D in nature; furthermore, the transition to the magnetic and charge-ordered state decouples from the transition to the insulating state at these thicknesses, whereas they coincide in the bulk. In the single atomic layer case, all forms of order are found to vanish. Through first-principles and model calculations, we establish that reduced dimensionality plays the prominent role in the multilayer structures, while the phase behavior in the single-layer case is dominated by interfacial effects. We identify a unique interfacial reconstruction based on ionicity mismatch that leads to the observed unordered ground state.