Some of the most physically fascinating materials and, often, some of the most challenging systems to understand are found within the class of perovskite transition metal oxides. Perovskite-type oxides built of transition metal cations with 3d-electron valence states are now well known to exhibit strongly correlated phase behavior largely driven by the on-site Coulomb repulsion (U) between electrons. This results in a variety of emergent phenomena ranging from high temperature superconductivity to colossal magnetoresistive phases that stem fundamentally from a carrier-doped Mott insulating ground state.
As the 3d transition metal cations in these systems are replaced with their heavier 5d cousins, the larger spatial extent/overlap of the electronic radii and the correspondingly reduced U should naively destabilize this parent Mott state; however, surprisingly, a growing class of 5d transition metal oxides built from Ir4+ ions shows that this is not always the case. Instead, the interplay between the amplified spin-orbit interaction intrinsic to these 5d-electron iridates and their residual U conspires to stabilize a novel class of spin-orbit assisted insulators with a proposed Jeff=1/2 Mott insulating ground state.
The idea of this spin-orbit Mott state has been the focus of recent interest due to its potential of hosting a variety of new phases driven by correlated electron phenomena (such as high temperature superconductivity or enhanced ferroic behavior) in a strongly spin-orbit coupled setting. Currently, however, there remains little to no consensus regarding the relative importance of electron-electron interactions in governing the ground state properties of these spin-orbit Mott insulators—an essential ingredient for realizing many of their hoped-for properties.
Here I will present our group’s recent experimental work exploring one such spin-orbit driven Mott material (Sr3Ir2O7) with the ultimate goal of determining the relevance of U and electron correlation effects within the system’s ground state. Our results suggest that U is not only critical to the parent state’s insulating phase formation but also that it remains essential as in-plane carriers are introduced, resulting in an electronically phase separated ground state. I’ll argue that the resulting experimental picture of a doped spin-orbit Mott phase where correlations remain relevant largely validates many of the theoretical hopes of realizing novel electronic phases in this class of perovskite iridates.