Slater and Mott Insulating States in Os- and Ir-Based Transitional Metal Oxides

The discovery of a novel J$_{\mathrm{eff}}=$1/2 electronic configuration and spin-orbit assisted insulating state in Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ has stimulated a fresh look at metal-insulator transitions where relativistic effects participate on an even footing with other energy scales such as crystal field splitting and electron-electron correlations. There are several view points on the origin of the insulating state in Sr$_{2}$IrO$_{4}$, but the most prominent is that spin-orbit coupling modifies the electronic configuration such that a Mott insulting state emerges despite the relatively modest electron-electron correlations within the 5$d$ orbitals. An alternative viewpoint is that magnetic effects enable the opening of the electronic gap giving rise to the insulating state or a Slater metal-insulator transition. Here we describe realizations of both Mott and Slater insulators in the context of Os- and Ir-based 5$d$ transition metal oxides. NaOsO$_{3}$, exhibits a continuous phase transition at 410 K where antiferromagnetism appears in conjunction with the onset of insulating behavior. A combination of neutron diffraction and magnetic resonant x-ray scattering enables the conclusion that G-type magnetic order occurs at the metal-insulator transition providing microscopic evidence that NaOsO$_{3}$ is the first three dimensional realization of a Slater insulator. On the other hand we have probed the robustness of the J$_{\mathrm{eff}}=$1/2 Mott insulating state though studies of Sr$_{2}$Ir$_{\mathrm{1-x}}$T$_{\mathrm{x}}$O$_{4}$ (T$=$Mn, Ru). For both Mn and Ru doping we find that despite qualitative changes in the magnetic order the J$_{\mathrm{eff}}=$1/2 electronic configuration remains robust. In particular, for Ru-doping the signatures of the J$_{\mathrm{eff}}=$1/2 state are observed for all concentrations where magnetic order is present. Finally, we have investigated Ca$_{4}$IrO$_{6}$ which appears to exhibit a nearly ideal J$_{\mathrm{eff}}=$1/2 state which is unperturbed by deviations from cubic crystal field level splitting.