Interplay between Spin-Orbit Coupling and Strong Correlation Effects: Comparison of Three Osmate Double Perovskites: Ba$_{2}A$OsO$_{6}$ ($A=$Na, Ca, Y)

High formal valence Os-based double perovskites are a focus of current interest because they display strong interplay of large spin orbit coupling and strong electronic correlation. Here we present the electronic and magnetic characteristics of a sequence of three cubic Os based double perovskites Ba$_{2}$$A$OsO$_{6}$, ($A=$Na, Ca, Y), with formal valences (charge states) of Os$^{+7}~(d^{1})$, Os$^{+6}(d^{2})$, Os$^{+5}(d^{3})$. For these first principles based calculations we apply an ``exact exchange for correlated electrons'' functional, with exact exchange applied in a hybrid fashion solely to the Os($5d$) states. While Ba$_{2}$NaOsO$_{6}$ is a ferromagnetic Dirac-Mott insulator studied previously, the other two show antiferromagnetic ordering while all retain the cubic (undistorted) structure. For comparison purposes we have investigated only the ferromagnetic ordered phase. A metal-insulator transition is predicted to occur upon rotating the direction of magnetization in all three materials, reflecting the central role of spin orbit coupling in these small gap osmates. Surprises arising from comparing formal charge states with the radial charge densities are discussed. Chemical shielding factors and orbital susceptibilities are provided for comparison with future nuclear magnetic resonance data.

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