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Microscopic theory of magnetism in Sr3Ir2O7

An intriguing idea of spin-orbit Mott insulator has been proposed to explain magnetic insulating behavior in various iridates. This scenario relies on the strength of the spin-orbit coupling being comparable to electronic correlations, and it is not a priori obvious whether this picture is valid for all iridates. In particular, Sr3Ir2O7 exhibits a small charge gap and magnetic moment compared to Sr2IrO4, questioning the validity of such hypothesis. To understand the microscopic mechanism for magnetism in Sr3Ir2O7, we construct a tight binding model taking into account the full t2g- orbitals, the staggered rotation of the local octahedra, and the bilayer structure. The bands near the Fermi level are mainly characterized by the total angular momentum Jeff = 1/2, except below the Γ point, supporting a reasonably strong spin-orbit coupling picture. A first order transition to a collinear antiferromagnet via multi-orbital Hubbard interactions is found within the mean field approximation. The magnetic moment jump at the transition is consistently smaller than Sr2IrO4, originated from the underlying band structure of a barely band insulator. Given the small charge gap and moment observed in Sr3Ir2O7, the system is close to a magnetic transition. A comparison to a spin-model is presented and connection to the Mott insulator is also discussed.