Motivated by the novel 5d Mott insulator Sr2IrO4, we have studied the three-orbital Hubbard model with a spin-orbit coupling (SOC) term in a two-dimensional square lattice. We have found the transition from a paramagnetic metal to an antiferromagnetic (AF) insulator at U = UMI. A large SOC, which is characteristic of 5d electron systems, greatly reduces UMI and even small U can lead the system into the Mott insulator. Moreover, the Hund's coupling induces the anisotropy and stabilizes the in-plane AF order. These mechanisms are considered to be valid for Sr 2IrO4.
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