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Spin-Orbital Locking, Emergent Pseudo-Spin, and Magnetic order in Na2IrO3

In this talk, I shall discuss the nature of magnetic order in the honeycomb lattice Iridate Na2IrO3 is by considering trigonal crystal field effect and spin-orbit coupling.  I shall derive an effective Hamiltonian in terms of an emergent pseudo-spin-1/2, resulting from a spin-orbital locking, which is different from j_eff = 1/2 that is obtained when the spin-orbit coupling dominates. The resulting Hamiltonian is anisotropic and frustrated. Mean field theory suggests a ground state with 4-sublattice zig-zag magnetic order in the relevant parameter regime, in conformity with experiments. This approach contrasts with the recent proposals of topological insulator or a Heisenberg-Kitaev system for this material, which fail to explain the observed magnetic properties.