iridates and the electric control of the physics properties*

The unique competition between spin-orbit interactions (SOI) and Coulomb correlation (U) in 5d elements and their compounds drives unusual physical behaviors such as j_eff=1/2 Mott insulating state. Due to the entanglement of spin and orbital degrees of freedom, the form of magnetic interactions depends closely on the underlying lattice geometry. In the case of a 180◦ Ir-O-Ir bond, the Hamiltonian is governed by an isotropic Heisenberg term plus a weak dipolar-like anisotropy term due to Hund’s coupling, while for a 90◦ bond the anisotropic term due to the off-diagonal hopping matrix results in a quantum analog of the compass model. The strong SOI limit assumes local cubic symmetry of the IrO₆ octahedra, which is rare in real materials. I will talk about the magnetic ground states of the square-lattice Sr₂IrO₄, honeycomb-lattice Na₂IrO₃, and the triangle lattice Ca₂Sr₂IrO₆. Our latest work of controlling the physical properties using electric current will also be discussed.