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5d transition-metal oxides have recently been revealed to give rise to novel electronic phases due to the interplay between strong spin-orbit coupling, modest electronic correlation and unique lattice topologies. Layered perovskite iridate Sr2IrO4, for example, was found to be a Jeff = ½ Mott insulator where strong spin-orbit coupling of Ir entangles the spin and orbital degree of freedom with including complex phase i. In such Jeff = ½ Mott insulators, magnetic coupling can be critically affected by the existence of complex phase. We studied two representative iridates, Sr2IrO4 and A2IrO3. Sr2IrO4 exhibits 2D Heisenberg antiferromagnetism even with strong spin-orbit coupling, whereas we propose that Li2IrO3 with a honeycomb network of Ir exemplifies Kitaev-Heisenberg antiferromagnet with a sizable ferromagnetic interaction. Besides Mott insulators, we also visited metallic systems in which strong spin-orbit coupling critically influences their nature. Orthorhombic perovskite SrIrO3 was found to be a semimetal unlike a half-filled Jeff = ½ metal naively expected. The semimetallic state is formed by the interplay between strong spin-orbit coupling and band crossings around the Fermi energy. |
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