Semi-metallic ground state of hyper-kagome iridate Na₃Ir₃O₈ produced by a competion of molecular orbital split and spin-orbit coupling

A new complex iridium oxide Na₃Ir₃O₈ with the chiral hyper-kagome lattice of Ir ions was synthesized. The average valence of Ir is 4.33+ and, therefore, Na₃Ir₃O₈ can be viewed as a doped analogue of the hyper-kagome spin liquid Na₄Ir₃O₈ with Ir⁴⁺. The transport measurements, combined with the electronic structure calculations, indicate that the ground state of Na₃Ir₃O₈ is a low carrier density semi-metal. We argue that the semi-metallic state is produced by a competition of the molecular orbital splitting of t_2g orbitals on Ir₃ triangles with strong spin-orbit coupling of heavy Ir ions.