Na 2 IrO 3 as a Novel Relativistic Mott Insulator with a 340-meV Gap

Abstract

We study Na 2 IrO 3 by angle-resolved photoemission spectroscopy, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5 d - t 2 g manifold highlights the importance of structural distortions and spin-orbit (SO) coupling in driving the system closer to a Mott transition. We detect an insulating gap Δ gap ≃ 340

meV which, at variance with a Slater-type description, is already open at 300 K and does not show significant temperature dependence even across T N ≃ 15

K . An LDA analysis with the inclusion of SO and Coulomb repulsion U reveals that, while the prodromes of an underlying insulating state are already found in LDA + SO , the correct gap magnitude can only be reproduced by LDA + SO + U , with U = 3

eV . This establishes Na 2 IrO 3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing.