Na2IrO3 as a Novel Relativistic Mott Insulator with a 340-meV Gap

We study Na2IrO3 by angle-resolved photoemission spectroscopy, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t(2g) 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 Delta(gap) similar or equal to 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 similar or equal to 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 Na2IrO3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing. DOI: 10.1103/PhysRevLett.109.266406