Na2IrO3 as a spin-orbit-assisted antiferromagnetic insulator with a 340 meV gap

TL;DR

Na2IrO3 is identified as a Mott-like insulator with a 340 meV gap, where spin-orbit coupling and Coulomb interactions are crucial for its insulating state, stable across a wide temperature range.

Contribution

The study demonstrates that Na2IrO3's insulating gap arises from combined spin-orbit and Coulomb effects, establishing it as a novel correlated insulator.

Findings

Insulating gap of 340 meV detected at 300 K.

Gap remains stable across the Néel temperature.

LDA+SO+U calculations reproduce the experimental gap.

Abstract

We study Na2IrO3 by ARPES, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t2g manifold highlights the importance of structural distortions and spin-orbit coupling (SO) in driving the system closer to a Mott transition. We detect an insulating gap {\Delta}_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 Na2IrO3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing.