Na2IrO3 as a molecular orbital crystal

TL;DR

Na2IrO3 is best described as a molecular orbital crystal with quasi-molecular orbitals dominating its electronic structure, challenging previous models based on spin-orbit coupling and Heisenberg-Kitaev interactions.

Contribution

This work introduces the QMO model for Na2IrO3, highlighting the importance of quasi-molecular orbitals over traditional spin-orbit coupling models.

Findings

Na2IrO3's insulating behavior is explained by the QMO model.

The zigzag antiferromagnetism in Na2IrO3 naturally arises from the QMO framework.

The electronic structure features substantial quenching of orbital moments.

Abstract

Contrary to previous studies that classify Na2IrO3 as a realization of the Heisenberg-Kitaev model with dominant spin-orbit coupling, we show that this system represents a highly unusual case in which the electronic structure is dominated by the formation of quasi-molecular orbitals (QMOs), with substantial quenching of the orbital moments. The QMOs consist of six atomic orbitals on an Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of such QMOs in solids invokes very different physics compared to the models considered previously. Employing density functional theory calculations and model considerations we find that both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow from the QMO model.