1205.0434 (I. I. Mazin et al.)

Formation of quasi-molecular orbitals and suppression of spin-orbit
coupling in Na2IrO3    [PDF]

I. I. Mazin, H. O. Jeschke, R. Valenti, D. I. Khomskii

Spin-orbit (SO) coupling can lead to many nontrivial effects in such fields as spintronics (Rashba effect), topological insulators, or to the formation of topologically protected states in systems de- scribed by the Kitaev or the Heisenberg-Kitaev model. This last situation was recently proposed to be realized in the honeycomb Na2IrO3. The philosophy behind this proposal is that the SO coupling for the heavy Ir ions is very strong, and cannot be quenched by the small trigonal crystal field. We show, however, that Na2IrO3 represents a highly unusual example, in which the electronic structure is dominated by the formation of quasi-molecular composite orbitals (QMOs), as opposed to atomic orbitals (AOs). The QMOs consist of six AOs on an Ir hexagon, and the orbital moment of each QMO is quenched even in the absence of any trigonal field. The concept of such composite orbitals, resembling molecular orbitals in hexagonal molecules such as benzene, is completely new for Na2IrO3 and similar solids, and invokes very different physics compared to the models considered previously. Yet such QMOs necessarily form around each honeycomb as long as the nearest neighbor Ir-O-Ir hopping is the dominant interaction (which it is in Na2IrO3). These orbitals form narrow well-separated subbands. This leads to numerous nontrivial consequences which can be also relevant for other systems with orbital degrees of freedom. For instance, one has to account for Hubbard correlations among the QMOs, and not individual AOs. In particular, both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow from this model, without the need to include the effects of Hubbard U.

View original: http://arxiv.org/abs/1205.0434