Ab initio analysis of the tight-binding parameters and magnetic interactions in Na2IrO3

By means of density functional theory (DFT) calculations [with and without inclusion of spin-orbit (SO) coupling] we present a detailed study of the electronic structure and corresponding microscopic Hamiltonian parameters of Na₂IrO₃. In particular, we address the following aspects: (i) We investigate the role of the various structural distortions and show that the electronic structure of Na₂IrO₃ is exceptionally sensitive to structural details. (ii) We discuss both limiting descriptions for Na₂IrO₃ - quasimolecular orbitals (small SO limit, itinerant) versus relativistic orbitals (large SO limit, localized) - and show that the description of Na₂IrO₃ lies in an intermediate regime. (iii) We investigate whether the nearest neighbor Kitaev-Heisenberg model is sufficient to describe the electronic structure and magnetism in Na₂IrO₃. In particular, we verify the recent suggestion of an antiferromagnetic Kitaev interaction and show that it is not consistent with actual or even plausible electronic parameters. Finally, (iv) we discuss correlation effects in Na₂IrO₃. We conclude that while the Kitaev-Heisenberg Hamiltonian is the most general expression of the quadratic spin-spin interaction in the presence of spin-orbit coupling (neglecting single-site anisotropy), the itinerant character of the electrons in Na ₂IrO₃ makes other terms beyond this model (including, but not limited to, 2nd and 3rd neighbor interactions) essential.