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

TL;DR

This study uses density functional theory to analyze the electronic structure and magnetic interactions in Na2IrO3, revealing its intermediate regime between localized and itinerant electrons and the insufficiency of simple models.

Contribution

It provides a detailed ab initio analysis of the microscopic Hamiltonian parameters and challenges the adequacy of the nearest neighbor Kitaev-Heisenberg model for Na2IrO3.

Findings

Electronic structure is highly sensitive to structural distortions.

Na2IrO3 lies in an intermediate regime between localized and itinerant electrons.

Nearest neighbor Kitaev-Heisenberg model is insufficient to describe magnetic interactions.

Abstract

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; quasi-molecular 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 Na2IrO3 makes other terms beyond this model (including, but not limited to 2nd and 3rd neighbor interactions) essential.