Pseudospin exchange interactions in d^7 cobalt compounds: Possible realization of the Kitaev model

TL;DR

This paper proposes that d^7 cobalt compounds can host Kitaev model physics, with unique exchange interactions influenced by e_g electrons, potentially stabilizing a spin-liquid phase in cobalt-based materials.

Contribution

It introduces a new class of materials, d^7 cobalt compounds, as candidates for realizing the Kitaev model, expanding beyond previously studied d^5 systems.

Findings

Kitaev interactions derived for d^7 cobalt compounds.

Presence of e_g electrons alters the balance of magnetic couplings.

Charge-transfer regime favors spin-liquid phase stabilization.

Abstract

The current efforts to find the materials hosting Kitaev model physics have been focused on Mott insulators of d^5 pseudospin-1/2 ions Ir^{4+} and Ru^{3+} with t_{2g}^5(S=1/2, L=1) electronic configuration. Here we propose that the Kitaev model can be realized in materials based on d^7 ions with t_{2g}^5e_g^2(S=3/2, L=1) configuration such as Co^{2+}, which also host the pseudospin-1/2 magnetism. Considering possible exchange processes, we have derived the d^7 pseudospin-1/2 interactions in 90^{\circ} bonding geometry. The obtained Hamiltonian comprises the bond-directional Kitaev K and isotropic Heisenberg J interactions as in the case of d^5 ions. However, we find that the presence of additional, spin-active e_g electrons radically changes the balance between Kitaev and Heisenberg couplings. Most remarkably, we show that the exchange processes involving e_g spins are highly sensitive to whether the system is in Mott (U<\Delta) or charge-transfer (U>\Delta) insulating regime. In the latter case, to which many cobalt compounds do actually belong, the antiferromagnetic Heisenberg coupling J is strongly suppressed and spin-liquid phase can be stabilized. The results suggest cobalt-based materials as promising candidates for the realization of the Kitaev model.