Strength of Kitaev Interaction in Na$_3$Co$_2$SbO$_6$ and Na$_3$Ni$_2$BiO$_6$

TL;DR

This paper develops a new method to determine the strength of Kitaev interactions in low spin-orbit coupling 3d systems and applies it to Na$_3$Co$_2$SbO$_6$ and Na$_3$Ni$_2$BiO$_6$, revealing different interaction strengths and properties.

Contribution

The paper introduces a state-dependent anisotropy method for rapidly assessing Kitaev interaction strength from first-principles calculations.

Findings

Na$_3$Co$_2$SbO$_6$ exhibits significant ferromagnetic Kitaev interaction.

Na$_3$Ni$_2$BiO$_6$ has negligible Kitaev interaction but matches experimental properties.

Heavy elements Sb or Bi do not contribute to Kitaev interaction through proximity.

Abstract

Kitaev spin liquid is proposed to be promisingly realized in low spin-orbit coupling $3d$ systems, represented by Na$_3$Co$_2$SbO$_6$ and Na$_3$Ni$_2$BiO$_6$. However, the existence of Kitaev interaction is still debatable among experiments, and obtaining the strength of Kitaev interaction from first-principles calculations is also challenging. Here, we report the state-dependent anisotropy of Kitaev interaction, based on which a convenient method is developed to rapidly determine the strength of Kitaev interaction. Applying such method and density functional theory calculations, it is found that Na$_3$Co$_2$SbO$_6$ with $3d^7$ configuration exhibits considerable ferromagnetic Kitaev interaction. Moreover, by further applying the symmetry-adapted cluster expansion method, a realistic spin model is determined for Na$_3$Ni$_2$BiO$_6$ with $3d^8$ configuration. Such model indicates negligible small Kitaev interaction, but it predicts many properties, such as ground states and field effects, which are well consistent with measurements. Furthermore, we demonstrate that the heavy elements, Sb or Bi, located at the hollow sites of honeycomb lattice, do not contribute to emergence of Kitaev interaction through proximity, contradictory to common belief. The presently developed anisotropy method will be beneficial not only for computations but also for measurements.