Kitaev materials are of great interest due to their potential in realizing quantum spin liquid (QSL) states and applications in topological quantum computing. In the pursuit of realizing Kitaev QSL, a Mott insulator with strong bond-dependent frustration and weak geometric frustration is highly desirable. Here we explore Kitaev physics in the van der Waals triangular antiferromagnet (AF) ${\mathrm{CoI}}_2$, through the spin-orbital states and Wannier function analyses, exact diagonalization and density matrix renormalization group study of the electronic structure and magnetic properties. We find that the high-spin ${\mathrm{Co}}^{2+}$ ion is in the $J_{\mathrm{eff}}=1/2$ state because of strong spin-orbit coupling, and the weak trigonal elongation and crystal field contribute to the observed weak in-plane magnetic anisotropy. The strong $t_{2g}\text{−}{e}_g$ hopping via the strong Co $3d$-I $5p$ hybridization gives rise to a strong Kitaev interaction ($K_1$) at the first nearest neighbors (1NN), and the long Co-Co distance and the weak $t_{2g}\text{−}{t}_{2g}$ hoppings determine a weak Heisenberg interaction $J_1$. The resultant $|K_1/J_1|$ = 6.63 confirms a strong bond-dependent frustration, while the geometric frustration due to the 3NN Heisenberg interaction $J_3$ gets involved, and they all together result in the experimental helical AF order in ${\mathrm{CoI}}_2$. We then propose to suppress the $J_3$ using a partial Mg substitution for Co, and indeed we find that ${\mathrm{Co}}_{2/3}{\mathrm{Mg}}_{1/3}{\mathrm{I}}_2$ has the much reduced geometric frustration but hosts the robust bond-dependent frustration, and thus it would be a promising Kitaev material being so far closest to the QSL state.

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• Magnetism
• Quantum spin liquid
• Density functional theory
• Density matrix renormalization group
• Exact diagonalization
• Kitaev-Heisenberg model
• Wannier function methods